Principles and Practice of Agricultural Analysis

PART FIRST. INTRODUCTION. — PROPERTIES OF THE ELEMENTS. — THE MINERALS OCCURRING IN ROCKS. — ROCKS AND ROCK DECAY. — AUTHORITIES CITED IN PART FIRST.

1. Definitions.—The term soil, in its broadest sense, is used to designate that portion of the surface of the earth which has resulted from the disintegration of rocks and the decay of plants and animals, and which is suited, under proper conditions of moisture and temperature, to the growth of plants. It consists, therefore, chiefly of mineral substances, together with some products of organic life, and of certain living organisms whose activity may influence vegetable growth either favorably or otherwise. The soil also holds varying quantities of gaseous matter and of water, which are important factors in its functions.

2. Origin Of Soil.—Agriculturally considered, the soil proper is the older and more thoroughly disintegrated superficial layer of the earth, which has been longest exposed to weathering and the influences of organic life. It is usually from six to twelve inches, but occasionally several feet in depth. The subsoil, which lies directly under this, is not as a rule so thoroughly disintegrated, since it is protected in a measure by the overlying soil. It usually contains less organic matter than the soil. There is a freer circulation of air in the soil than in the subsoil, and the metallic elements usually exist therein as higher oxids. There is usually a notable difference in color between the soil and subsoil, and frequently a very sharp color line separating the two.

Geologically considered, the soil is that portion of the earth’s crust which has been more or less thoroughly disintegrated by weathering and other forces from the original 2rock formations, or from the sedimentary rocks, or from the unconsolidated sedimentary material. The soil has, therefore, the same essential constitution as the general mass of the earth, except that this débris has been subjected to the solvent action of water and the influence of vegetable growth.

Preliminary to the proper understanding of the methods of the analysis of soils, there should be some definite knowledge concerning the composition of the earth’s crust, so that the analyst may understand more thoroughly the origin and nature of the material he has to deal with, and thereby be better equipped for his work.

3. The Chemical Elements Present in the Soil.—The chemical elements present in the soil are naturally some or all of those which were present in the original rocks. For analytical purposes relating to agriculture, it is not necessary to take into account the rare elements which may occur in the soil, but only those need be considered which are present in some quantity and which enter as an important factor into plant growth. Of the whole number of chemical elements less than twenty are of any importance in soil analysis. These elements may be grouped into two classes, the non-metals, and the metals as follows:

4. Atomic Masses.—For the purpose of facilitating the calculation of results the latest revised table of atomic masses is given below. All the known elements are included in this table for the convenience of analysts who may have to study some of the rarer elements in the course of their work.

This table represents the latest and most trustworthy results reduced to a uniform basis of comparison with oxygen = 16 as starting point of the system. No decimal places representing large uncertainties are used. When values vary, with equal probability on both sides, so far as our present knowledge goes, as in the case of cadmium (111.8 and 112.2), the mean value is given in the table.

Following is a brief description of the most important elements occurring in the earth’s crust in respect of their relations to agriculture.

5. Oxygen exists in the free gaseous state in the atmosphere of which it constitutes about one-fifth by bulk, whilst in combination with other elements it forms nearly half the weight of the solid earth, and eight-ninths by weight of water. It enters into combination with all the other elements, except fluorin, forming what are known as oxids, and with many of the elements it unites in several proportions, forming oxids of different composition. Combined with silicon, carbon, sulfur, and phosphorus, it forms an essential part of the silicates, carbonates, 4sulfates, and phosphates, most of which are very abundant and all of which are very widely distributed in the earth’s crust. In this form it is exceedingly stable and is rarely set free. With the exception of the oxids of silicon these oxids seldom occur uncombined with the metals as constituents of rocks or soils. The oxids of iron very commonly occur as such in rocks and soils, and play a very important part in organic life. The several oxids of iron very frequently determine the color of soils; as the iron in a soil is more or less oxidized, or as it is exposed more or less to access of air, the color of the soil changes. These oxids of iron also play an important part in the absorption capacities of soils for moisture and other physical conditions of soils, and also in the oxidation of organic matters in the soil. Many organic substances, and even the roots of growing plants when deprived of free access of air, can readily secure oxygen from the iron oxid, thus reducing the iron to a lower form of oxidation, the oxygen being used for the oxidation of the organic matter or for the needs of the growing plant; while the lower oxid of iron can more readily take up oxygen of the air and again be converted into a higher oxid, ready again to give up a part of its oxygen and thus serve as a carrier.

6. Silicon never occurs in the free state, but combined with oxygen it forms silica, which constitutes more than one-half of the earth’s crust. The oxid of silicon occurs in the very common form of quartz, and likewise, as silicate of alumina, lime or magnesia. Silicon forms an essential part of many minerals, such as the feldspars, amphiboles, pyroxenes, and the micas, besides being an essential ingredient of many other minerals. Silica is relatively very slightly affected by the ordinary forces concerned in the decay of rocks, and even after the crystals of feldspars, micas, and other common minerals occurring in rocks have been disintegrated the silica remains as hard grains of sand, forming the bulk of most soils. By far the larger part of silicon in soils is in the form of grains of quartz or silica. This form, however, is probably chemically inert in regard to plant growth, but it plays a very important part in the physical structure of soils and in the physical relation of soils to plant growth.

57. Carbon as an elementary substance occurs as diamond and graphite and in an impure form as anthracite and bituminous coals. In peats and mucks carbon is the chief constituent. This substance is also contained in the organic matters of the soil known as humus, and the relation of the carbon to nitrogen often throws important light upon the amount and character of the nitrogenous matters. In composition with oxygen it forms the chief food of growing plants, the carbon of the carbon dioxid of the air being elaborated into the tissue of the plants and the oxygen returned to the atmosphere. The content of carbon dioxid in the air is from three to five parts per thousand by volume. As carbonates this element helps to form some of the most important ingredients of the earth’s crust, namely, limestones, marbles, dolomites, etc., and in an organic form it is found in the shells of the crustaceans. The calcareous matter of the soil, that is, the carbonates of the earths therein found, are of the highest importance from an agricultural point of view. The carbonates in the soil not only favor the process of converting nitrogenous bodies into forms suitable for plant food, but also exert a most potent influence on the physical state of the soil and its capacity for holding water and permitting its flow to and from the rootlets of the plant.

8. Sulfur occurs in nature in both the free and combined state. In the free state it is found in volcanic regions such as Sicily, Iceland, and the western United States. Its usual form of occurrence is in combination with the metals to form sulfids, or with oxygen and a metal to form sulfates. Sulfur and iron combine to form iron pyrites or iron disulfid (FeS₂), while sulfur, oxygen, and calcium are found in gypsum, an important fertilizing compound.

Sulfur plays an important part in the nourishment of plants, being found in them both as sulfuric acid and in organic compounds. Methods for estimating the sulfur in both forms will be found in another part of this manual.

9. Hydrogen is a colorless, invisible gas, without taste or smell. It occurs free in small proportions in certain volcanic gases, and in natural gas, but its most common form is 6in combination with oxygen as water (H₂O), of which it forms 11.13 per cent by weight. It also occurs in combination with carbon to form the hydrocarbons, such as the mineral oils (petroleum, etc.) and gases. Hydrogen is of no importance to agriculture in a free state, but water is the most important of all plant foods.

10. Chlorin occurs free in nature only in limited amounts and in volcanic vents. Its most common form is in combination with hydrogen, forming hydrochloric acid, or with the metals to form chlorids. It combines with sodium to form sodium chlorid or common salt (NaCl), which is the most abundant mineral ingredient in sea water and which can usually be detected in rain and ordinary terrestrial waters. In this form, also, it exists as extensive beds of rock salt, which is mined for commercial purposes.

Chlorin is found uniformly in plants and must be regarded as an essential constituent thereof. Common salt applied to a soil modifies its power of attracting and holding water.

11. Phosphorus never occurs in nature in a free state but exists in combination in greater or less quantities in all soils. Its combinations are also found in large deposits of minerals known as phosphorite and apatite and as so-called pebble deposit and phosphate rock. Phosphorus in some sort of combination is one of the most essential elements in animal and plant food. In animals its compounds form almost all of the mineral matter of the bones, and in plants they are the chief constituents of the ash of seeds.

The mineral deposits of phosphorus, as well as bones, are chiefly tri-calcium phosphate, while the slag compound resulting from the basic treatment of iron ores rich in phosphorus is a tetra-calcium salt.

The pebble deposits and some rock phosphates are supposed to be of organic origin, derived from the remains of marine, terrestrial, and aerial animals.

Cereal crops remove about twenty pounds of phosphoric acid per acre from the soil annually and grass crops about twelve pounds. The total phosphoric acid removed annually by the cereal and grass crops in the United States is nearly four billion pounds.

7Gautier^[1] calls attention to the fact that the oldest phosphates are met with in the igneous rocks such as basalt, trachyte, etc., and even in granite and gneiss. It is from these inorganic sources, therefore, that all phosphatic plant food must have been drawn. In the second order in age Gautier places the phosphates of hydro-mineral origin. This class not only embraces the crystalline apatites but also those phosphates of later formation formed from hot mineral waters in the jurassic, cretaceous, and tertiary deposits. These deposits are not directly suited to nourish plants.

The third group of phosphates in order of age and assimilability embraces the true phosphorites containing generally some organic matter. They are all of organic origin. In caves where animal remains are deposited there is an accumulation of nitrates and phosphates.

Not only do the bones of animals furnish phosphates but they are also formed in considerable quantities by the decomposition of substituted glycerids such as lecithin. The ammonia produced by the nitrification of the albuminoid bodies combines with the free phosphoric acid thus produced, forming ammonium or diammonium phosphates. The presence of ammonium phosphates in guanos was first noticed by Chevreul more than half a century ago.

If such deposits overlay a pervious stratum of calcium carbonate, such as chalk, and are subject to leaching a double decomposition takes place as the lye percolates through the chalk. Acid calcium phosphate and ammonium carbonate are produced. By further nitrification the latter becomes finally converted into calcium nitrate. In like manner aluminum phosphates are formed by the action of decomposing organic matter on clay.

Davidson,^[2] explains the origin of the Florida phosphates by suggesting that they arose chiefly through the influx of animals driven southward during the glacial period. According to his supposition the waters of the ocean, during the cenozoic period contained more phosphorus than at the present time. The waters of the ocean over Florida were shallow and the shell fish existing therein may have secreted phosphate as well as carbonate 8of lime. This supposition is supported by an analysis of a shell of lingula ovalis, quoted by Dana, in which there were 85.79 per cent of lime phosphate. In these waters were also many fishes of all kinds and their débris served to increase the amount of phosphatic material. As the land emerged from the sea came the great glacial epoch driving all terrestrial animals southward. There was, therefore, a great mammal horde in the swamps and estuaries of Florida. The bones of these animals contributed largely to the phosphatic deposits. In addition to this, the shallow sea contained innumerable sharks, manatees, whales, and other inhabitants of tropical waters, and the remains of these animals added to the phosphatic store.

While these changes were taking place in the quaternary period, the Florida Peninsula was gradually rising, and as soon as it reached a considerable height the process of denudation by the action of water commenced. Then there was a subsidence and the peninsula again passed under the sea and was covered with successive layers of sand. The limestones during this process had been leached by rain water containing an excess of carbon dioxid. In this way the limestones were gradually dissolved while the insoluble phosphate of lime was left in suspension. During this time the bones of the animals before mentioned by their decomposition added to the phosphate of lime present in the underlying strata, while some were transformed into fossils of phosphate of lime just as they are found to-day in vast quantities.

Wyatt,^[3] explains the phosphate deposits somewhat differently. According to him, during the miocene submergence there was deposited upon the upper eocene limestones, more especially in the cracks and fissures resulting from their drying up, a soft, finely disintegrated calcareous sediment or mud. The estuaries formed during this period were swarming with animal and vegetable life, and from this organic life the phosphates were formed by decomposition and metamorphism due to the gases and acids with which the waters were charged.

After the disappearance of the miocene sea there were great disturbances of the strata. Then followed the pliocene and tertiary periods and quaternary seas with their deposits and drifts 9of shells, sands, clays, marls, bowlders, and other transported materials supervening in an era when there were great fluctuations of cold and heat.

By reason of these disturbances the masses of the phosphate deposits which had not been infiltrated in the limestones became broken up and mingled with the other débris and were thus deposited in various mounds or depressions. The general result of the forces which have been briefly outlined, was the formation of bowlders, phosphatic débris, etc. Wyatt therefore classifies the deposits as follows:

1. Original pockets or cavities in the limestone filled with hard and soft rock phosphates and débris.

2. Mounds or beaches, rolled up on the elevated points, and chiefly consisting of huge bowlders of phosphate rock.

3. Drift or disintegrated rock, covering immense areas, chiefly in Polk and Hillsboro counties, and underlying Peace River and its tributaries.

Darton,^[4] ascribes the phosphate beds of Florida to the transformation of guano. According to this author two processes of decomposition have taken place. One of these is the more or less complete replacement of the carbonate by the phosphate of lime. The other is a general stalactitic coating of phosphatic material. Darton further calls attention to the relation of the distribution of the phosphate deposits as affecting the theory of their origin, but does not find any peculiar significance in the restriction of these deposits to the western ridge of the Florida peninsula.

As this region evidently constituted a long narrow peninsula during early miocene time it is a reasonably tentative hypothesis that during this period guanos were deposited from which was derived the material for the phosphatization of the limestone either at the same time or soon after.

Darton closes his paper by saying that the phosphate deposits in Florida will require careful, detailed geologic exploration before their relations and history will be fully understood.

According to Dr. N. A. Pratt the rock or bowlder phosphate had its immediate origin in animal life and to his view the phosphate bowlder is a true fossil. He supposes the existence 10of some species in former times in which the shell excreted was chiefly phosphate of lime. The fossil bowlder, therefore, becomes the remains of a huge foraminifer which had identical composition in its skeleton with true bone deposits or of organic matter.

Perhaps the most complete exposition of the theory of the recovery of waste phosphates, with especial reference to their deposit in Florida, has been given by Eldridge.^[5] He calls attention to the universal presence of phosphates in sea water and to the probability that in earlier times, as during the miocene and eocene geologic periods, the waters of the ocean contained a great deal more phosphate in solution than at the present time. He cites the observations of Bischof, which show the solubility of different phosphates in waters saturated with carbon dioxid. According to these observations apatite is the most insoluble form of lime phosphate, while artificial basic phosphate is the most soluble. Among the very soluble phosphates, however, are the bones of animals, both fresh and old. Burnt bones, however, are more soluble than bones still containing organic matter. Not only are the organic phosphates extremely soluble in water saturated with carbon dioxid, but also in water which contains common salt or chlorid of ammonium. The presence of large quantities of common salt in sea water would, therefore, tend to increase its power of absorbing lime phosphates of organic origin. It is not at all incredible, therefore, to suppose that at some remote period the waters of the ocean, as indicated by these theories, were much more highly charged with phosphates in solution than at the present time.

According to Eldridge, the formation of the hard-rock and soft phosphates may be ascribed to three periods: First, that in which the primary rock was formed; second, that of secondary deposition in the cavities of the primary rock; third, that in which the deposits thus formed were broken up and the resulting fragments and comminuted material were redeposited as they now occur.

“The first of these stages began probably not later than the close of the older miocene, and within the eocene area it may 11have begun much earlier. Whether the primary phosphate resulted from a superficial and heavy deposit of soluble guanos, covering the limestones, or from the concentration of phosphate of lime already widely and uniformly distributed throughout the mass of the original rock, or from both, is a difficult question. In any event, the evidence indicates the effect of the percolation of surface waters, highly charged with carbonic and earth acids, and thus enabled to carry down into the mass of the limestone dissolved phosphate of lime, to be redeposited under conditions favorable to its separation. Such conditions might have been brought about by the simple interchange of bases between the phosphate and carbonate of lime thus brought together, or by the lowering of the solvent power of the waters through loss of carbonic acid. The latter would happen whenever the acid was required for the solution of additional carbonate of lime, or when, through aeration, it should escape from the water. The zone of phosphate deposition was evidently one of double concentration, resulting from the removal of the soluble carbonate thus raising the percentage of the less soluble phosphate, and from the acquirement of additional phosphate of lime from the overlying portions of the deposits.”

“The thickness of the zone of phosphatization in the eocene area is unknown, but it is doubtful if it was over twenty feet. In the miocene area the depth has been proved from the phosphates in situ to have been between six and twelve feet.”

The deposits of secondary origin, according to Eldridge, are due chiefly to sedimentation, although some of them may have been due to precipitation from water. This secondary deposition was kept up for a long period, until stopped by some climatic or geologic change. The deposits of phosphates thus formed in the Florida peninsula are remarkably free from iron and aluminum, in comparison with many of the phosphates of the West Indies.

The third period in the genesis of the hard rock deposits embraces the time of formation of the original deposits and their transportation and storage as they are found at the present time. The geologic time at which this occurred is somewhat uncertain but it was probably during the last submergence of the peninsula.

12In all cases the peculiar formation of the Florida limestone must be considered. This limestone is extremely porous and therefore easily penetrated by the waters of percolation. A good illustration of this is seen on the southwestern and southern edges of Lake Okeechobee. In following down the drainage canal which has been cut into the southwest shore of the lake the edge of the basin, which is composed of this porous material may be seen. The appearance of the limestone would indicate that large portions of it have already given way to the process of solution. The remaining portions are extremely friable, easily crushed, and much of it can be removed by the ordinary dredging machines. Such a limestone as this is peculiarly suited to the accumulation of phosphatic materials, due to the percolation of the water containing them. The solution of the limestone and consequent deposit of the phosphate of lime is easily understood when the character of this limestone is considered.

Shaler, as quoted by Eldridge in the work already referred to, refers to this characteristic of the limestone and says that the best conditions for the accumulation of valuable deposits of lime phosphate in residual débris appear to occur where the phosphatic lime marls are of a rather soft character; the separate beds having no such solidity as will resist the percolation of water through innumerable incipient joints such as commonly pervade stratified materials, even when they are of a very soft nature.

Eldridge is also of the opinion that the remains of birds are not sufficient to account for the whole of the phosphatic deposits in Florida. He ascribes them to the joint action of the remains of birds, of land and marine animals and to the deposition of the phosphatic materials in the waters in the successive subsidences of the surface below the water line.

12. Nitrogen as a mineral constituent of soils, is found chiefly in the form of nitrates, but, owing to their solubility, they can not accumulate in soils exposed to heavy rain-falls. The gaseous nitrogen in the soil is also of some importance, since it is in this material that the anaerobic organisms which accumulate on the rootlets of some plants probably act in the process of the fixation of atmospheric nitrogen in a 13form accessible to plants. Nitrogen in the free state, it is believed, is not directly absorbed into the tissues of plants. It is necessary that it be oxidized in some way to nitric acid before it can be assimilated. The importance of nitrogen as a plant food can not be too highly estimated. It is as necessary to plant growth and development as water, phosphoric acid, lime, and potash, and far more costly. While a large quantity of nitrogen exists in the air in an uncombined state, it is, nevertheless, one of the least abundant of the elements of high importance in plant nutrition.

The conservation and increase of the stores of available nitrogen in the soil is one of the chief problems occupying the attention of agricultural chemistry. Nitrogen, which is not immediately available for the growth of plants, is conserved and restored by natural processes in various ways.

The waste nitrogen finds its way sooner or later to the sea, and is restored therefrom in many forms. Sea-weeds of all kinds are rich in recovered nitrogen. Many years ago Forchhammer^[6] pointed out the agricultural value of certain fucoids. Many other chemists have contributed important data in regard to the composition of these bodies.

Jenkins^[7] has shown from the analyses of several varieties of sea-weeds that in the green state they are quite equal in fertilizing value to stall manure, and are sold at the rate of five cents per bushel. These data are fully corroborated by Goessmann.^[8]

Wheeler and Hartwell^[9] give the fullest and most systematic discussion which has been published of the agricultural value of sea-weeds. Sea-weed was used as a fertilizer as early as the fourth century, and its importance for this purpose has been recognized more and more in modern days, especially since chemical investigations have shown the great value of the food materials contained therein.

To show the commercial importance of sea-weed, it is only necessary to call attention to the fact that in 1885 its value as a fertilizer in the State of Rhode Island was $65,044, while the value of all other commercial fertilizers was $164,133. While sea-weed, in a sense, can only be successfully applied to littoral agriculture, yet the extent of agricultural lands bordering on 14the sea is so great as to render its commercial importance of the highest degree of interest.

A large amount of nitrogen is also recovered from the sea in fishes. It is shown by Atwater^[10] that the edible part of fishes has an unusually high percentage of protein. In round numbers, about seventy-five per cent of the water free edible parts of fish are composed of albuminoids. Some kinds of fish are taken chiefly for their oil and fertilizing value, as the menhaden. Squanto,^[11] an American Indian, first taught the early New England settlers the manurial value of fish.

Immense quantities of waste nitrogen are further secured, both from sea and land, by the various genera of birds. The well-known habit of birds in congregating in rookeries during the night and at certain seasons of the year tends to bring into a common receptacle the nitrogenous matters which they have gathered and which are deposited in their excrement and in the decay of their bodies. The feathers of birds are particularly rich in nitrogen, and the nitrogenous content of the flesh of fowls is also high. The decay of remains of birds, especially if it take place largely excluded from the leaching of water, tends to accumulate vast deposits of nitrogenous matter. If the conditions in such deposits be favorable to the processes of nitrification, the whole of the nitrogen, or at least the larger part of it, which has been collected in this débris, becomes finally converted into nitric acid and is found combined with appropriate bases as deposits of nitrates. The nitrates of the guano deposits and of the deposits in caves arise in this way. If these deposits be subject to moderate leaching the nitrate may become infiltered into the surrounding soil, making it very rich in this form of nitrogen. The bottoms and surrounding soils of caves are often found highly impregnated with nitrates.

While for our purpose, deposits of nitrates only are to be considered which are of sufficient value to bear transportation, yet much interest attaches to the formation of nitrates in the soil even when they are not of commercial importance.

In many of the soils of tropical regions not subject to heavy rain-falls, the accumulation of these nitrates is very great. Müntz and Marcano^[12] have investigated many of these soils to 15which attention was called first by Humboldt and Boussingault. They state that these soils are incomparably more rich in nitrates than the most fertile soils of Europe. The samples which they examined were collected from different parts of Venezuela and from the valleys of the Orinoco as well as on the shore of the Sea of Antilles. The nitrated soils are very abundant in this region of South America where they cover large surfaces. Their composition is variable, but in all of them carbonate and phosphate of lime are met with and organic nitrogenous material. The nitric acid is found always combined with lime. In some of the soils as high as thirty per cent of nitrate of lime have been found. Nitrification of organic material takes place very rapidly the year round in this tropical region. These nitrated soils are everywhere abundant around caves, as described by Humboldt, caves which serve as the refuge of birds and bats. The nitrogenous matters, which come from the decay of the remains of these animals, form true deposits of guano which is gradually spread around, and which, in contact with the limestone and with access of air, suffers complete nitrification with the fixation of the nitric acid by the lime.

Large quantities of this guano are also due to the débris of insects, fragments of elytra, scales of the wings of butterflies, etc., which are brought together in those places by the millions of cubic meters. The nitrification, which takes place in these deposits, has been found to extend its products to a distance of several kilometers through the soil. In some places the quantity of the nitrate of lime is so great in the soils that they are converted into a plastic paste by this deliquescent salt.

The theory of Müntz and Marcano in regard to the nitrates of soils, especially in the neighborhood of caves, is probably a correct one, but there are many objections to accepting it to explain the great deposits of nitrate of soda which occur in many parts of Chile. Another point, which must be considered also, is this: That the processes of nitrification can not now be considered as going on with the same vigor as formerly. Some moisture is necessary to nitrification, inasmuch as the nitrifying ferment does not act in perfectly dry soil, and in many localities 16in Chile where the nitrates are found it is too dry to suppose that any active nitrification could now take place.

The existence of these nitrate deposits has long been known.^[13] The old Indian laws originally prohibited the collection of the salt, but nevertheless it was secretly collected and sold. Up to the year 1821, soda saltpeter was not known in Europe except as a laboratory product. About this time the naturalist, Mariano de Rivero, found on the Pacific coast, in the Province of Tarapacá, immense new deposits of the salt. Later the salt was found in equal abundance in the Territory of Antofogasta and further to the south in the desert of Atacama, which forms the Department of Taltal.

At the present time the collection and export of saltpeter from Chile is a business of great importance. The largest export which has ever taken place in one year was in 1890, when the amount exported was 927,290,430 kilograms; of this quantity 642,506,985 kilograms were sent to England and 86,124,870 kilograms to the United States. Since that time the imports of this salt into the United States have largely increased.

According to Pissis^[14] these deposits are of very ancient origin. This geologist is of the opinion that the nitrate deposits are the result of the decomposition of feldspathic rocks; the bases thus produced gradually becoming united with the nitric acid provided from the air.

According to the theory of Nöllner^[15] the deposits are of more modern origin and due to the decomposition of marine vegetation. Continuous solution of soils, gives rise to the formation of great lakes of saturated water, in which occurs the development of much marine vegetation. On the evaporation of this water, due to geologic isolation, the decomposition of nitrogenous organic matter causes generation of nitric acid, which, coming in contact with the calcareous rocks, attacks them, forming nitrate of calcium, which, in presence of sulfate of sodium, gives rise to a double decomposition into nitrate of sodium and sulfate of calcium.

The fact that iodin is found in greater or less quantity in Chile saltpeter is one of the chief supports of this hypothesis of marine origin, inasmuch as iodin is always found in sea 17and not in terrestrial plants. Further than this, it must be taken into consideration that these deposits of nitrate of soda contain neither shells nor fossils, nor do they contain any phosphate of lime. The theory, therefore, that they were due to animal origin is scarcely tenable.

13. Boron occurs chiefly in volcanic regions, but is much more widely distributed in the soil than formerly believed. It is a regular constituent of the ash of many plants,^[16] and is, therefore, thought to be a true plant food. It is one of the least abundant of the elements, not occurring in sufficient quantity to find a place in the table showing their relative abundance, which is to follow. Boracic acid is used to some extent as a preservative.

14. Fluorin does not occur free in nature, but it exists chiefly in combination with calcium, forming fluorspar, and traces of it are found in sea water. It occurs in bone, teeth, blood, and the milk of mammals. It is the only element that does not combine with oxygen, and it can be isolated only with the greatest difficulty. Only very small traces of it are found ordinarily and it is usually not considered in the chemical analysis of soils. Fluorin is found, however, in considerable quantities in certain phosphate deposits.

15. Aluminum is, probably, next to oxygen and silicon, the most abundant element of the earth’s crust, of which it is estimated to form about one-twelfth. It has never been found, in nature, in the free state, but commonly occurs in combination with silicon and oxygen, in which form it is an abundant constituent of feldspar, mica, kaolin, clay, slate, and many other rocks and minerals.

By the weathering of feldspar, mica, and other minerals containing aluminum, kaolin or true clay is formed, which is of the greatest importance in the constitution of the soil. The compounds of aluminum are not so important as plant food as they are as the constituents of the soil, forming a large part of its bulk, and modifying in the most profound degree its physical properties. It is the custom of some authors to use the word clay to designate the fine particles of soil which have 18in general the same relations to moisture and tilth as the particles of weathered feldspar, etc. In a strict chemical sense, however, the term clay is applied only to the hydrated silicate of alumina formed as indicated above. The fertility of a soil is largely dependent on the quantity of clay which it contains, its relations to moisture and amenability to culture being chiefly conditioned by its clay content. The determination of the percentage of clay in soils is an operation of the highest utility in forming an opinion of the value of a soil on analytical data alone.

16. Calcium is one of the commonest and most important elements of the earth’s crust, of which it has been estimated to compose about one-sixteenth. It does not occur free in nature, but its most common form is in combination with carbon dioxid, forming the mineral calcite, marble, and the very abundant limestone rocks. In this form it is slightly soluble in water containing carbon dioxid, and hence lime has become a universal component of all soils and is very generally found in natural waters, in which it furnishes the chief ingredient necessary for the formation of the shells and skeletons of the various tribes of mollusca and corals. In combination with sulfuric acid calcium forms the rock gypsum. Lime is not only a necessary plant food, but influences in a marked degree the physical condition of the soil and the progress of nitrification. Many stiff clay soils are rendered porous and pulverulent by an application of lime, and thus made far more productive. On account of its great abundance and low price, it has not commanded the degree of attention from farmers and agricultural chemists which its merits deserve. It forms an essential ingredient of plants and animals, in the latter being collected chiefly in the bones, while in plants it is rather uniformly distributed throughout all the tissues.

17. Magnesium occurs chiefly in combination with carbon dioxid or with lime and carbon dioxid in the mineral dolomite. It is intimately associated with calcium and a trace of it is nearly always found where lime occurs in any considerable quantity. The bitter taste of sea water and some mineral waters is due to the presence of salts of magnesia. In combination 19with silica it forms an essential part of such rocks as serpentine, soapstone, and talc. Magnesia is not of much importance as a plant food nor as a fertilizing material.

18. Potassium combined with silica is an important element in many mineral silicates as, for instance, orthoclase. Granitic rocks usually contain considerable quantities of potassium, and on their decomposition this becomes available for plant food. In the form of chlorid, potassium is found in small quantities in sea water, and as a nitrate it forms the valuable salt known as niter or saltpeter. Potassium, as is the case with phosphorus, is universally distributed in soils, and forms one of the great essential elements of plant food. Under the form of kainite and other minerals large quantities of potassium are used for fertilizing and for the manufacture of pure salts for commercial and pharmaceutical purposes. The ordinary potassium salts are very soluble and for this reason they can not accumulate in large quantities in soils exposed to heavy rain-fall. In the form of carbonate, potassium forms one of the chief ingredients of hard wood ashes, and in this form of combination is especially valuable for fertilizing purposes. Potash salts, being extremely soluble, are likely to be held longest in solution. Some of them, are recovered in animal and vegetable life, but the great mass of potash carried into the sea still remains unaccounted for. The recovery of the waste of potash is chiefly secured by the isolation of sea waters containing large quantities of this salt and their subsequent evaporation. Such isolation of sea waters takes place by means of geologic changes in the level of the land and sea. In the raising of an area above the water level there is almost certain to be an enclosure, of greater or less extent, of the sea water in the form of a lake. This enclosure may be complete or only partial, the enclosed water area being still in communication with the main body of the sea by means of small estuaries. If this body of water be exposed to rapid evaporation, as was doubtless the case in past geologic ages, there will be a continual influx of additional sea water through these estuaries to take the place of that evaporated. The waters may thus become more and more charged with saline constituents. Finally a point is reached in the evaporation 20when the less soluble of the saline constituents begin to be deposited. In this way the various formations of mineral matter, produced by the drying up of enclosed waters, take place.

The most extensive potash deposits known are those in the neighborhood of Stassfurt, in Germany. The following description probably represents the method of formation of these deposits:^[17]

“The Stassfurt salt and potash deposits had their origin, thousands of years ago, in a sea or ocean, the waters of which gradually receded, leaving near the coast, lakes which still retained communication with the great ocean by means of small channels. In that part of Europe the climate was then tropical, and the waters of these lakes rapidly evaporated but were constantly replenished through these small channels connecting them with the main body. Decade after decade this continued, until by evaporation and crystallization, the various salts present in the sea water were deposited in solid form. The less soluble material, such as sulfate of lime or ‘anhydrite,’ solidified first and formed the lowest stratum. Then came common rock salt with a slowly thickening layer which ultimately reached 3000 feet, and is estimated to have been 13,000 years in formation. This rock salt deposit is interspersed with lamellar deposits of ‘anhydrite,’ which gradually diminish towards the top and are finally replaced by the mineral ‘polyhalite,’ which is composed of sulfate of lime, sulfate of potash, and sulfate of magnesia. The situation in which this polyhalite predominates is called the ‘polyhalite region’ and after it comes the ‘kieserite region,’ in which, between the rock salt strata, kieserite (sulfate of magnesia) is imbedded. Above the kieserite lies the ‘potash region,’ consisting mainly of deposits of carnallite, a mineral compound of muriate of potash and chlorid of magnesia. The carnallite deposit is from 50 to 130 feet thick and yields the most important of the crude potash salts and that from which are manufactured most of the concentrated articles, including muriate of potash.”

“Overlying this region is a layer of impervious clay which acts as a water-tight roof to protect and preserve the very soluble potash and magnesia salts, which, had it not been for the 21very protection of this overlying stratum, would have been long ages ago washed away and lost by the action of the water percolating from above. Above this clay roof is a stratum, of varying thickness of anhydrite, and still above this a second salt deposit, probably formed under more recent climatic and atmospheric influences or possibly by chemical changes in dissolving and subsequent precipitation. This salt deposit contains ninety-eight per cent (often more) of pure salt, a degree of purity rarely elsewhere found. Finally, above this are strata of gypsum, tenacious clay, sand, and limestone, which crop out at the surface.”

“The perpendicular distance from the lowest to the upper surface of the Stassfurt salt deposits is about 5000 feet (a little less than a mile), while the horizontal extent of the bed is from the Harz Mountains to the Elbe River in one direction, and from the city of Madgeburg to the town of Bernburg in the other.”

According to Fuchs and DeLauny^[18] the saline formation near Stassfurt is situated at the bottom of a vast triassic deposit surrounding Madgeburg. The quantity of sea water which was evaporated to produce saline deposits of more than 500 meters in thickness must have been enormous and the rate of evaporation great. It appears that a temperature of 100° would have been quite necessary, acting for a long time, to produce this result.

These authors therefore admit that all the theories so far advanced to explain the magnitude of these deposits are attended with certain difficulties. What, for instance, could have caused a temperature of 100°? The most reasonable source of this high temperature must be sought for in the violent chemical action produced by the double decompositions of such vast quantities of salts of different kinds. There may also have been at the bottom of this basin some subterranean heat such as is found in certain localities where boric acid is deposited.

Whatever be the explanation of the source of the heat it will be admitted that at the end of the permian period there was thrown up to the northeast of the present saline deposits a ridge extending from Helgoland to Westphalia. This dam established throughout the whole of North Germany saline lagoons 22in which evaporation was at once established, and these lagoons were constantly fed from the sea.

There was then deposited by evaporation, first of all a layer of gypsum and afterwards rock salt, covering with few exceptions the whole of the area of North Germany.

But around Stassfurt there occurred at this time geologic displacements, the saline basin was permanently closed and then by continued evaporation the more deliquescent salts, such as polyhalite, kieserite, and carnallite, were deposited.

These theories account with sufficient ease for the deposition of the saline masses, but do not explain why in those days the sea water was so rich in potash and why potash is not found in other localities where vast quantities of gypsum and common salt have been deposited. It may be that the rocks composing the shores of these lagoons were exceptionally rich in potash and that this salt was, therefore, in a certain degree, a local contribution to the products of concentration.

19. Sodium is never found free in nature, but its most common form is in combination with chlorin as common salt, an important ingredient of sea water. Combined with silica sodium is an important element in many silicates. Sodium, although closely related to potassium chemically, cannot in any case be substituted therefor in plant nutrition. In combination with nitrogen it forms soda or Chile saltpeter which is a valuable fertilizer on account of its content of nitric acid.

20. Iron is the most abundant of the heavy metals, and occurs in nature both free and combined with other elements. In the free state it is found only to a limited extent in basaltic rocks and meteorites, but in combination with oxygen it is one of the most widely diffused of metals, and forms the coloring matter of a large number of rocks and minerals. In this form, too, it exists as the valuable ores of iron known as magnetite and hematite. In combination with sulfur it forms the mineral pyrite, FeS₂. The yellow and red colors of soils are due chiefly to iron oxids. It is an important plant food, although not taken up in any great quantity by the tissues of plants.

2321. Manganese, next to iron, is the most abundant of the heavy metals. It occurs in nature only in combination with oxygen, in which form it is associated in minute quantities with iron in igneous rocks or in the forms known mineralogically as pyrolusite, psilomelane and wad. As the peroxid of manganese it occurs in concretionary forms scattered abundantly over the bottom of the deep sea. It is found in the ash of some plants but is not believed to be an essential to plant growth.

22. Barium occurs in nature combined with sulfuric acid, forming the mineral barite, or heavy spar, or with carbon dioxid forming the mineral witherite. It is of small importance from an agricultural standpoint.

23. Relative Abundance of the More Important Chemical Elements.—It will be of interest to the agricultural analyst to know as nearly as possible the relative abundance of the more important chemical elements. This subject has been carefully studied by Prof. F. W. Clarke in a paper read before the Philosophical Society of Washington.^[19] The materials considered in these calculations are the atmosphere, the water, and the solid crust of the earth to the depth of ten miles below the sea level. Of these materials the relative quantities of the three constituents named are as follows:

According to these calculations the relative abundance of the important elements composing the atmosphere, the water of the ocean and the solid crust of the earth to the depth given is as follows:

24. Fluorin is not mentioned in this table but it is stated that its probable percentage is 0.02 to 0.03 making it thus slightly more abundant than nitrogen.

One of the chief points of interest in connection with this table is that the nitrogen which is regarded by most persons as one of the most abundant of the elements is almost the least abundant of those mentioned.

25. The Soil, as before stated, being comprised almost exclusively of decayed rocks, its characteristics would naturally be determined by the character of the minerals contained in the rocks.

A rock may be composed of a single mineral or an aggregation of several minerals.

According to the authority of the National Museum^[20] it may occur, either in the form of stratified beds, eruptive masses, sheets or dikes, or as veins and other chemical deposits of comparatively little importance as regards size and extent. The mineral composition of rocks is greatly simplified by the wide range of conditions under which the commonest minerals can be formed. Thus quartz, feldspar, mica, the minerals of the hornblende, or pyroxene group, can be formed from a mass cooling from a state of fusion; they may be crystallized from solution, or be formed from volatilized products. They are therefore the 25commonest of minerals and are rarely excluded from rocks of any class, since there is no process of rock formation which determines their absence.

Most of the common minerals, like the feldspars, micas, hornblendes, pyroxenes, and the alkaline carbonates possess the capacity of adapting themselves to a very considerable range of compositions. In the feldspars, for example, lime, soda, or potash may replace one another almost indefinitely, and it is now commonly assumed that true species do not exist, but all are but isomorphous admixtures passing into one another by all gradations, and the names albite, oligoclase, anorthite, etc., are to be used only as indicating convenient stopping and starting points in the series. Hornblende or pyroxene, further, may be pure silicate of lime and magnesia, or iron and manganese may partially replace these substances. Lime carbonate may be pure, or magnesia may replace the lime in any proportion.

These illustrations are sufficient to show the reason for the great simplicity of rock masses as regards their chief mineral constituents.

Whatever may be the conditions of the origin of a rock mass, the probabilities are that it will be formed essentially of one or more of a half a dozen minerals in some of their varieties.

But however great the adaptability of these few minerals may be they are, nevertheless, subject to very definite laws of chemical equivalence. There are elements which they cannot take into their composition, and there are circumstances which retard their formation while other minerals may be crystallizing. In a mass of rock of more or less accidental composition formed under these widely varying conditions it may, therefore, be expected that other minerals will form, in considerable numbers, but minute quantities. It is customary to speak of those minerals which form the chief ingredients of any rock, and which may be regarded as characteristic of any particular variety, as the essential constituents, while those which occur in but small quantities, and whose presence or absence does not fundamentally affect its character, are called accessory constituents. The accessory mineral which predominates, and which is, as a rule, present in such quantities as to be recognizable by the unaided 26eye, is the characterizing accessory. Thus a biotite granite is a stone composed of the essential minerals quartz and potash feldspar, but in which the accessory mineral biotite occurs in such quantities as to give a definite character to the rock.

26. Classification Of Minerals.—The minerals of rocks may also be conveniently divided into two groups, according as they are products of the first consolidation of the mass or of subsequent changes. This is the system here adopted. We thus have:

(1) The original or primary constituents, those which formed upon its first consolidation. All the essential constituents are original, but on the other hand all the original constituents are not essential. Thus, in granite, quartz and orthoclase are both original and essential, while beryl and zircon or apatite, though original, are not essential.

(2) The secondary constituents are those which result from changes in a rock subsequent to its first consolidation, changes which are due in great part to the chemical action of percolating water. Such are the calcite, chalcedony, quartz, and zeolite deposits which form in the druses and amygdaloidal cavities, of traps and other rocks.

Below is given a list of the more common, original and secondary minerals occurring in rocks. It will be observed that the same mineral may, in certain cases, occur in both original and secondary forms. The tables following were prepared by Dr. George P. Merrill.

1.

Quartz, SiO₂.

2.

The Feldspars:

2a.	Orthoclase.	Anhydrous silicate of alumina with varying amounts of lime, potash, or soda and rarely barium.
2b.	Microcline.
2c.	Albite.
2d.	Oligoclase.
2e.	Andesite.
2f.	Labradorite.
2g.	Bytownite.
2h.	Anorthite.

3.

The Amphiboles:

3a.	Hornblende.	Anhydrous silicates of lime and magnesia with iron and alumina in the dark varieties.
3b.	Tremolite.
3c.	Actinolite.
3d.	Arfvedsonite.
3e.	Glaucophane.
3f.	Smaragdite.

4.

The Monoclinic Pyroxenes:

4a.	Malacolite.	Anhydrous silicates of magnesia and lime with alumina and iron in the dark varieties.
4b.	Diallage.
4c.	Augite.
4d.	Acmite.
4c.	Aegerite.

5.

The Rhombic Pyroxenes:

5a.	Enstatite (bronzite).	Silicates of magnesia and iron.
5b.	Hypersthene.

6.

The Micas:

6a.	Muscovite.	Anhydrous silicates of alumina with potash, soda, and iron.
6b.	Biotite.
6c.	Phlogopite.

7.

Calcite,

8.

Dolomite.

9.

Gypsum.

10.

Olivine.

11.

Beryl.

12.

Tourmaline.

13.

Garnet, variable common form.

14.

Vesuvianite.

15.

Epidote.

16.

Zoisite.

17.

Allanite.

18.

Andalusite.

19.

Staurolite.

27

20.

Fibrolite.

21.

Cyanite.

22.

Scapolite.

23.

Apatite.

24.

Elaeolite and Nepheline.

25.

Leucite.

26.

Cancrinite.

27.

The Sodalite Group:

27a.

Sodalite.

27b.

Haüyn (noseau).

28.

Zircon.

29.

Chondrodite.

30.

Cordierite.

31.

Topaz.

32.

Corundum.

33.

Titanite (sphene).

34.

Rutile.

35.

Menaccanite.

36.

Magnetite.

37.

Hematite.

38.

Chromite.

39.

The Spinels:

39a.

Pleonast.

39b.

Picotite.

40.

Pyrolusite.

41.

Halite (common salt).

42.

Fluorite.

43.

The Elements:

43a.

Graphite.

43b.

Carbon.

43c.

Iron.

43d.

Copper.

44.

The Metallic Sulfids:

44a.

Galena.

44b.

Sphalerite.

44c.

Pyrrhotite.

44d.

Marcasite.

44e.

Pyrite.

44f.

Chalcopyrite.

44g.

Arsenopyrite.

1.

Quartz:

1a.

Chalcedony.

1b.

Opal.

1c.

Tridymite.

2.

Albite.

3.

The Amphibole Group:

3a.

Hornblende.

3b.

Tremolite.

3c.

Actinolite.

3d.

Uralite.

4.

Muscovite (sericite).

5.

The Chlorites:

5a.

Jefferisite.

5b.

Ripidolite.

5c.

Penninite.

5d.

Prochlorite.

6.

Calcite (and aragonite).

7.

Wollastonite.

8.

Scapolite.

9.

Garnet.

10.

Epidote.

11.

Zoisite.

12.

Serpentine.

13.

Talc.

14.

Kaolin.

15.

The Zeolites:

15a.

Pectolite.

15b.

Laumontite.

15c.

Prehnite.

15d.

Thomsonite.

15e.

Natrolite.

15f.

Analcite.

15g.

Datolite.

15h.

Chabazite.

15i.

Stilbite,

15k.

Heulandite.

15l.

Harmotome.

16.

Magnetite.

17.

Hematite.

18.

Limonite.

19.

Siderite.

20.

Pyrite.

21.

Pyrrhotite.

27. Types Of Rocks.—Rocks may be divided in reference to their structure into four types: First, crystalline; second, vitreous; third, colloidal; fourth, fragmental.

Of these classes there may be selected, as types of the first order, granite and crystalline limestone.

The second class is typically represented by obsidian. Rocks of this kind are confined to a volcanic origin.

The third class of rocks is completely amorphous in its structure and is less common than the others. It is found only in rocks of chemical origin. Types of this class are the siliceous sinters, opals, flint nodules, and many serpentines.

Of the fourth class of rocks, sandstone is typical, being comprised wholly of fragments of rocks pre-existing. The particles may be held together either by cohesion or by a cement composed of silica, iron oxids, carbonate of lime or clayey matter.

28. The Microscopical Structure of Rocks.—A great deal more light is thrown upon the nature of rock materials by microscopical study than by their study in bulk. The requisites for a microscopical study of rock are that the material should be cut into extremely thin laminae with parallel sides and polished so as to transmit the light freely. The study of the crystalline structure of the material is then conducted by means of a microscope furnished with polarizing and analyzing appliances. The light before passing through the mineral film is polarized by a Nicol prism. After passing through the film it is analyzed by a second Nicol prism. In this way the crystalline structure of the rock as affecting polarized light is distinctly brought out. The thickness of the films examined should be from ¹⁄₅₀₀ to ¹⁄₆₀₀ of an inch.

Fig. 1. Microstructure of granite.
Fig. 2. Microstructure of micropegmatite.
Fig. 3. Microstructure of quartz porphyry.
Fig. 4. Microstructure of porphyritic obsidian.
Fig. 5. Microstructure of trachyte.
Fig. 6. Microstructure of serpentine.

29The method of rock study by thin microscopic sections is one of comparatively recent origin. It is scarcely more than a dozen years since the process was fairly adopted by mineralogists. The value of the method is based upon the fact that every crystalline mineral has certain definite optical properties. Therefore, when a crystalline mineral is distorted or misshapen so as to be incapable of identification by the ordinary method, it can be at once identified by its optical examination in the manner just described. In this way not only can one mineral be distinguished from another, but the crystalline system to which it belongs can be accurately pointed out. The value of the method is well summed up by Merrill,^[21] who says that it is not merely an aid in determining the mineralogical composition of a rock, but also, which is often much more important, its structure and the various changes which have taken place in it since its first consolidation. Rocks are not the definite and unchangeable mineral compounds they were once considered, but are rather ever varying aggregates of minerals which even in themselves undergo structural and chemical changes almost without number.

Another valuable result of such a study is illustrated by the discovery that the structural features of a rock are not dependent upon its chemical composition or geologic age, but upon the conditions under which it cooled from the molten magma. Portions of the same rock may vary all the way from a wholly crystalline to a pure vitreous form.

Some typical microstructures of crystalline rocks are shown in the accompanying figures 1–6.^[22]

Although this method of study has thus far been confined mainly to crystalline rocks, its efficiency is by no means limited to them. The fragmental rocks and their decomposed débris to which the name soil is given are equally worthy of study by this method. Indeed, the full value of a chemical analysis of any rock or soil can not be ascertained unless such an analysis is accompanied by a microscopic examination. It is desirable to know not merely what there is in any soil, but in what form these compounds exist. To this latter question the chemical analysis as ordinarily made will give no clew. In Germany a beginning has been made in this line of work, and American scientists are beginning to realize its importance. An outline of this method of analysis will be given in the proper place.

3029. Specific Gravity.—Much information in regard to the properties of a rock, or mineral constituent thereof, may be derived from its specific gravity.

The internal structure of a rock may have much to do with its apparent specific gravity. As an instance of this, it may be stated that an obsidian pumice will float upon water, buoyed up by the air contained in its vesicles, while a compact obsidian of the same composition will sink immediately. A careful discrimination must, therefore, be made between apparent and true specific gravity. In general it may be said that crystalline rocks have a higher specific gravity than those of a vitreous nature. The specific gravity is, therefore, largely dependent upon chemical and crystallographic properties; for instance, among siliceous rocks those which contain the largest amount of silica are the lightest, while those with a comparatively small amount, but rich in iron, lime, and magnesia, are heaviest.

30. Chemical Composition of Rocks.—Rocks are often classified with respect to the chief mineral constituent which they contain. Rocks which are composed largely of lime are termed calcareous; of silica, siliceous; of iron, ferruginous; and of clay, argillaceous. In respect of eruptive rocks, it is customary to speak of those which show above sixty per cent of silica as acidic, while those containing less than fifty per cent of silica and a correspondingly larger amount of iron, lime, and magnesia, are spoken of as basic. Illustrations of the classification of rocks on the above principles are given below.^[23]

31. Color Of Rocks.—The color of rocks is determined chiefly by the oxids of metals which they contain and the degree of oxidation of the mineral in each particular case. There are, however, many colors of rocks which seem to depend not upon any particular mineral ingredient which they contain, but upon some particular crystalline structure or physical condition.

The chief coloring matters in minerals are those which form colored bases such as iron, manganese, chromium, etc. The yellow, brown, and red colors, common to fragmental rocks, are due almost wholly to free oxids of iron. The gray, green, dull brown, and even black colors of crystalline rocks are due to the presence of free iron oxids or to the prevalence of silicate mineral rich in iron, as augite, hornblende, or black mica. Rarely copper and other metallic oxids than those of iron are present in sufficient abundance to impart their characteristic hues. As a rule, a white or light-gray color denotes an absence of an appreciable amount of iron in any of its forms. The bluish and black colors of many rocks, particularly the limestones and slates, are due to the presence of carbonaceous matter.

In still other cases, and particularly the feldspar-bearing rocks, the color may be due in part to the physical condition of the feldspar.

Inasmuch as the color of rocks is due so largely to metallic 32oxids, it is easy to see that they may undergo changes when exposed to weathering, or the degree of oxidation may change, and either, together with changes in the physical structure of the rock, may cause a distinct change in color. Luster is often considered in connection with color, and is due almost exclusively to physical conditions.

32. Kinds of Rocks.—The rocks which form any essential part of the earth’s crust are grouped under four main heads, the distinction being based upon their origin and structure.^[24] Each of the main divisions may be subdivided into groups or families, the distinction being based mainly upon chemical composition, structure, and mode of occurrence. The four chief families are:

First, aqueous rocks, formed mainly through the agency of water as chemical precipitates or as sedimentary beds.

Second, aeolian rocks formed from wind-drifted materials.

Third, metamorphic rocks, changed from their original condition through dynamic or chemical agencies, and which may have been partly of aqueous and partly of igneous origin.

Fourth, igneous or eruptive rocks, which have been brought up from below in a molten condition, and which owe their present structural peculiarities to variations in composition and conditions of solidification.

33. Aqueous Rocks.—Aqueous rocks may be divided into the following general classes:

First, rocks formed as chemical precipitates.

Second, rocks formed as sedimentary deposits and fragmental in structure. The second class may again be subdivided into rocks formed by mechanical agencies and mainly of inorganic materials; and second, rocks composed mainly of the débris of plant and animal life.

In regard to the first form of aqueous rocks, namely, those formed as chemical precipitates, it may be said that while their quantity is not large they are yet of considerable importance from an agricultural point of view. They embrace those substances which, having once been in a condition of vapor or aqueous solution, have been deposited or precipitated, either by 33cooling or by the evaporation of the liquor holding them in solution, or by coming in contact with chemical substances capable of precipitating them. The influence of water as a solvent is perhaps not fully appreciated. Its solvent influence will be noted particularly under the head of weathering or decay of rocks. Its importance, however, in producing stratified rocks has been very great. Water, especially when under great pressure and at a high temperature, has the power of dissolving many minerals. This power is often greatly increased by the mineral matter previously in solution in the water or by the gases which it may contain. As an illustration of the latter property, the solvent action of water charged with carbon dioxid on limestone may be cited.

When mineral matters have been dissolved by the water in the ways mentioned and carried with the water beyond the condition where the solution has taken place, new conditions are found favorable to the precipitation of the dissolved matters. The water, which before may have been very hot, may reach a place where it cools, and being a supersaturated solution, the excess of the material is thrown down as the water cools.

On the other hand, if the solution be due to the presence of carbon dioxid and the water reach a place where it is exposed to the air or where the pressure under which the abundance of the gas has been due is diminished, the carbon dioxid will escape and the mineral matters which have been dissolved thereby will be precipitated.

The incrustations which often appear round the mouth of springs and the occurrence of stalagmites and stalactites in caves are illustrations of this action.

In respect of the formation of rocks as precipitates from a state of vapor we have scarcely any illustrations excepting in volcanic regions. Rocky materials with which we are generally acquainted are practically non-volatile at the highest temperature which can be secured on the earth’s surface, but it is possible that in the interior of the earth the temperature may be so high as to maintain many substances in a state of vapor.

They may, in this case, become disassociated so that the 34compounds or elements exist distinctly in a vaporous condition. Such a vapor transported to regions of diminished temperature would first of all on cooling permit a union of the chemical elements forming new compounds less volatile, which, of course, would be at once precipitated.

The rocks and minerals formed in this way which are of some agricultural importance may be classified as follows:

Oxids, carbonates, silicates, sulfur, sulfids, sulfates, phosphates, chlorids, and hydrocarbon compounds, the most important from an agricultural point of view being the phosphates.

The second group of rocks, namely those formed as sedimentary deposits, differ from those just described in that they are comprised mainly of fragmental materials derived from the breaking down of pre-existing rocks. The formation of fragmental rocks includes, therefore, the same processes as are active in the formation of arable soil. They are deposited from water, and are as a rule distinctly stratified.

Through the action of pressure and the heat thereby generated, or simply through the chemical action of percolating solutions, such rocks pass over into the crystalline sedimentary forms known as metamorphic. All metamorphic rocks, however, are not of a sedimentary origin. For instance, by pressure, heat, and the chemical changes thereby induced, granite may be changed into gneiss and the latter would then be a metamorphic rock.

This group of sedimentary rocks and of sedimentary material, either unchanged or metamorphosed, is of vast extent and includes materials of widely varying chemical and mineralogical nature. They form by far the greater portion of the present surface of the earth, even the mountain ranges being composed mainly of this sedimentary material. Indeed, in the whole of this country there is only a comparatively very small extent of igneous or irruptive rocks. They are of great importance from a purely scientific, as well as agricultural standpoint, since they contain the fossil records of past geologic ages. From them it is possible to study the variations in climate, the meteorological conditions in circumstances and periods far remote, and thus form some idea of the process by which the crust of the earth 35has been modified by natural forces from its original form to the present time.

The sedimentary rocks may be divided, with sufficient accuracy for our purposes, into two great classes: First, rocks formed by mechanical agencies and mainly of inorganic materials. These are subdivided again as follows:

(a) The arenaceous group.

(b) The argillaceous group.

(c) The volcanic group.

The second class of sedimentary rocks is formed largely, or in part at least, by mechanical agencies, but is comprised chiefly of the débris of plant and animal life. It may be subdivided as follows:

(a) The siliceous group, such as infusorial earth.

(b) The calcareous group, fossiliferous formations, limestone, etc.

(c) The carbonaceous group, such as peat, lignite, coals, etc. The different classes of rock described above are distinguished by special qualities represented largely by the name. The first division, the arenaceous group, is composed mainly of the siliceous or coarsely granular materials derived from the disintegration of older crystalline rocks, which have been rearranged in beds of varying thickness through the mechanical agency of water. They are, in short, consolidated or unconsolidated beds of sand and gravel. In composition and texture they vary almost indefinitely. Many of them having suffered little during the process of disintegration and transportation are composed essentially of the same materials as the rocks from which they were derived.

The sandstones, which are the type of these rocks, vary greatly in structure as well as in composition, in some the grains being rounded while in others they are sharply angular.

The microscopic structure of sandstone is shown in figure 7.^[24]

The material by which the individual grains of a sandstone are bound together is usually the material of some of the other classes. The calcareous, ferruginous, and siliceous cements being the chief ones. This cementing substance is deposited among the granules forming the sandstone by percolating water.

36The colors of sandstone are dependent usually upon iron oxids. Especially is this true of the red, brown, and yellow colors. In some of the light grey varieties, the color is that of the minerals comprising the stone. Some of the darker colored sandstones contain organic matter.

Fig. 7.

Microscopic Structure of Sandstone.

The rocks of the argillaceous group are composed essentially of a hydrous silicate of alumina, which is the basis of common clay, and varying amounts of free silica, oxids of iron and manganese, carbonates of lime and magnesia, and small quantities of organic matter. They may have originated in situ from the decomposition of feldspars or as deposits of fine mud or silt at the bottom of large bodies of water. The older formations of these rocks are known as shales, argillites, and slates and the fissile structure which enables this to be split into thin sheets is probably due to the conditions under which they have been formed and not to any properties of the clays themselves.

One of the purest forms of this rock is kaolin, which is almost a pure hydrous silicate of alumina formed from the decomposition of feldspathic rocks from which the alkalies, iron oxids and other soluble constituents have been removed by water.

Under the volcanic group are included the materials ejected 37from volcanic vents in a more or less finely comminuted condition and which through the drifting power of atmospheric currents may be scattered over many miles of territory. Various names are applied to such products, names dependent in large part upon their state of subdivision. Volcanic dust and sand, or ashes, includes the finer dust-like or sand-like materials, and lapilli, or rapilli the coarser. The general name tuff includes the more or less compacted and stratified beds of this material, while trass, peperino, and pozzuolano are local varietal names given to similar materials occurring in European volcanic regions.

The second division, namely sedimentary rocks composed of the débris of plant and animal life includes many forms of great agricultural importance.

The first subdivision of this group is the infusorial or diatomaceous earth. It forms a fine white or yellowish pulverulent rock composed mainly of minute shells, or tests of diatoms, and is often so soft and pliable as to crumble readily between the thumb and fingers. According to Whitney the beds are of comparatively limited extent and for this reason are of little agricultural value, although the weathering of this diatomaceous material gives rise to a light yellow clay forming very fertile agricultural lands.

The second subdivision of this group includes the rocks of a calcareous nature derived from animal life; that is to say, what are properly called limestones. They vary in color, structure, and texture almost indefinitely, and include all possible grades of materials from those which can be used only as a flux, or for lime burning, through ordinary building materials to the finest marbles. These rocks are world-wide in their distribution and limited to no one particular geologic horizon, but are found in stratified beds among rocks of all ages from the most ancient to the most recent.

Owing to the fact that their chief constituent, carbonate of lime, is soluble in ordinary meteoric waters, the rocks have undergone extensive decomposition, their lime being removed, while their less soluble constituents or impurities remain to form soil. A single ton of residual soil represents not infrequently a loss of 100 tons of original rock matter. As this mass of lime 38carbonate is removed by solution the residual soil settles, and as the limestone rocks are more soluble than the adjacent rock formations limestone formations usually form valley lands with ridges on either side. Caves are frequently found in such formations. Furthermore, as the lime is almost all in the form of the easily soluble lime carbonate it can be very completely removed and the fertile “limestone soils” are often very deficient in lime and respond readily to an application of burnt lime, which, not infrequently, is quarried from the same field. From an agricultural standpoint this group is of very great interest and importance.

The third subdivision of this group, namely, that of vegetable origin, includes peat, lignite, coals, etc. Rocks of this group are made up of more or less fragmental remains of plants. In many of them, as the peats and lignites, the traces of plant structure are still apparent. In others, as the anthracite coals, these structures have been wholly obliterated by metamorphisms.

Plants when decomposing on the surface of the ground give off their carbon to the atmosphere in the shape of carbon dioxid gas leaving only the strictly inorganic or mineral matter behind. When, however, they are protected from the oxidizing influence of the air, by water or by other plant growth, decomposition is greatly retarded, and a large portion of the carbonaceous and volatile matters is retained, and by this means together with pressure from the overlying mass, the material becomes slowly converted into coal. When this process goes on near the surface of the earth, and without much pressure, peat or muck is the product.

The fourth subdivision of this group, the phosphatic, forms a class of rocks limited in extent but of the greatest economic importance. Guano, coprolites, and phosphatic rocks (the phosphorites) come under this head.

34. Aeolian Rocks.—This class of rocks is of less importance than the others, either geologically or agriculturally. It is formed from materials drifted by the winds and this material has various degrees of compactness. Usually the components of these drifts form rocks or deposits of a friable texture and of a fragmental nature. The very extensive deposits of 39loess in China, forming their most fertile lands, are admitted now to have been formed in this way, but it is now generally admitted that similar deposits in this country are of subaqueous origin.

Chief among these rocks, are the volcanic ashes which are often carried to a long distance by the wind before they are deposited and consolidated into rock masses. Many loose soils may be carried to great distances by the wind, the deposits forming new aggregates. This is particularly the case in arid regions.

35. Metamorphic Rocks.—This class of rocks includes all sedimentary or eruptive rocks, which, after their deposition and agglomeration, have been changed in their nature through the action of heat, pressure, or by chemical means. Sometimes these changes are so complete that no indication of the character of the original rocks remains. At other times the changes may be found in all the stages of progress, so that they can be traced from the original fragmental or irruptive to the completely metamorphosed deposit. This is especially true of rocks containing large quantities of lime. In those containing silica, the changes are less readily traced.

Fig. 8.

Microstructure of Crystalline Limestone.

(West Rutland, Vermont.)

40The metamorphic rocks may be divided into two subclasses, namely, stratified or bedded, and foliated or schistose.

The rocks of the first class are represented by the crystalline limestones and dolomites. The microstructure of a crystalline limestone is shown in Fig. 8.^[25] When lime and magnesia occur together in combination with carbon dioxid, the substance is known as dolomite. The chemical nature of these rocks and their soil-forming properties are the same as those of the ordinary, non-metamorphosed limestones and dolomites to which reference has already been made. The subject need not, therefore, be further dwelt upon here.

Fig. 9.

Microstructure of Gneiss.

(West Andover, Massachusetts.)

At a a are shown plagioclase crystals broken and rounded by the sheering force producing the foliation.

The second variety of metamorphic rocks is represented by the gneisses and crystalline schists. Gneiss has essentially the same composition as granite and can frequently hardly be distinguished from it, except by a microscopic study of its sections, and even thus it is sometimes difficult to determine. Frequently a number of new minerals is formed in the metamorphic changes. The microstructure of a gneiss is shown in Fig. 9.^[26] The schists include an extremely variable class of rocks, of which quartz 41is the prevailing constituent, and which, as a rule, are deficient in potash and other important ingredients.

36. Rocks Formed Through Igneous Agencies, or Eruptive Rocks.—This group includes all those rocks, which, having been at some time in a state of igneous fusion, have been solidified into their present form by a process of cooling. It may be stated, as a general principle, that the greater the pressure under which a rock solidifies and the slower and more gradual the cooling the more perfect will be found the crystalline structure. Hence, it follows that the older and more deep-seated rocks which are forced up in the form of dikes, bosses, or intrusive sheets, into the overlying masses, and which have become exposed only through erosion and removal of the overlying rocks, are the more highly crystalline.

The eruptive rocks are divided into two main groups, viz.:

(a) Intrusive or plutonic rocks, and

(b) Effusive or volcanic rocks.

Among the more important of the first division of the plutonic form, from an agricultural point of view, are the granites. The essential constituents of granite are quartz, potash feldspar, and plagioclase. One or more minerals of the mica, amphibole or pyroxene groups are also commonly present, and in microscopic proportions apatite and particles of magnetic iron. The more valuable constituents, from an agricultural standpoint, are the minerals potash feldspar, and apatite, which furnish by their decomposition the essential potash and phosphoric acid.

In addition to the granites, which have already been mentioned, the group includes the syenites, the nepheline syenites, the diorites, the gabbros, the diabases, the theralites, the peridotites, and the pyroxenites.

The second group, the effusive or volcanic rocks, includes those igneous rocks, which, like the first group, have been forced up through the overlying rocks, but which were brought to the surface, flowing out as lavas. These, therefore, represent, in many cases, only the upper or surface portions of the first group, differing from them structurally, because they have 42cooled under little pressure more rapidly, and hence are not so distinctly crystalline. These comprise the following groups:

 

(a) Quartz porphyries. (b) Liparites. (c) Quartz-free porphyries. (d) Trachytes. (e) Phonolites. (f) Porphyrites. (g) Andesites. (h) Melaphyrs and augite porphyrites. (i) Basalts. (j) Tephrites and Basanites. (k) Picrite porphyrites. (l) Limburgites and augitites. (m) Leucite rocks. (n) Nepheline rocks. (o) Melilite rocks.

It is, in most cases, impossible to state which of the above classes is of most importance from an agricultural standpoint, since, in the process of soil formation, both chemical and physical processes are involved, whereby the character of the resultant soil is so modified as to but remotely resemble its parent rock. In most soils, the prevailing constituent is but the least soluble one of the rock mass from which it was derived. Thus a limestone soil may contain upwards of ninety per cent of silica and alumina, while the original limestone itself may not have carried more than one or two per cent of these substances. Of course, if a rock mass contains none of the constituents essential to plant growth, its resultant soil must by itself alone be quite barren. It does not absolutely follow, however, that those rocks containing the highest percentages of valuable constituents will yield the most fertile soils, since much depends on the manner in which they have been formed, the amount of leaching, etc., they may have undergone. Nevertheless, the study of the composition of the rocks in their relation to soils, is an extremely interesting and by no means unimportant one.

A comparative table of rock compositions is here given. It will be observed that there is a considerable range of variation even among rocks of the same class, a fact due to the varying abundance of their mineral constituents. The figures given are 43not those of actual analyses on any one particular rock, but are selected from a number of comparatively typical cases; and, it is thought, fairly well represent the composition of the class of rocks indicated.

37. Origin of Soils.—The soils in which crops grow and which form the subject of the analytical processes to be hereinafter described have been formed under the combined influences of rock decay and plant and organic growth. The mineral matters of soils have had their origin in the decay of rocks, while the humic and other organic constituents have been derived from living bodies. It is not the object of this treatise to discuss in detail the processes of soil formation, but only to give such general outlines as may bear particularly on the proper conceptions of the principles of soil investigation.

38. The Decay of Rocks.—The origin and composition of rocks are fully set forth in works on geology and mineralogy. Only a brief summary of those points of interest to agriculture has been given in the preceding pages. The soil analyst should be acquainted with these principles, but for practical purposes he has only to understand the chief factors active in securing the decay of rocks and in preparing the débris for plant growth.

44The following outline is based on the generally accepted theories respecting the formation of soils.[7]

The forces ordinarily concerned in the decay of rocks are:

 

1. Changes of temperature, including the ordinary daily and monthly changes, and the conditions of freezing and thawing.

 

2. Moving water or ice.

 

3. Chemical action of water and air.

 

4. Influence of vegetable and animal life:

 

(a) Shades the rock or soil surface.

 

(b) Penetrates the rock or soil material with its roots, thus admitting air.

 

(c) Solvent action of roots.

 

(d) Chemical action of decaying organic matter.

 

5. Earth worms.

 

6. Bacteria.

39. The Action of Freezing and Thawing.—In those parts of a rock stratum exposed near the surface of the earth the processes of freezing and thawing have perhaps had considerable influence in rock decay. The expansive force of freezing water is well known. Ice occupies a larger volume than the water from which it was formed. The force with which this expansion takes place is almost irresistible. The phenomenon of bursted water pipes which have been exposed to a freezing temperature is not an uncommon one. While the increase in volume is not large, yet it is entirely sufficient to produce serious results. The way in which freezing affects exposed rock is easily understood. The effect is unnoticeable if the rock be dry. If, on the other hand, it be saturated with water, the disintegrating effect of a freeze must be of considerable magnitude. This effect becomes more pronounced if the intervals of freezing and thawing be of short duration. The whole affected portion of the rock may thus become thoroughly decayed. But even in the most favorable conditions this form of disintegration must be confined to a thin superficial area. Even in very cold climates the frost only penetrates a few feet below the surface, and therefore the action of ice cannot in any way be connected with those changes at great depths, to which attention has already 45been called. Nevertheless, certain building stones seem very sensitive to this sort of weathering, and the crumbling of the stone in the Houses of Parliament is due chiefly to this cause.

On the whole it appears that the action of ice in producing rock decay has been somewhat overrated, although its power must not by any means be denied. But on the other hand a freeze extending over a long time tends to preserve the rocks, and it therefore appears that the entire absence of frost would promote the process of rock decay.

At best it must be admitted that frost has affected the earth’s crust only to an insignificant depth, but its influence in modifying the arable part of the soil is of the utmost importance to agriculture.

40. The Action of Glaciers.—The action of ice in soil formation is not confined alone to the processes just described. At a period not very remote geologically, a great part of our Northern States was covered with a vast field of moving ice. These seas of ice crept down upon us from more northern latitudes and swept before them every vestige of animal and vegetable life. In their movement they leveled and destroyed the crests of hills and filled the valleys with the débris. They crushed and comminuted the strata of rocks which opposed their flow and carried huge boulders of granite hundreds of miles from their homes. On melting they left vast moraines of rocks and pebbles which will mark for all time the termini of these empires of ice. When the ice of these vast glaciers finally melted the surface which they had leveled presented the appearance of an extended plain. No estimate can be made of the enormous quantities of rock material which were ground to finest powder by these glaciers. This rock powder forms to-day no inconsiderable part of those fertile soils which are composed of glacial drift. The rich materials of these soils probably bear a more intimate relation to the rocks from which they were formed than of any other kinds of soil in the world. The rocks were literally ground into a fine powder, and this powder was intimately mixed with the soils which had already been formed in situ. The melting ice also left exposed to disintegrating forces large surfaces of unprotected rocks in which decay would go 46on much more rapidly than when covered with the débris which protected them before the advent of the ice. The area of glacial action extended over nearly all of New England and over the whole area of the northern tier of States. It extended southward almost to the Ohio river, and in some places crossed it. The effect of the ice age in producing and modifying our soil must never be forgotten in a study of soil genesis. It is not a part of our purpose here to study the causes which produced the age of ice. Even a brief reference to some of the more probable ones might be entirely out of place. Before the glacial period it is certain that a tropical climate extended almost, if not quite, to the North Pole. The fossil remains of tropical plants and animals which have been found in high northern latitudes are abundant proofs of this fact.

In the opinion of Sterry Hunt,^[27] rock decay has taken place largely in preglacial and pretertiary times. The decay of crystalline rocks is a process of great antiquity. It is also a universal phenomenon. The fact that the rocks of the southern part of this country seem to be covered with a deeper débris than those further north is probably due to the mechanical translation of the eroded particles towards the south. The decay and softening of the material were processes necessarily preceding the erosion by aqueous and glacial action.

It is possible that a climate may have existed in the earlier geologic ages more favorable to the decay of rocks than that of the present time.

41. Progress of Decay as Affected by Latitude.—Extensive investigations carried on along the Atlantic side of the country show wide differences in the rate of decay in the same kind of rocks in different latitudes. In general, the progress of decay is more marked toward the south. The same fact is observed in the great interior valleys of the country; at least, everywhere except in the arid and semi-arid regions. Wherever there is a deficiency of water the processes of decay have been arrested. Where the rock strata have been displaced from a horizontal position the progress of decay has been more rapid. This is easily understood. The percolation of water is more easy as the displacement approaches a vertical position.

47A most remarkable example of this is seen in the rocks of North Carolina.^[28] A kind of rock known as trap is found in layers called dikes in the Newark system of rocks in that State. These dikes have been so completely displaced from the horizontal position they at first occupied as to have an almost vertical dip. The edges thus exposed vary from a few feet to nearly 100 feet in thickness. The trap rock in those localities is composed almost exclusively of the mineral dolerite, which is so hard and elastic in a fresh state as to ring like a piece of metal when struck with a hammer. In building a railroad through this region these dikes were in some places uncovered to a depth of forty feet and more. At this depth they were found completely decomposed and with no indications of having reached the lower limit of disintegration. The original hard bluish dolerite has been transformed into a yellowish clay-like mass that can be molded in the fingers and cut like putty. Similar geologic formations in New Jersey and further North do not exhibit anything like so great a degree of decomposition, thus illustrating in a marked degree the fact that freezing weather for a part of the year is a protection against rock decay. The ice of winter at least protects the rocks from surface infiltration, although it can not stop the subterranean solution which must go on continuously.

Other things being equal, therefore, it appears that as the region of winter frost is passed the decay of the rocks has been more rapid than in the North, because the chief disintegrating forces act more constantly.

42. The Solvent Action of Water.—The water of springs and wells is not pure. It contains in solution mineral matters and often a trace of organic matter. The organic matter comes from contact with vegetable matter and other organic materials near the surface of the earth. The mineral matter is derived from the solvent action of the water and its contents on the soil and rocks.

The expressions “hard” and “soft” applied to water indicate that it has much or little mineral matter in suspension, as the case may be. When surface and spring waters are collected into streams and rivers they still contain in solution the greater part of the mineral matters which they at first carried.

48When well or spring waters have more than forty grains of mineral matter per gallon they are not suitable for drinking waters. Mineral waters, so called, are those which carry large quantities of mineral matter, or which contain certain comparatively rare mineral substances which are valued for their medicinal effects.

The analysis of spring, well, or river waters will always give some indication of the character of the rocks over which they have passed.^[29] The vast quantities of mineral matters carried into oceans and seas are gradually deposited as the water is evaporated. If, however, these matters be very soluble, such as common salt, sulfate of magnesia, etc., they become concentrated, as is seen with common salt in sea waters. In small bodies of waters, such as inland seas, which have no outlet, this concentration may proceed to a much greater extent than in the ocean. As an instance of this, it may be noted that the waters of the Dead Sea and Great Salt Lake are impregnated to a far greater degree with soluble salts than the water of the ocean. The solvent action of water on rocks is greatly increased by the traces of organic or carbonic acids which it may contain. When surface water comes in contact with vegetable matter it may become partially charged with the organic acids which the growing vegetable may contain or decaying vegetable matter produce. Such acids coming in contact with limestone under pressure will set free carbon dioxid. Water charged with carbon dioxid acts vigorously on limestone and other mineral aggregates. If such solutions penetrate deeply below the surface of the earth their activity as solvents may be greatly increased by the higher temperature to which they are subjected. Hence, all these forces combine to disintegrate the rocks, and through such agencies vast deposits of original and secondary rocks have been completely decomposed.

The gradual passing of the firm rock into an arable soil is beautifully shown in Fig. 10, a print from a negative taken by Mr. Geo. P. Merrill, near Washington, D. C.

Figure 10.

View on the Broad Branch of Rock Creek, Washington, D. C.

The fresh but badly decomposed granitic rock is shown passing upward into material more and more decomposed until it becomes sufficiently pulverulent and soluble to support plant life. The roots showing in the upper part of the picture formerly penetrated the decomposed rock, but have been exposed through quarrying operations. Photograph by George P. Merrill, 1891.

4943. Action Of Vegetable Life.—The preliminary condition to vegetation is the formation of soil, but once started, vegetation aids greatly in the decomposition of rocks. Some forms of vegetation, as the lichens, have apparently the faculty of growing on the bare surface of rocks, but the higher order requires at least a little soil. The vegetation acts by shading the surface and thus rendering the action of water more effective, by mechanically separating the rock particles by means of its penetrating roots and by the positive solvent action of the root juices. The rootlets of plants in contact with limestone or marble dissolve large portions of these substances, and while their action on more refractory rocks is slower, it must be of considerable importance. The organic matter introduced into the soil by vegetation also promotes decay still further both directly and by the formation of acids of the humic series. This matter also furnishes a considerable portion of carbon dioxid which is carried by the water to assist in its solvent action.

44. Action of Earth-Worms.—Of animal organisms those most active in the formation of soil are earth-worms. The work of earth-worms has been exhaustively studied by Darwin.^[30] The worms not only modify the soil by bringing to the surface portions of the subsoil, but also influence its physical state by making it more porous and pulverulent. According to Darwin the intestinal content of worms has an acid reaction, and this has an effect on those portions of the soil passing through their alimentary canal. The acids, which are formed in the upper part of the digestive canal are neutralized by the carbonate of lime secreted by the calciferous glands of the worms thus neutralizing the free acid and changing the reaction to alkaline in the lower part of the canal. There is a fair presumption that the acids of the worm are of a humic nature.

The worms further modify the composition of the soil by drawing leaves and other organic matter into their holes, and leaving therein a portion of such matter which is gradually converted into humus. Stockbridge^[31] gives a striking illustration of this process due to an experiment by von Hensen. Darwin estimates that about eleven tons of organic matter per acre are annually added to the soil in regions where worms abound. A considerable portion of the ammonia in the soil at 50any given time may also be due to the action of worms, as much as 0.18 per cent of this substance having been found in their excrement. It is probable that nearly the whole of the vegetable matter in the soil passes sooner or later through the alimentary canals of these ceaseless soil builders, and is converted into the form of humus.

45. Action of Bacteria.—The intimate relations which have been found to subsist between certain minute organisms and the chemical reactions which take place in the soil is a sufficient excuse for noting the effect of other similar organisms in the formation of soils.

In addition to the usual forces active in decomposing rocks Müntz^[32] has described the effects of a nitrifying bacillus contributing to the same purpose.

According to him the bare rock usually furnishes a purely mineral environment where organisms cannot be developed unless they are able to draw their nourishment directly from the air. Some nitrifying organisms belong to this class. It has been shown that these bodies can be developed by absorbing from the ambient atmosphere carbonate of ammonia and vapors of alcohol, the presence of which has been determined in the air. According to the observations of Winogradsky, they assimilate even the carbon of the carbon dioxid just as vegetables do which contain chlorophyl. Thus even in the denuded rocks of high mountains the conditions for the development of all these inferior organisms exist. In examining the particles produced by attrition, it is easily established that they are uniformly covered by a layer of organic matter evidently formed by microscopic vegetations. Thus we see, in the very first products of attrition, appearing upon the rocky particles the characteristic element of vegetable soil, viz., humus, the proportion of which increases rapidly with the products of disaggregation collected at the foot of declivities until finally they become covered with chlorophyliferous plants.

In a similar manner the presence of nitrifying organisms has been noted upon rocky particles received in sterilized tubes, and cultivated in an appropriate environment where they soon produce 51nitrification. The naked rocks of the Alps, the Pyrenees, the Auvergnes and the Vosges, comprise mineralogical types of the most varied nature, viz., granite, porphyry, gneiss, micaschist volcanic rocks and limestones and all these have shown themselves to be covered with the nitrifying ferment. It is known that below a certain temperature these organisms are not active; their action upon the rock is, therefore, limited to the summer period. During the cold season their life is suspended but they do not perish, inasmuch as they have been found living and ready to resume all their activity after an indefinite sleep on the ice of the glaciers where the temperature is never elevated above zero.

The nitrifying ferment is exercised on a much larger scale in the normal conditions of the lower levels where the rock is covered with earth. This activity is not limited to the mass of rock but is continued upon the fragments of the most diverse size scattered through the soil and it gradually reduces them to a state of fine particles. It is therefore a phenomenon of the widest extension.

The action of these micro-organisms according to Müntz is not confined to the surface but extends to the most interior particles of the rocky mass. Where, however, there is nothing of a nitrogenous nature, to nitrify such an organism must live in a state of suspended animation.

When the extreme minuteness of these phenomena is considered there may be a tendency to despise their importance, but their continuity and their generality in the opinion of Müntz place them among the geologic causes to which the crust of the earth owes a part of its actual physiognomy and which particularly have contributed to the formation of the deposits of the comminuted elements constituting arable soil.

The general action of nitrifying organisms in the soil, the nature of these bodies, and the method of isolating and identifying them will be fully discussed in another part of this work.

46. Action of the Air.—The air itself takes an active part in rock decay. Wherever rocks are exposed to decay, there air 52is found or, at least, the active principle of air, viz., oxygen. The air not only penetrates to a great depth in the earth, but is also carried to much greater depths by water which always holds a greater or less quantity of air in solution. The oxygen of the air is thus brought into intimate contact with the disintegrating materials and in a condition to assist wherever possible in the decomposing processes.

The oxygen acts vigorously on the lower oxids of iron, converting them into peroxids, and thus tends to produce decay.

There are other constituents of rocks which oxygen affects injuriously and thus helps to their final breaking up. It is true that, as a rule, the constituents of rocks are already oxidized to nearly as high a degree as possible, and on these constituents of course the air would have no effect. But on others, especially when helped by water with the other substances it carries in solution, the air may greatly help in the work of destruction.

In a general view, the action of the air in soil formation may be regarded as of secondary importance, and to depend chiefly on the oxidation of the lower to the higher basic forms. These processes, while they seem of little value, have, nevertheless, been of considerable importance in the production of that residue of rock disintegration which constitutes the soil.

47. Classification of Soils According to Deposition.—In regard to their deposition soils are divided into five classes:

1. Those which are formed from the decomposition of crystalline or sedimentary rocks or of unconsolidated sedimentary material in situ.

2. Those which have been moved by water from the place of their original formation and deposited by subsidence (bottom lands, alluvial soils, lacustrine deposits, etc.).

3. Those which have been deposited as débris from moving masses of ice (glacial drift).

4. Soils formed from volcanic ashes or from materials moved by the wind and deposited in low places or in hills or ridges.

5. Those formed chiefly from the decay of vegetable matter, (tule, peat, muck).

48. Qualities of Soils.—In respect of quality, soils have 53been arbitrarily divided into many kinds. Some of the more important of these divisions are as follows:

1. Sand. Soils consisting almost exclusively of sand.

2. Sandy Loams. Soils containing some humus and clay but an excess of sand.

3. Loams. Soils inclining neither to sand nor clay and containing some considerable portions of vegetable mold, being very pulverulent and easily broken up into loose and porous masses.

4. Clays. Stiff soils in which the silicate of alumina and other fine mineral particles are present in large quantity.

5. Marls. Deposits containing an unusual proportion of carbonate of lime, with often some potash or phosphoric acid resulting from the remains of sea-animals and plants.

6. Alkaline. Soils containing carbonate and sulfate of soda, or an excess of these alkaline and other soluble mineral substances.

7. Adobe. A fine grained porous earth of peculiar properties hereinafter described.

8. Vegetable. Soils containing much vegetable débris in an advanced state of decomposition. When such matter predominates or exists in large proportion in a soil the term tule, peat or muck is applied to it.

With the exception of numbers six, seven and eight these types of soil are so well-known as to require no further description for analytical purposes. The alkaline, adobe and vegetable soils on the contrary demand further study.

49. Alkaline Soils.—The importance of a more extended notice of this class of soils for analytical purposes is emphasized by their large extent in the United States.

Chiefly through the researches of Hilgard attention has been called to the true character of these soils which are found throughout a large part of the Western United States and which are known by the common name of alkali. The following description of the origin of these soils is compiled chiefly from Hilgard’s papers on this subject. Wherever the rain-fall is scanty, and especially where the rains do not come at any one time with sufficient force to thoroughly saturate the soil and 54carry down through the subsoil and off through the drainage waters the alkali contained therein, favorable conditions exist for the production of the alkaline soil mentioned above. The peculiar characteristic of this soil is the efflorescence which occurs upon its surface and which is due to the raising of soluble salts in the soil by the water rising through capillary attraction and evaporating from the surface, leaving the salts as an efflorescence.

Soils which contain a large amount of alkali are usually very rich in mineral plant food, and if the excess of soluble salt could be removed, these lands under favorable conditions of moisture would produce large crops.

The formation of the alkali may be briefly described as follows: By the decomposition of the native rocks, certain salts soluble in water are formed. These salts in the present matter are chiefly sodium and potassium sulfates, chlorids and carbonates. The salts of potash together with those of lime are more tenaciously held by the soil than the soluble salts of soda, and the result of this natural affinity of the soil for soluble potash, lime and magnesian salts is seen in the formation at the surface of the earth, by the process of evaporation above described, of a crust of alkaline material which is chiefly composed of the soluble salts of soda. In countries which have a sufficient amount of rain-fall, these soluble salts are carried away either by the surface drainage or by the percolation of water through the soil, and the sodium chlorid is accumulated in this way in the waters of the ocean. But where a sufficient amount of rain-fall does not occur, these soluble salts carried down by each shower only to a certain depth rise again on the evaporation of the water, reinforced by any additional soluble material which may be found in the soil itself. The three most important ingredients of the alkali of the lands referred to are sodium chlorid, sulfate, and carbonate. When the latter salt, namely, sodium carbonate, is present in predominant quantity, it gives rise to what is popularly known as black alkali. This black color is due to the dark colored solution which sodium carbonate makes with the organic matters or humus of the soil. The black alkali is far more injurious to growing vegetation 55than the white alkali composed chiefly of sodium sulfate and chlorid.

This black alkali has been very successfully treated by Hilgard^[33] by the application of gypsum which reacting with the sodium carbonate produces calcium carbonate and sodium sulfate, thus converting the black into the white alkali and adding an ingredient in the shape of lime carbonate to stiff soils which tends to make them more pulverulent and easy of tillage.

This method of treatment, however, as can be easily seen, is only palliative, the whole amount of the alkaline substances being still left in the soil, only in a less injurious form.

The only perfect remedy for alkaline soils, as has been pointed out by Hilgard, is in the introduction of underdrainage in connection with irrigation. The partial irrigation of alkaline soils, affording enough moisture to carry the alkali down to and perhaps partially through the subsoil, can produce only a temporary alleviation of the difficulties produced by the alkali. Subsequent evaporation may thus increase the amount of surface incrustation. For this reason in many cases the practice of irrigation without underdrainage may completely ruin an otherwise fertile soil by slowly increasing the amount of alkali in the soil by the total amount of the alkaline material added in the waters of irrigation.

As Hilgard has pointed out, if a soil can be practically freed from alkali by underdrainage connected with a thorough saturation by irrigation, it may be centuries before the alkali will accumulate in that soil again when ordinary irrigation only is practiced. It may thus become possible to reclaim large extents of alkaline soil little by little by treating them with an excess of irrigation water in connection with thorough underdrainage. The composition of the alkali on the surface of the soil due to the causes above set forth is thoroughly illustrated by the analyses of Hilgard and Weber, which follow:

5750. Adobe Soils.—In many parts of the arid regions of this country which can be recovered for agricultural purposes by irrigation the soil has peculiar characteristics.

The name adobe as commonly used applies to both the sundried bricks of the arid regions of the West and Southwest, and to the materials of which they are composed. The material is described by Russell^[34] as a fine grained porous earth, varying in color through many shades of gray and yellow, which crumbles between the fingers, but separates most readily in a vertical direction. The coherency of the material is so great that vertical scarps will stand for many years without forming a noticeable talus slope.

Distribution.—The area over which adobe forms a large part of the surface has not been accurately mapped, but enough is known to indicate that it is essentially co-extensive with the more arid portions of this country. In a very general way it may be considered as being limited to the region in which the mean annual rain-fall is less than twenty inches. It forms the surface over large portions of Colorado, New Mexico, Western Texas, Arizona, Southern California, Nevada, Utah, Southern Oregon, Southern Idaho, and Wyoming. Adobe occurs also in Mexico and may there reach a greater development than in the United States, but observations concerning it south of the Rio Grande are wanting.

In the United States it occurs from near the sea-level in Arizona, and even below the sea-level in Southern California, up to an elevation of at least six or eight thousand feet, along the eastern border of the Rocky Mountains, and in the elevated valleys of New Mexico, Colorado, and Wyoming. It occupies depressions of all sizes up to valleys having an area of hundreds of square miles. Although occurring throughout the arid region, it can be studied to best advantage in the drainless and lakeless basins in Nevada, Utah, and Arizona.

Composition.—When examined under the microscope, the adobe is seen to be composed of irregular, unassorted flakes and grains, principally quartz, but fragments of other minerals are also present. An exhaustive microscopic study has not been made, but the samples examined from widely-separated localities 58were very similar. The principal characteristics observed were the extreme angularity of the particles composing the deposit and the undecomposed condition of the various minerals entering into its composition. It is to be inferred from this that the material was not exposed even to a very moderate degree of friction, and had not undergone subaerial decay before being deposited. Adobe collected, at typical localities is so fine in texture that no grit can be felt when it is rubbed between the fingers; in other instances it contains angular rock fragments of appreciable size.

The composition of the material is illustrated by the following analyses:

51. Vegetable Soils.—The heavy soils whose origin has been described are essentially of a mineral nature. The quantity of organic matter in such soils may vary from a mere trace to a few per cent, but they never lose their mineral predominance. When a soil on the other hand is composed almost exclusively of vegetable mold it belongs to quite another type. Such soils are called tule, peat or muck. In this country there are thousands of acres of peat or muck soils; the largest contiguous deposits being found in Southern Florida. The origin of these soils is 59easily understood. Whenever rank vegetation grows in such a location as to secure for the organic matter formed a slow decay there is a tendency to the accumulation of vegetable mold in shallow water or on marshy ground and where conditions are favorable to such accumulations. In Florida the muck soils have been accumulated about the margins of lakes. During the rainy season the marshes bordering these are partly covered with water, but the vegetation is very luxuriant. The water protects the vegetable matter from being destroyed by fire. It therefore accumulates from year to year and is gradually compacted into quite a uniform mass of vegetable mold.

The composition of the muck is illustrated in the following table which shows the character of the layers at one, two and three feet in depth:^[35]

In this sample, No. 3, the muck was only three feet deep, resting on pure sand. As the bottom of the deposit is approached the admixture of sand becomes greater and the percentage of organic matter less.

No reliable estimate of the time which has been required to form these deposits can be given, but in the Okeechobee region in Florida the deposit of vegetable mold in some places exceeds ten feet in depth.

The purest muck or peat soils contain only small quantities of potash and phosphoric acid, and especially is this true of the Florida mucks which have been formed of vegetable growth containing very little mineral matter.

It is not at all probable that the flora now growing on any particular area of virgin peat contains all the plants that have contributed to its formation. The principal vegetable growths now going to make up the muck soils of Florida are the following:

The above are only the plants growing in the greatest profusion and do not include all which are now contributing to increase the store of vegetable débris.

52. Humus.—The active principle of vegetable mold is called humus, a term used to designate in general the products of the decomposition of vegetable matter as they are found in soils. In peat and muck are found a mixture of humus with undecomposed or partially decomposed vegetation.

According to Kostytchoff^[36] vegetable matter decays under the influence of molds and bacteria. Molds alone produce the dark colored matters which give soils rich in vegetable matter, their color. One chief characteristic of humus is its richness in nitrogen. Black Russian soil contains from 4 to 6.65 per cent of nitrogen. This soil is estimated to contain sixty million organisms per gram and much of the nitrogen which it holds must be in the form of proteids. The first development in decaying vegetable matter is of bacteria and there is a tendency of the decaying matter to become acid. This causes a decay of the bacteria and the ammonia produced by this neutralizes the acid. The various kinds of mold grow when the reaction becomes neutral. Afterwards the bacteria and the molds develop together. This statement of Kostytchoff is not a very satisfactory explanation of even our limited knowledge of the decomposition of organic matters in the soil. Ammonia and ammonia salts are formed not by the decay of some forms of bacteria but by the activities of other forms. Warington found that in nitrification there were three distinct forms of bacteria concerned in the final products of ammonia, nitrites, and nitrates. Humus always contains easily decomposable matter and consequently the rate of decay at any observed periods is nearly the same. In humus which is produced above the water-level Kostytchoff states that all trace of the vegetable structure is destroyed by the leaves being gnawed and passed through the bodies of caterpillars and 61wire-worms. Under the water-level the vegetable structure is preserved and peat results. The decay of humus is most rapid in drained and open soils. For this reason the presence of clay in a soil promotes the accumulation of humus. Inferior organisms are the means of diffusing organic matter through the soil. The mycelia of fungi grow on a dead root for instance, ramify laterally and thus carry organic matter outward and succeeding organisms extend this action and the soil becomes darkened in proportion. Humic acid in black soil is almost exclusively in combination with lime.

A more common view of the difference between the formation of humus above and below the water-level is that above the water-level there is a very free access of air and even the harder parts of the leaf skeleton can be oxidized through the agency of bacteria, while under the water-level there is a very limited supply of air and this oxidation cannot proceed as rapidly. The harder parts of the leaf skeleton are preserved, and from the freer access of air humus is oxidized more readily in drained and open soils, and accumulates in clay soils where there is less circulation of air.

The real composition of humus is a matter which has never been definitely determined. Composed of many different but closely related substances it has been difficult to isolate and determine them.

Stockbridge^[37] gives the following composition of the bodies which form the larger part of humus:

He further states that there are, aside from these humus compounds, others still less known and the action of which is not yet understood; among them xylic acid, C₂₄H₃₀O₁₇, saccharic acid, C₁₄H₁₈O₁₁, glucinic acid, C₁₂H₂₂O₁₂, besides a brown humus acid containing carbon, 65.8 per cent, and hydrogen, 6.25 per cent, and a black humus acid yielding carbon, 71.5 per cent, and hydrogen, 5.8 per cent.

According to Mulder humic acid has the following composition, C₆₀H₅₄O₂₇, while Thenard^[38] ascribes to it the formula, C₂₄H₁₀O₁₀.

At the present time we can only regard the various forms of humus bodies as mixtures of many substances mostly of an acid nature and resulting from a gradual decomposition of organic matter under conditions which partially exclude free access of oxygen.

For analytical purposes it is only necessary to separate these bodies by the best approved processes. A further knowledge of their composition can then be derived by determining the percentages of carbon dioxid and water which they yield on combustion.

53. Soil and Subsoil.—Many subdivisions have been made of the above varieties of soil, but they have little value for analytical purposes. For convenience in description for agricultural purposes, the soil, however, is further divided into soil and subsoil. In this sense the soil comprises that portion of the surface of the ground, usually from four to nine inches deep, containing most of the organic remains of plants and animals and in which air circulates more or less freely for the proper humification of the organic matter, which usually gives a darker color to the soil than to the subsoil. The subsoil proper lies below this, and has usually more characteristic properties, especially in respect of color and texture, as it has been less influenced by artificial conditions of cultivation and the remains of vegetation.

63The subsoil extends to an indefinite depth and is limited usually by deposits of undecomposed or partly decomposed rock matter, or by layers of clay, sand or gravel.

Inasmuch, however, as the influence of the subsoil on growing crops is of little importance below the depth of eighteen inches the analysis of samples from a greater depth has more of a geologic than agricultural value.

Hilgard regards as subsoil whatever lies beneath the line of change, or below the minimum depth of six inches. But should the change of color occur at a greater depth than twelve inches, the soil specimen should nevertheless be taken to the depth of twelve inches only, which is the limit of ordinary tillage; then another specimen from that depth down to the line of change, and then the subsoil specimens beneath that line. The depth to which the last should be taken will depend upon circumstances. It is always desirable to know what constitutes the foundation of a soil to the depth of three feet at least, since the question of drainage, resistance to drought, etc., will depend essentially upon the nature of the substratum. But in ordinary cases ten or twelve inches of subsoil will be sufficient. The sample should be taken in other respects precisely like that of the surface soil, while that of the material underlying this subsoil may be taken with less exactness, perhaps at some ditch or other easily accessible point, and should not be broken up like the other specimens.

In the method of soil sampling adopted by the Royal Agricultural College of England, the soil is regarded as that portion of the surface of the ground which is reached by ordinary tillage operations, generally being from six to nine inches deep; the subsoil is that portion which is ordinarily not touched in plowing.

PART SECOND. TAKING SAMPLES OF SOIL FOR ANALYSIS. — TREATMENT OF SAMPLE IN THE LABORATORY. — AUTHORITIES CITED IN PART SECOND.

54. General Principles.—It would be unwise to attempt to give any single method of taking soil samples as the only one to be practiced in all circumstances. In the methods which follow it is believed will be found directions for every probable case. The particular method to be followed will in each case have to be determined by circumstances.

The sole object in taking a sample of soil should be to have it representative of the type of soils to which it belongs. Every precaution should be observed to have each sample measure up to that standard.

The physical and chemical analyses of soils are long and tedious processes and are entirely too costly to be applied to samples which represent nothing but themselves.

The particular place selected for taking the samples as well as the method employed are also largely determined by the point of view of the investigations. The collection of samples to illustrate the geologic or mineralogical relations of soils is quite a different matter from gathering portions to represent their agricultural possibilities. In a given area the sum of plant food in the soil would only be determined by the analyses of samples from that particular field, while samples illustrating geologic relations could or should be taken at widely distant points. Again the chemist is content with a sample of a few grams in weight while the physicist would require a much larger quantity. Much popular ignorance exists respecting the importance of the collection 66of soil samples. As an illustration of this may be cited a recent instance in which a sample of soil was received by the author with a request for a complete analysis and a statement of the kinds of crops it was suited to grow. No data relating to the locality in which the sample was taken accompanied this request. The sample itself, which weighed a little less than 3.6 grams, was not a soil at all in an agricultural sense but a highly ferruginous sand.

The collector of samples who understands the purpose for which he is working will find among the approved methods which follow some one or some combination of methods, by means of which his work can be made successful. In these cases it is the collector rather than the method on which reliance must be placed to secure properly representative samples.

55. General Directions for Sampling.—The locality having been selected which presents as nearly as possible the mean composition of the field a square hole is dug with a sharp spade to the depth of eighteen inches. The walls of this hole should be smooth and perpendicular. The soil to the depth of six to nine inches is then removed from the sides of the hole in a slice about four inches thick; or the sample of soil may be taken to the depth indicated by a change of color. Any particles which fall into the bottom of the hole are carefully collected and added to the parts adhering to the spade. The whole is thrown into a suitable vessel for removal to the laboratory. The sample of soil having been thus secured, the subsoil is taken in the same way. To insure uniformity in the samples, it is well to take several of them from the same field. Where more than one sample is taken it is advisable to mix all the sub-samples in the field, remove large sticks, stones, roots, etc., and take a general sample of from three to five kilograms. The character of the débris, etc., removed should be carefully noted.

It is sometimes desirable to take samples of the subsoil to a greater depth than eighteen inches. A post-hole auger or large wood auger will be found very useful for this purpose. It is rarely necessary to take samples of subsoil to a greater depth than six feet. In taking samples the geologic formation and 67the general topography of the field should be noted, also the character of the previous crops, kind and amount of fertilizers employed, character of drainage and any other data of a nature to give a more accurate idea of the forces which have determined the physical and chemical properties of the sample.

56. Method Of Hilgard.—Hilgard^[39] recommends that samples should not be taken indiscriminately from any locality you may chance to be interested in, but that you should consider what are the two or three chief varieties of soil which, with their intermixtures, make up the cultivable area of your region, and carefully sample these first of all.

As a rule, and whenever possible, samples should be taken only from spots that have not been cultivated, or are otherwise likely to have been changed from their original condition of virgin soils and not from ground frequently trodden over such as roadsides, cattle paths, or small pastures, squirrel holes, stumps, or even the foot of trees, or spots that have been washed by rains or streams, so as to have experienced a noticeable change, and not be a fair representative of their kind. He further suggests that the normal vegetation, trees, herbs, grass, etc., should be carefully observed and recorded, and spots showing unusual growth be avoided whether in kind or quality, as such are likely to have received some animal manure or other outside addition.

Specimens should be taken from more than one spot judged to be a fair representative of the soil intended to be examined as an additional guarantee of a fair average.

After selecting a proper spot pull up the plants growing on it, and scrape off the surface lightly with a sharp tool to remove half-decayed vegetable matter not forming part of the soil. Dig a vertical hole, like a post-hole, at least 20 inches deep. Scrape the sides clean so as to see at what depth the change of tint occurs which marks the downward limit of the surface soil, and record it. Take at least half a bushel of the earth above this limit, and on a cloth (jute bagging should not be used for this purpose, as its fibers, etc., become intermixed with the soil) or paper break it up and mix thoroughly, and put up at least a pint of it in a sack or package for examination. 68This specimen will, ordinarily, constitute the soil. Should the change of color occur at a less depth than six inches the fact should be noted, but the specimen taken to that depth nevertheless, since it is the least to which rational cultures can be supposed to reach.

In case the difference in the character of a shallow surface soil and its subsoil should be unusually great, as may be the case in tule or other alluvial lands or in rocky districts, a separate example of that surface soil should be taken, besides the one to the depth of six inches.

Specimens of salty or alkali soils should, as a rule, be taken only toward the end of the dry season, when they will contain the maximum amount of the injurious ingredients which it may be necessary to neutralize.

Whatever lies beneath the line of change, or below the minimum depth of six inches, will constitute the subsoil. Should the change of color occur at a greater depth than twelve inches the soil specimen should nevertheless be taken to the depth of twelve inches only, which is the limit of ordinary tillage; then another specimen from that depth down to the line of change, and then the subsoil specimen beneath that line.

Hilgard justly calls attention to the fact that all peculiarities of the soil and subsoil, their behavior in wet and dry seasons, their location, position and every circumstance in fact, which can throw any light on their agricultural qualities or peculiarities should be carefully noted and the notes sent with the samples. Unless accompanied by such information, samples can not ordinarily be considered as justifying the amount of labor involved in their examination.

57. Whitney^[40] suggests that a geologic map of the region to be sampled should always be at hand and that all samples should be rejected from spots showing local discrepancies, washings or other disturbances.

The kind of analyses to which a sample is to be subjected also largely determines the method to be pursued in selecting it: For instance, a sample to be used for determining the size of the particles therein, may be taken in quite a different manner from that designed only for the determination of moisture.

6958. In the directions collated by Richards^[41] and which have been largely followed by the correspondents of the Department of Agriculture, it is recommended to select in a field, four or five places, at least, per acre, taking care that these places have an homogeneous aspect, and represent as far as possible the general character of the whole ground. If the field, however, present notable differences, either in regard to its aspect or its fertility, the samples gathered from the different parts must be kept separate.

The sampling of arable soil should be made only after the raising of the crop and before it has received any new manure. In other soils the sample should be taken only from spots that have not been cultivated.

59. In the method of soil sampling adopted by the German Experiment Stations^[42] it is directed that the samples of soil should be taken according to the extent of the surface to be sampled, in three, five, nine, twelve or more places at equal distances from each other. They should be taken in perpendicular sections to the depth turned by the plow; and for some studies of the subsoil to a depth of sixty to ninety centimeters. The single samples can be either examined separately or carefully mixed and an average portion of the mixture taken.

60. Method of the Official French Commission.—The official French commission^[43] emphasizes the fact that the sample of soil taken for analysis, should represent a layer of equal thickness through the total depth of its arable part. An analysis of the subsoil taken in the same way, will often be useful to complete the data of the soil study.

First of all, according to this authority, it is necessary to determine the point of view from which the sample is to be taken. If the object is a general study, having for its aim the determination of the general composition of the soils of a definite geologic formation, the sample should be taken in such a way that the different characteristics of the soil alone should enter into consideration without paying any attention to its accidental components, which have been determined by local causes, such as are produced by continued high cultivation, the application 70of abundant fertilizers, or the practice of a particular line of agriculture. The samples of soil therefore, with such an object in view, should be taken on parts of the earth which are beyond the reach of the causes mentioned above and which tend to modify the nature of the primitive soil. In such a case it is the soil which has not been modified, or better still, virgin soil, such as is found in the woodlands and prairies, which should be taken for a sample, choosing those places in which the geologic formation is most perfectly characterized. In such a case a soil taken in one spot corresponding to the conditions before mentioned, would be the best for the purposes in view. The sample would thus represent a true type to be studied, not one of a mean composition got by taking samples from different localities and mixing them into a homogeneous parcel. This last method of proceeding could introduce into the sample earth modified by culture or by influences purely accidental. However, it would be wise, in a region characterized by the same geologic formation, to take a certain number of samples in different localities, and examine singly each one of them in order to be assured that there is a uniformity of composition in the whole of the soils.

If, on the contrary, it is the purpose of the investigation to furnish information to the cultivator concerning the fields which are worked, it is necessary to approach the problem from a different point of view. In this case the earth which is under cultivation should be first of all considered with all the modifications which nature causes or practical culture has caused, in it. But it often happens that upon the same farm the natural soil is variable, caused either by the washings from the adjacent soils, by the accumulation at certain points of deposits formed from standing water, or from other reasons. In such a case it would be necessary to take samples from every part of the field which exhibited any variation from the general type, in order to get a complete mean sample of the whole. It is necessary to be on guard against making a mixture of these different lots which would neither represent the different soils constituting the farm nor their mean composition. It would be better to examine each of these samples alone and then from those parts which appear 71to have a similar composition, to take a general sample for the mean analysis.

Most often it is necessary to confine our studies to the really important part of the farm the composition of which would have a practical interest. The aspect of the spontaneous vegetation in such a case, will often serve as a guide to determine the parts of the farms which are similar in nature. The sample should represent the arable layer, properly so-called, that is, that part of it which is stirred by the agricultural implements in use and in which the root system of the plant takes its greatest development and which is the true reservoir of the fertilizing materials.

When a trench is dug in the soil it is easy to distinguish the arable layer from the subsoil. In the first place, its color is different, generally being modified by vegetable débris which forms the supply of humus. The depth of the arable layer is variable, but it is most frequently between 200 and 300 millimeters. In the analysis the depth and layers should be indicated since the chemical composition of the earth varies according as the sample is taken to a greater or less depth. As an example of this it may be said that the quantity of nitrogen decreases in general in proportion as the depth of the layer is increased. The sample, therefore, should be limited exactly to the arable layer of soil.

61. Caldwell^[44] advises that according to the purpose of the analysis samples be taken:

a, from one or from several spots in the field, in order to subject each sample to a separate analysis; or

b, for an average representation of the soil of the whole field; in this case, several portions of earth are taken from points distributed in a regular manner over the field, all of which are most carefully mixed together, and 4–6 kilograms of the mixture, free from any large stones, are preserved as the average sample.

An excavation in the soil 30–50 centimeters deep, or through to the subsoil, and 30–50 centimeters square, with one side as nearly vertical as possible is made and a slice taken from this side of uniform thickness throughout, weighing 4–5 kilograms. If the subsoil is to be examined, a sample of it should be taken out in the same manner as directed for the upper soil, to the depth of about 60 centimeters.

72If the character of the soil varies materially in different parts of the field, samples from several spots should be analyzed separately.

A small portion of the sample should be put at once in a well-stoppered bottle; the remainder may be allowed to become air-dried, by exposing it in a thin layer, in summer, to the common temperature in the shade, or, in winter, to that of a warm room, or a moderately warm drying-chamber, heated to 30°–40°; in either case it should be carefully protected from dust.

At the time of taking the sample of the soil, observations should be made in regard to the following points:

a. The geognostic origin of the soil.

b. The nature of the underlying strata, to the depth of 1–2 meters, if practicable.

c. The meteorology of the locality, by consulting meteorological records, if possible; otherwise, by the general opinion of the neighborhood; in this connection, the height of the locality above the level of the sea should be noted also.

d. The management and rotation of crops in previous years.

e. The character of the customary manuring.

f. The amount of the crops removed in the preceding year, and, if possible, the average amount of each of the more important crops yielded by the field.

g. The practical judgment of neighboring farmers in regard to the field.

Caldwel’s method is practically identical with that of Wolff^[45] which was one of the earliest of the systematic schemes for taking soil samples.

62. Wahnschaffe^[46] insists on rather a fuller preliminary statement to accompany soil samples but gives essentially the method of Wolff with some unimportant variations which add little to the value of the process.

63. Method Of Peligot.—According to Peligot^[47] the taking of samples of soil of which the physical and chemical properties are to be determined is a delicate operation.

These samples should represent as nearly as possible both the good and bad qualities of a soil.

73In the field selected are chosen a certain number of places at least four or five per hectare. The spots selected should have a homogeneous appearance—resembling as nearly as possible the general aspect of the field.

By means of a spade a few kilograms of earth are removed to the depth of the subsoil being careful to include in the sample no accidental detritus which the upper part of the soil especially may contain.

The samples should be taken immediately after the crop is harvested and before any fresh fertilizer is applied. The samples are carefully mixed and placed in a glass bottle or flask.

The sample of subsoil is obtained in the same manner. If the field presents notable differences in surface or fertility all the samples taken should be examined separately.

64. Method of Whitney.^[48]—An ordinary wood auger, 2½ inches in diameter is so arranged as to admit of additions to the stem to enable the operator to take samples at different depths. It may be fitted with a short piece of gas pipe for a handle and the several pieces of which it is composed may be taken apart and carried in a knapsack.

In taking a soil sample the boring is continued until a change in color shows that the subsoil has been reached. The auger cuts a very clean sample save in excessively sandy soil. After the soil sample is secured the hole is cleaned out and the sample of subsoil taken by the same instrument. The soil is conveniently preserved in heavy cloth bags of which the usual size is 6 by 8½ inches. Where larger samples are required the size of the bag is correspondingly increased. Each bag is to be tagged or labeled to correspond with the entry in the note book.

Samples to determine the amount of empty space in a soil are taken as follows: The sampler is a piece of brass cylinder about nine inches long and about 1½ inches in diameter. A piece of clock spring is soldered in one end and sharpened to give a good cutting edge. This arrangement permits the sample to pass into the cylinder without much friction. The area enclosed by the clock spring is accurately determined and a mark is placed in the cylinder six inches from the cutting edge. The 74cylinder is driven into the soil to a depth of six inches, a steel cap being used to prevent the hammer from injuring the cylinder. The earth is next removed from about the cylinder with a trowel, and the separated cylinder of earth is cut smoothly off by a sharp knife and removed together with its brass envelope. The sample is taken to the laboratory in a cloth bag, dried and weighed.

65. Taking Samples for Moisture Determination.—A number of brass tubes is provided nine inches long and ¾ inch in diameter and with a mark six inches from the bottom.

The tube is pushed down into the soil to the mark and the sample of soil removed with the tube. There is but little danger of the sample dropping out of the tube even in sandy soils. When the tube is withdrawn each end is capped with a rubber finger tip making a perfectly air tight joint. The tubes containing the samples can be kept several days with no fear of losing moisture. This method is especially useful in having samples taken by observers in different localities who can enclose the tubes in a cloth sack and send them to the laboratory by mail daily or at stated intervals. A tube of the size given holds about fifty grams of soil.

66. Taking Samples to Determine the Permeability of Soil to Water or Air.—Whitney^[49] determines the permeability of soil or subsoil to water or air in the following manner:

An excavation two feet square and eighteen inches deep is made in the soil. On one side of this hole the sample of soil or subsoil is taken by means of a narrow saw blade and a sharp carving knife. The sample of soil taken should be two inches square and 3½ to 4 inches long.

It is placed in a brass cylinder three inches long and 3¼ inches in diameter. The open space in the cylinder is filled with paraffin heated just to its melting point. As the paraffin cools the upper surface should be kept stirred to prevent the mass when set from receding from the square column of soil. Care must be taken to keep the paraffin from the ends of the soil columns and these should be left, as far as possible in their natural condition.

75The rate of percolation of the water may be determined at the time the sample is taken. For this purpose an additional section of brass tube two inches deep is secured to the one holding the sample by a rubber band. An iron rod is driven into the earth carrying a retort stand ring supporting a funnel filled with fine gravel. The lower end of the soil column in the brass cylinder is placed on this gravel. Water is next carefully poured upon the top of the sample of soil being careful not to disturb the surface. The surface of the sample may be protected with a little fine sand. The water should be poured on the paraffin thus affording an additional protection to the soil surface. When the water begins to drop from the funnel a graduated glass is set under it and the time required for a given volume to pass through under an initial pressure of two inches is noted. The volume required represents one inch in depth over the four square inches of soil surface, viz.: four cubic inches.

67. Sampling of Soil for Staple Crops.—Some variations from the usual methods are recommended by Whitney when the samples are taken from fields growing staple crops.

The immediate object of the work, for which these samples are desired, is to make a thorough study of the physical and chemical properties of a number of typical soils adapted to the different staple crops, such as grass, wheat, truck, and the different types of tobacco. They should be taken for a careful study of the texture of the soils, the relative amount and arrangement of sand and clay, the relation of the soils to moisture and heat, and the ease with which they can maintain a proper water supply for the different staple crops under existing climatic and cultural conditions. The ultimate object of such a study is to see how these conditions can be changed so as to make the soils more productive, and make them yield a better quality of crop, or to change the conditions in other soils, which differ from these, so that the culture of the different staple crops can be extended over wider areas by improved methods of cultivation and manuring.

The soil selected for sampling for these investigations should be typical, should represent fairly well a considerable area 76of land. It should represent either the very best type of land for the staple crop or crops of the locality, or the very poorest lands for these same crops. Both of these extremes are desired for contrast. For example, if the staple crop of the locality is wheat or a certain type of tobacco, select the soil best adapted to this staple crop, and another soil, if possible, in the same locality, representing considerable area of land upon which this staple crop cannot be successfully grown on account of the inferior yield, quality, or the time of ripening of the crop. The soil sampled should be, or should recently have been, under actual cultivation in the crop or crops best adapted to it, so that the real agricultural value of the land can be accurately known.

The samples should be taken inside the field, some distance away from fences, roads, or trees. If there are plants growing in the field, the sample should be taken about midway between two plants. The samples should be taken where they will typify fairly well the average soil of the field and of the large area of land which they are to represent.

The samples are taken in some one of the ways described herein. Each sample should be carefully labelled at the time of taking. The following blank form will be found convenient for this purpose:

68. Method of the Royal Agricultural Society.^[50]—Have a wooden box made, six inches long and wide, and from nine to twelve inches deep, according to the depth of soil and subsoil in the field. At one of the selected places mark out a space of 77twelve inches square; dig around it in a slanting direction a trench, so as to leave undisturbed a block of soil, with its subsoil, from nine to twelve inches deep; trim this block to make it fit into the wooden box, invert the open box over it, press down firmly, then pass a spade under the box and lift it up and gently turn it over.

In the case of very light, sandy, and porous soils, the wooden box may be at once inverted over the soil and forced down by pressure, and then dug out.

Proceed in the same way for collecting the samples from all the selected places in the field, taking care that the subsoil is not mixed with the surface soil. The former should be sampled separately.

In preparing the plot for the gathering of the sample, take care to have it lightly scraped so as to remove any débris which may be accidentally found there.

The different samples thus procured are emptied on a clean, boarded surface, and thoroughly mixed, so as to incorporate the different samples of the same field together. The heap is then divided into four divisions, and the opposite quarters are put aside, taking care to leave the two remaining ones undisturbed; these are thoroughly mixed together, the heap divided into quarters, and the opposite ones taken away as before. This operation of mixing, dividing into quarters and taking away the opposite quarter is continued until a sample is left weighing about ten or twelve pounds.

Thus is obtained the average sample of the soil. Of course where only a single sample is taken from the field this method of quartering is not resorted to, but the bottom of the box is nailed directly on and sent to the laboratory, where the soil is to be analyzed.

69. Grandeau^[51] suggests that in taking soil samples there are two cases to be considered; first a homogeneous soil and second, a soil variable in its appearance and composition. First, if the soil is homogeneous, being of the same geologic formation it will be sufficient to take a mean sample in accordance with the following directions:

78The field is first divided by diagonals or by transverse lines the direction of which need not be fixed in advance but as inspection of the form and configuration of the field may indicate. In the ordinary conditions, of homogeniety (marly, granite, argillaceous or silicious soils) it will be sufficient to select about five points per hectare from which the samples are to be taken. These points having been determined the surface is cleaned in such a way as to remove from it the detritus which may accidentally cover it; such as dry leaves, fragments of wood, foreign bodies, etc. The surface having been prepared, (five to six square meters) a hole is dug four-tenths of a meter long and as wide as the spade employed. The sides should be as nearly vertical as possible. As to depth it varies with the usage of the country in regard to tillage. The layer of arable earth is what in effect properly constitutes the soil. It ought not to be mixed with any fragments of the subsoil. When the hole is properly cleaned the samples are secured with a spade from the sides of the excavations. About five kilograms are taken. The soil is placed in a proper receptacle as it is removed from the hole.

This operation is repeated on as many points as may be necessary to obtain a mean sample of the soil of the whole field.

All the samples are now collected on a table sufficiently large, and intimately mixed together. Two samples, each of about five kilograms, are then taken from the mixed material. One sample is immediately placed in bottles and carefully stoppered and sealed; the other is dried in the sun or on the hearth of a furnace. When sufficiently dry the second sample is also placed in bottles and well stoppered. While mixing the samples, pebbles, etc., of the size of a nut and larger are removed, the weight of the rejected matter being determined. The nature of the pebbles should also be noted; whether silicates, limestone, etc.

The sample of subsoil is taken in exactly the same manner, using the same holes from which the samples of soil were taken. The nature, the arrangement and the appearance of the strata will indicate the depth to which the subsoil should be taken. In general, a depth equal to that of the sample of soil will be sufficient. The depth to which the roots of cultivated plants reach 79is also a good indication in taking a sample of the subsoil. In forests the sample of subsoil should be taken from four to five-tenths of a meter below the surface.

If the soil in respect of its geologic formation, its fertility or its physical aspect presents great differences, special samples should be taken in each part in accordance with the directions given above.

70. Method of the Official Agricultural Chemists.—In the directions given by the Association of Official Agricultural Chemists^[52] it is stated that the soil selected should be as far as possible in its natural condition, not modified by recent applications of manure, or changed by the transporting action of water or wind. Surface accumulations of decaying leaves, etc., should be removed before taking the sample.

To eliminate accidental variations in the soil, select specimens from five or six places in the field which seem to be fair averages of the soil, remove two or three pounds of the soil, taking it down to the depth of nine or ten inches^[53] so as to include the whole depth. Mix these soils intimately, remove any stones, shake out all roots and foreign matter, and dry the soil until it-becomes friable.^[54] Break down any lumps in a mortar with a wooden pestle, but avoid pulverizing any mineral fragments; pass eight to ten pounds of the soil through a sieve, having circular perforations one twenty-fifth of an inch in diameter, rejecting all pebbles and materials too coarse to pass through the sieve. Once more mix intimately the sifted soil. Expose in thin layers in a warm room till thoroughly air dry (or dry it in an air-bath at a temperature of 40°), place six to eight pounds in a clean bottle, with label of locality and date, and cork the bottle containing the soil, for analysis.

The soil is rapidly dried to arrest nitrification; it is not heated above 40° lest there should be dissipation of ammonia compounds, or a change in solubility. The normal limit to which the soil may be heated in place by the sun’s rays should not be exceeded in preparing a sample for an agricultural chemical analysis.

80The relative amount of fragments too coarse to pass through the sieve should be made a matter of record. They are soil material, but not yet soil, so far as agricultural purposes are concerned.

71. Method of Lawes.—In a late method of sampling proposed by Sir J. B. Lawes^[55] a steel frame ten by twelve inches, and nine inches deep open at top and bottom is driven into the earth until its upper edge is level with the surface of the soil. All above-ground vegetation is then cut off as closely as possible with scissors. The soil within the frame is then removed exactly to the depth of the frame, and immediately weighed. It is then partially dried, and mechanically separated by a series of sieves, all visible vegetable matter being at the same time picked out. The stones and roots and the remaining soil are thus separated, and the determinations of dry matter, nitrogen, etc., are made in the separated soil after being finely powdered. The loss of water at each stage of preparation and on drying the samples as analyzed is also carefully determined. This method, which requires the soil to be taken to an arbitrary depth of nine inches, could not be used when samples of strictly arable soil are to be taken.

72. In taking a sample by the French commission^[56] method it is necessary to remove from the surface, the living and dead vegetation which covers the soil. With a spade a square hole is then dug to the depth of about 500 millimeters; in other words, to a depth considerably exceeding that of the arable layer. Afterwards on each of the four sides of the hole there is removed by the spade, a prismatic layer of the arable portion of a thickness equal to its depth. The samples thus obtained are united together and carefully mixed for the purpose of forming a sample for analysis. If there are large stones they are removed by hand and their proportion by weight determined.

In all cases it would prove useful to take a sample of the subsoil which is far from playing a secondary rôle. The rootlets bury themselves deeply in it and seek there a part of their nourishment. The subsoil, therefore, furnishes an important addition to the alimentation of the plants. For taking a sample 81of the subsoil a ditch is dug of sufficient depth, say one meter, and the arable soil carefully removed from the top portion. Afterwards pieces are taken from the four sides of the hole at variable depths, which should always be indicated, and which should extend in general, from six to eight-tenths of a meter below the arable soil since it is demonstrated that the roots of nearly all plants go at least to this depth. The analysis of the subsoil, however, is less important than that of the soil, properly so-called, because the agronomist does not act directly upon it and takes no thought of modifying it and enriching it as he does the layer of arable soil. But the composition of the subsoil is a source of information capable of explaining certain cultural results and capable sometimes, of leading to the correct way of improving the soil, as in cases where the subsoil can be advantageously mixed with the superficial layer.

73. Wolff^[57] suggests that a hole thirty centimeters square be dug perpendicularly and a section from one of the sides taken for the sample. To the depth of thirty centimeters the sample shall be taken as soil and to the additional depth of thirty centimeters as subsoil. The thickness of the section taken may vary according to the quantity of the sample desired. For analytical purposes, five kilograms will usually be sufficient. When culture experiments are also contemplated a larger quantity will be required.

74. Method of Wahnschaffe.—The method of sampling advised by Wahnschaffe^[58] is but little different from that of Wolff already mentioned.

A square sample hole is dug with a spade having its sides perpendicular to the horizon. The soil which is removed is thrown on a cloth and carefully mixed. From the whole mass a convenient amount is next removed care being taken not to include any roots. In a similar manner it is directed to proceed for the sample of subsoil. At first the subsoil should be removed to a depth of two to three decimeters. The number and depth of subsequent samples will depend chiefly upon the character of the soil. Where samples are taken to the depth of two meters the use of a post-hole auger is recommended.

82The samples taken should not be too small. In general from two to three kilograms should remain after all preliminary sampling is finished.

75. Method of König.—The directions given by König^[59] for taking soil samples are almost identical with those prescribed by Wahnschaffe and do not require any further illustration.

76. Special Instruments Employed in Taking Samples.—In general a sharp spade or post-hole auger is quite sufficient for all ordinary sampling but for certain special purposes other apparatus may be used.

The instrument which is used by King^[60] consists of a thin metal tube of a size and length suited to the special object in view, provided with a point which enables it to cut a core of soil smaller than the internal bore of the tube and at the same time make a hole in the ground larger than its outside diameter. Its construction is shown in figure 11, in which A B represent a soil tube intended to take samples down to a depth of four feet. A′ is a cross-section of the cutting end of the tube, which is made by soldering a heavy tin collar, about three inches wide, to the outside of a large tube allowing its lower end to project about one-half an inch. Into this collar a second one is soldered with one edge projecting about one-quarter of an inch and the other abutting directly against the end of the soil tube. Still inside of this collar is a third about one-half an inch wide which projects beyond the second and forms the cutting edge of the instrument.

Figure 11.

The construction of the head of the tube is shown at B′. It is formed by turning a flange on the upper end of the tube and then wrapping it closely with thick wire for a distance of about 83three inches, the wire being securely fixed by soldering. The soil tube should be of as light weight as possible not to buckle when being forced into the ground, and the cutting edge thin. The brass tubing used by gas fitters in covering their pipes has been found very satisfactory for ordinary sampling. With a one inch soil tube four feet long it is possible to get a clear continuous sample of soil to that depth by simply forcing the tube into the ground with the hand and withdrawing it, or the sample may be taken in sections of any intermediate length. Later in the season when the soil becomes dryer it is necessary to use a heavy wooden mallet to force the tube, and this should be done with light blows.

The closeness with which it is possible to duplicate the samples in weight by this method will be seen below, where from each of four localities three samples were taken from the surface to a depth of four feet.

Showing Variations in the Dry Weight of Triplicate Samples of Soil.

These four series of samples were taken at the four corners of a square twelve feet on a side and serve to show how much samples may vary in that distance. The large difference shown in III, A is due to the fact that the soil tube penetrated a hole left by the decay of a rather large root as shown by the bark in the sample.

77. Auger for Taking Samples.—It has already been said that the ordinary auger used for boring fence post-holes may be used to advantage in taking soil samples. Large wood augers can also be used to advantage for the same purpose. For special purposes, however, other forms of augers may be used.

Norwacki and Borchardt^[61] have described a new auger for taking samples of soil for analytical purposes.

Figure 12.

In figure 12, A, B and C show the general exterior and interior form of the instrument. The handle is hollow and made of iron gas pipe covered with leather. On the inside of this, in the middle, is fixed a wooden plug a, which leaves two compartments, one in each end for holding the brass plug bb,’ and the wicker lubricating wad cc.’ The stem of the auger a, is heavy and made of eight-sided steel and the under end is strengthened with a heavy casting fitting into the auger guide g g. The end of the auger I I′ is triangular and hardened. The auger guide g g, is made out of a single piece of drawn steel tubing. Above it is strengthened by a ring-shaped piece of iron or copper and its lower end is furnished with saw teeth as shown in K and is hardened. The fixing key e, is bent in the form of a hook and can be passed through the two holes o o, of the auger stem and through the one hole o′ in the strengthened part of the auger guide. It permits the auger guide to be fixed upon the auger stem in two different positions, higher and lower. On one end it is cut squarely across and on the other provided with a conical hole drilled into it. It fits on the one hand exactly in the auger guide and on the other loosely plays in the cavity of the handle at b, designed to hold it when not in use. The cap d′ is made of heavy sheet brass and is fastened upon the end of the handle at c c′ after the manner of a bayonet. The wicker cartridge is made of rolled and sewed wicker-work. At the upper end it is provided with a metallic button and before use it is saturated with paraffin oil. It fits on the one side firmly in the auger guide and on the other in the cavity, of the handle c where it is kept when not in use. The union h is made of a brass tube which 84below is closed with a piece of solid brass upon the inside of which a hole is bored. In this hole rests the end of the auger stem when the union is placed firmly upon the auger guide.

The auger is placed together as is shown in A B, the union h is taken off and it is driven with gentle blows, turning it back and forth, to the proper depth into the soil. After the key is loosened the auger is lifted high enough so that the second hole appears and then it is fixed in position by the key. Then the boring is continued, turning the auger to the right, by which the auger, eating its way with its saw teeth, presses deeper into the ground and withdraws the material for analysis. After the auger guide has been filled through any desired length, say five to ten centimeters with the sample of soil, the whole auger is drawn out of the soil, the key removed, the auger stem withdrawn from the auger guide, the apparatus opened by turning the bayonet fastening of the stopper on the handle, the brass plug placed in the end and then with the smooth part forward, from above, it is allowed to fall into the auger guide until it reaches the soil. The auger stem is then put back, the point of it fitting into the hole of the plug and the sample of soil shoved out of the auger guide. The auger guide is again fixed on the auger stem by the key and then the apparatus is ready for a second operation. When the borings cease the wicker cartridge is drawn out of the handle and shoved, the soft end forward, from above, into the auger guide and the brass plug after it and pushed through with the auger stem. By this process the wicker cartridge gives up a sufficient amount of paraffin oil to completely grease the inside of the auger guide and to protect it from rust. After use the instrument should be cleaned on the outside by means of a cloth, the plug and wicker wad replaced in their proper positions, the cap fixed on the handle and the union on the point of the instrument.

The length of the whole apparatus may reach one meter or more; the internal diameter sixteen millimeters. The apparatus weighs with a length of one meter, together with all its belongings, about two kilograms. For the investigation of peat and muck soils as well as sand, instead of the steel auger guide 85one of brass or copper can be used. For this purpose the length of the apparatus may reach three to four meters.

In comparison with other apparatus which are used for taking samples, it appears without doubt that with the one just described a better and less mixed portion of the soil can be obtained at great depths. The apparatus is said to have many advantages over a similar one known as Fraenkel’s, and is much more easy to clean. The advantages of the apparatus are said to be the following: The farmer with this piece of apparatus in a short time can go over his whole farm taking samples to the depth of ninety centimeters since a single boring does not take more than one minute. Geologists and others interested in the soil at greater depths can use an apparatus three to four meters in length and obtain unmixed samples from these lower depths. These are also interesting from a bacteriologic point of view. The entire apparatus is especially valuable for the investigation of the lower parts of peat and muck soils. The apparatus has been tried in the collection of samples for the laboratory of the Department of Agriculture and is too complicated to be recommended for ordinary use. When however samples are to be taken at great depths as in peat soils it is highly satisfactory.

78. Soil Sampling depends for its success more on the judgment and knowledge of the collector than on the method employed and the apparatus used. One skilled in the art and having correct knowledge of the purpose of the work will be able to get a fair sample with a splinter or a jack-knife while another with the most elaborate outfit might fail entirely in collecting anything of representative value.

There are some special kinds of soil sampling, however, which cannot be left to the method of the individual and it is believed that with the descriptions given above nearly all purposes for which samples are desired may be served.

For the study of nitrifying organisms, however, special precautions are required and these will be noted in a more appropriate place.

In taking samples for moisture determinations the method of 86Whitney is recommended as the best. For the general physical and chemical analytical work the standard methods are all essentially the same. The principles laid down by Hilgard will be found a sufficient guide in most cases.

79. The Sample, or mixed sample, taken by one of the methods above described, is placed on a hard smooth board, broken up by gentle pressure into as fine particles as possible and all pieces of stone and gravel carefully removed and weighed; all roots, particles of vegetable matter, worms, etc., are also to be weighed and thrown out. This can be done very well by using a sieve of from one to two millimeter mesh. Care should be taken that the soil be made to pass through, which can be accomplished by subjecting the lumps to renewed pressure with a rubber-tipped pestle. In the above operation the soil should be dry enough to prevent sticking. The relative weights of the pebbles, roots, etc., and the soil should be determined.

80. Order of Preliminary Examination.—Hilgard^[62] commences the examination of a soil sample by washing about ten grams of it into a beaker with a water current of definite velocity, stirring meanwhile actively the part carried into the vessel. The residue not carried by the current is examined macro- and microscopically to determine the minerals which may be present, and the condition in which the fragments exist—whether sharp or rounded edges, etc.

This examination will give some general idea of the parent rocks from which the sample has been derived and of the distance the particles have been transported. Next follows the hand test, viz., rubbing the soil between the thumb and fingers first in the dry state and afterwards kneading it with water and observing its plasticity. Following this should come a test of the relations of the sample to water, viz., its capacity for absorbing and retaining moisture. Finally the separation of the soil into particles of definite hydraulic value and a chemical examination of the different classes of soil concludes the analytical work.

8781. Air Drying.—The sifted soil should be thoroughly mixed and about one kilogram spread on paper and left for several days exposed in a room with free circulation of air and without artificial heat. The part of the sample to be used for the determination of nitrates should be dried more quickly as described in another place. The sample is then placed in a clean, dry glass bottle, corked, sealed, and labeled. The label or note book should indicate the locality where the sample was taken, the kind of soil, the number of places sampled, and other information necessary to proper description and identification.

82. Caldwell^[63] directs that having taken the sample to the laboratory, the stones and larger pebbles should be separated from the finer parts by the hand, or by sifting with a very coarse sieve, and examined with reference to their mineralogical character, weight and size, making note, in this last respect, of the number that are as large as the fist or larger, the number as large as an egg, a walnut, hazel-nut, and pea, or give the percentage of each by weight.

Pulverize the air-dried soil in a mortar with a wooden pestle, and separate the fine earth by a sieve with meshes three millimeters wide; this sieve should have a tightly fitting cover of sheepskin stretched over a loop, and it should be covered in the same manner underneath, so that no dust can escape during the process of sifting.

Wash the pebbles and vegetable fibers remaining on the sieve with water, dry and weigh the residue; the water with which this gravel was washed should be evaporated to dryness at a temperature not exceeding 50° towards the close of the evaporation, and the residue mixed with what passed through the dry sieve.

The sifted fine earth is reserved for all the processes hereinafter described, and is kept in well-stoppered bottles, marked air-dried fine earth. The sieve mentioned above is too coarse for the more modern methods of analysis.

83. Wolff^[64] directs that the air-dried earth (in summer dried in thin layers at room temperature, in winter in ovens at 30° to 50°) be freed from all stones, the latter washed, dried, and 88weighed. The soil is next passed through a three millimeter mesh sieve, the residual pebbles and fiber washed, dried and weighed. The fine earth passing the sieve is used for all subsequent examinations. It is air-dried at moderate temperatures and preserved in stoppered glass vessels.

84. The French^[65] commission calls especial attention to the method of subsampling, and prescribes that the sample of earth which has been taken in the manner indicated, and of which the weight should be greater as the material is less homogeneous should not be analyzed as a whole. It should be divided into two parts. The first includes the finer particles constituting the earth, properly so-called, with the elements which alone enter into play in vegetable nutrition and on which it is necessary to carry out the analysis. The second embraces the coarser particles to which only a superficial examination should be given and which may have a certain importance from a physical point of view but which cannot take any part from a chemical point of view, in the nutrition of plants. It is, however, useful to examine its mineralogical constitution and to look for the useful elements such as lime, potash, etc., which it may be able to furnish to the earth, and in proportion as it is decomposed, finer particles which may be useful in plant nutrition.

How are we to distinguish between the fine and coarse elements? All grades of fineness are observed in the soil, from the particles of hydrated silica so small that with the largest magnifying power of the microscope it is scarcely possible to distinguish them, up to grains of sand which are of palpable size and visible to the naked eye, and extending to pebbles of varying sizes. All intermediate stages are found between these and if it should be asked what is the precise limit at which it is necessary to stop in distinguishing the fine from the coarse elements of the soil, the answer is that this can only be determined by a common understanding among analysts. In general, it may be said, that the mark of distinction should be the separation which can be secured with a sieve having ten meshes per centimeter.

85. Loose Soils.—Having agreed upon a sieve of the above size, the process of separation in loose soils is as follows: The 89earth is exposed to the air and when the touch shows that it is sufficiently dry the conglomerated particles should be simply divided without breaking the rocky material which exists in a state of undivided fragments. There are some special precautions to be taken. Rubbing in a mortar must be forbidden since it reduces the earth to particles which are unnatural in size, by securing the breaking up of the fragments consisting of the débris of rocks. When it is possible the earth should be rubbed simply in the hand and after having separated that which passes the sieve, the large particles which have not passed should be again rubbed with the hand, until all the particles which can be loosened by this simple treatment have passed the sieve. The separation should be as complete as possible in order that a sample of the particles passing the sieve should represent as nearly as possible, a correct sample of the fine particles of the soil.

In regard to the pebbles, they should be washed with water upon the sieve in order to carry through the last of the particles of earth adhering to them. They are then dried and their weight taken. The fine part of the earth is also weighed. On an aliquot part, say 100 grams, the moisture is determined and then by simple calculation the whole sample of the air-dry soil can be calculated to the dry state. The sample is then placed in a glass flask.

The pebbles are examined with a view of determining their mineralogical constitution; as for instance, on being touched with a little hydrochloric acid it can be determined whether or not they are carbonate of lime. The nature of the rock from which they have been derived is often to be determined by a simple inspection.

86. Compact Soils.—If the soils are not sufficiently loose to be treated as before described, it is necessary to have recourse to other means of division, which should not, however, be sufficiently energetic to reduce the rocky elements to fine particles. For this purpose the earth may be broken by means of a wooden mallet, striking it lightly and separating the fine elements from time to time by sifting. A wooden roller may also be used with 90a little pressure, for breaking up the particles or a roller made out of a large glass bottle. These methods will permit of a sufficiently fine division of the soil without breaking up any of the pebbles. Sometimes, however, a soil can not be broken up by such treatment. It is then necessary to have recourse to the following process: The soil is thoroughly moistened and afterwards rubbed up with water. The paste which is thus formed, is poured upon the sieve and washed with a stream of water until all the fine particles are removed. The wash water and the fine particles are left standing until the silt is thoroughly deposited when the supernatant water is poured off and the deposited moist earth is transferred into a large dish and dried on a sand or water-bath. In this way a firm paste is formed which can be worked up with the hand until rendered homogeneous and afterwards an aliquot portion be taken to determine moisture.

87. Method of Peligot.—The method recommended by Peligot^[66] for the preparatory treatment of the sample is essentially that already described. The sample is at first dried in the air and then in an oven at 120°. When dry and friable 100 grams are placed in a mortar and rubbed with a wooden pestle. It is then passed through a sieve of ten meshes per centimeter. The largest particles which remain in the sieve should have about the dimensions of a pin’s head. The stones are separated by hand. They should be shaken with water in order to detach any pulverulent particles adhering thereto. The turbid water resulting from this treatment is added to that which is used in separating the sand from the impalpable part of the soil.

88. Wahnschaffe prescribes^[67] in the further preparation of the sample for analysis that the coarse pieces up to the size of a walnut be separated in the field where the sample is taken and their relative weight and mineralogical character determined. The soil sample is then to be placed in linen or strong paper bags and carefully labelled. In order to avoid any danger of loss of label the description or number of the sample should be put on the cloth or paper directly.

The sample when brought to the laboratory should be spread out to dry, in a room free of dust. In the winter the room 91should be heated to the usual temperature. The air drying should continue until there is no sensible loss of weight. The samples then are to be placed in dry, glass-stoppered glass bottles where they are kept until ready for examination. This method of keeping the samples avoids contact with ammonia or acid fumes with which a laboratory is often contaminated.

89. The Swedish chemists^[68] direct that samples which are to be used for chemical examination in the manner described below, are most conveniently brought to such a condition of looseness and humidity that the soil feels moist when pressed between the fingers without, however, sticking to the skin. To prepare the sample in this manner, spread it in a large porcelain dish or on a glass plate in a place where it is not reached by the laboratory atmosphere; stir it frequently till it assumes the mentioned humidity (if the sample when sent is too dry, moisten it with distilled water till its condition is as indicated); then pulverize carefully between the fingers and finally sift through a sieve with five millimeter holes. In this way free the sample from stones, undecayed roots and similar parts of plants, pieces of wood, and other matter strange to the soil, which remain on the sieve; mix the sample carefully and put it into a glass bottle provided with a stopper well ground in; keep it in a cool place. Samples prepared in this way will usually contain 20–30 per cent moisture; boggy soils 60–80 per cent and peat soils 50 per cent.

90. Petermann^[69] follows the method below in preparing samples of soil for analysis.

The soil is gently broken up by a soft pestle and all débris if of organic nature, cut fine with scissors. About 2500 grams of this soil are passed through a one millimeter mesh sieve. The organic débris is removed by forceps, washed free of adhering earth dried at 120° and weighed. The nature of the organic débris should be noted as carefully as possible.

The pebbles and mineral débris not passing the sieve are worked in a large quantity of water by decantation. They are also dried at 120° and weighed. This débris is examined mineralogically and thus some idea of the origin of the soil obtained.

9291. The various methods for the preliminary treatment as practiced by the best authorities have been somewhat fully set forth in the foregoing résumé. The common object of all these procedures is to get the soil into a proper shape for further physical and chemical examination and to determine the comparative weights of foreign bodies contained therein.

The essential conditions to be observed are the proper sifting of the material and avoidance of mechanical communition of the solid particles too large to pass the meshes of the sieve. If possible the material should be passed through a sieve of one millimeter mesh. In cases where this is impracticable a larger mesh may be used, but as small as will secure the necessary separation. Before final chemical analysis a half millimeter mesh sieve should be employed if the soil be of a nature which will permit its use. Over-heating of the sample should be avoided. Rapid drying is advisable when the samples are to be examined for nitrates.

The method recommended by the French commissions seems well adapted to the general treatment of samples, but the analyst must be guided by circumstances in any particular soil.

PART THIRD. PHYSICAL PROPERTIES OF SOILS. — DETERMINATION OF PHYSICAL PROPERTIES. — RELATION OF THE SOIL TO HEAT. — DETERMINATION OF SPECIFIC HEAT. — SOIL THERMOMETRY. — APPLICATIONS OF SOIL THERMOMETRY. — COHESION AND ADHESION OF SOILS. — THE ABSORPTIVE POWER OF SOILS FOR SALTS IN SOLUTION. — POROSITY AND ITS RELATIONS TO MOISTURE. — MOVEMENT OF WATER THROUGH SOILS: LYSIMETRY. — AUTHORITIES CITED IN PART THIRD.

92. The Soil as a Mass.—The soil constituted as indicated in the preceding pages, is now brought to the analyst for investigation. The properties with which he first becomes acquainted are those which impress his senses as mass characteristics. There is a perception of color, consistence, weight and other features which the soil possesses as a whole. The several constituents of the soil must first be considered as molecular and mole aggregates. In other words, the soil in its natural state is a mechanical mixture of particles which must first be considered as a whole. The physical properties of the soil, therefore, should engage the attention of the analyst before he proceeds to the investigation of the properties of its several constituents as classified by the relative size or hydraulic value of the particles of which they are composed, or to a chemical determination of the compounds or elements therein contained.

93. Color.—The color of a soil depends chiefly upon the proportion of organic matter and iron compounds which it contains and the state of subdivision of its particles. When a soil contains a large amount of organic matter, especially when this organic matter is in an advanced state of decay, it assumes more or less a black color when moist. This black color is to be distinguished from the black alkali tint which is produced by 95the action of carbonate of soda on organic matter. The naturally black color of a soil containing a large amount of organic matter depends, however, either upon the action of mineral matters upon this organic matter, as in the case of the black alkali mentioned, or upon the blackish color of carbon resulting from the slow combustion of the organic matter during the period of decay.

The presence of a large amount of ferric oxid in soil gives the well-known red color so well-marked in the soils of southwestern Kentucky and other portions of the United States. The preponderance of sand in a soil tends to produce a light yellow or whitish tint, while certain kinds of clay have a bluish tint probably due to the presence of ferrous salts. The influence of the color of the soil upon the color of the vegetation is also well-marked, the black soils as a rule producing a much deeper green tint of foliage than the light colored soils. This effect should not be attributed to color alone for as a matter of fact highly colored soils are usually very close and very retentive of moisture, which is one reason, probably, for their not being more highly oxidized. Such soils will produce a more vigorous and ranker growth of vegetation, but it is the texture of the soil and the more moist condition which it maintains, rather than the color, which produce the deeper green tint of foliage.

The color of a soil is also used as an index of its fertility, the black and red soils being usually the most fertile.

It may be well to add here the probable reason as given by Whitney for this, viz., that the deeper color shows that the oxids of iron and the organic compounds have less oxygen and indicate that the soils are quite retentive of moisture and rather tend to the exclusion of air, so that part of the oxygen of the iron compounds and of the organic matters has been used up in the oxidation processes within the soil. It is known, for example, that wood oxidizes much more rapidly around a rusty nail than where it is simply exposed to the air, the iron oxid acting as a carrier between the oxygen of the air and the organic matter. In a sandy soil, on the contrary, where there is usually less moisture and much freer circulation of air, the iron compounds have more oxygen and usually have a light yellow color. If this sand is 96heated, however, with the exclusion of air, and especially in the presence of organic matters, part of this oxygen will be given off and there will be the same red color as in the heavier clay soils. It is frequently noticed, also, in compact clays that where air gains access through cracks or root-holes, the color is altogether modified.

94. Determination of Color.—There is no process which will give experimentally and accurately the color of a soil sample. The changes which the color of a soil undergoes in passing from a saturated to an anhydrous state are well-marked. The analyst will have to be content with giving as nearly as possible a description of the color of the sample when taken and the changes which it undergoes in air drying or on heating in a bath to 100°–110°, or in heating to redness with or without exclusion of the air. These changes in color will give some indication of the character of the organic and mineral matters present.

95. Odoriferous Matters in Soil.—It is known that the soil emits a peculiar odor which is not disagreeable except when it has been recently wet, for instance, after a short rain. Several attempts have been made to discover the nature of this odor. These researches have established the fact that the essential principle of this odor resides in an organic compound of a neutral nature of the aromatic family and which is carried by the vapor of water after the manner of a body possessing a feeble tension. The odor is penetrating, almost piquant, and analogous to that of camphorated and quite distinct from other known substances. In regard to the quantity of this substance, it is extremely minute and can be regarded as being only a few millionths of a per cent.

According to Berthelot and André^[70] this new principle is neither an acid nor an alkali nor even a normal aldehyd. It is, in a concentrated aqueous solution, precipitable by potassium carbonate with the production of a resinous substance. Heated with potash it develops a sharp odor similar to the aldehyde resin. It does not reduce the ammoniacal nitrate of silver. Treated with potash and iodin it gives an abundant formation of iodoform, which, however, is a property common to a great 97number of substances. For the qualitative and quantitative estimation of the odoriferous matter the following process is employed:

About three kilograms of the soil are mixed with sand containing a small amount of carbonate of lime and some humic substance; after having freed it from all organic débris which is visible, it is placed in a glass alembic. The soil should contain from ten to twelve per cent of water at least. The alembic is placed in a sand bath and is kept at 60° for several hours. The water evaporated is condensed until about seventy-five cubic centimeters are distilled over. This distilled water is again rectified so as to obtain in all about twenty cubic centimeters. The odoriferous matter appears to be nearly all contained in this twenty cubic centimeters. The liquid thus obtained shows an alkaline reaction; it contains some ammonia and reduces ammoniacal silver nitrate. This last reaction is due to some pyridic alkali or analogue thereof, and is cause for it to be distilled anew with a trace of sulfuric acid which gives a neutral liquor deprived of all reducing action but which preserves the odor peculiar to the soil. The twenty cubic centimeters obtained as before are subjected to two additional distillations and in the final one only one cubic centimeter of liquid is distilled over.

The peculiar odor is intensified proportionately as the volume of the liquid is decreased. To this one cubic centimeter, is added some pure crystallized potassium carbonate. The liquor is immediately troubled and some hours are required for it to become clear again. Meanwhile there is formed upon its surface a resinous ring almost invisible, amounting at most to from ten to twenty milligrams of a matter which has not been identified with any known principle. The reactions described above, however, permit of its general character being known. This resinous matter contains the odoriferous principle, the composition of which is not yet definitely known.

96. Specific Gravity.—The density of a soil depends on its composition, the fineness of its particles and upon the packing which it has received. It has in other words an apparent and a real specific gravity. It is easy to see that a soil in good tilth 98would weigh less per cubic foot than one which had been pressed closely together, as in a road or well-pastured field. Ordinary soils in good tilth have an apparent specific gravity of about 1.2, and when entirely free from air, a real specific gravity of about 2.5. If the apparent specific gravity of a soil sample were 1.2 and the air were removed, leaving a vacuum in the interstices of the soil, the apparent specific gravity would not be sensibly increased. The figure 1.2 is the apparent specific gravity of a mixture of soil material which is about 2½ times heavier than water, and of an extremely small proportion by weight of air which is about 1000 times lighter than water. The figure 2.5 is about the true specific gravity of the real soil material, and shows that this material is about 2½ times heavier than an equal volume of water.

The weights of a cubic foot of different kinds of soil as given by Schübler^[71] are as follows;

In general the specific gravity of soil decreases inversely as its content of humus.

97. Determination of Specific Gravity.—The ordinary method of proceeding to determine the true specific gravity is by means of a pyknometer. The pyknometer should have a capacity of from twenty-five to fifty cubic centimeters.

From ten to fifteen grams of earth dried to constant weight at 100° are taken, boiled for a time with a few cubic centimeters of water to remove air and poured into the pyknometer. All soil particles are washed out of the vessel in which the boiling took place into the pyknometer with freshly boiled distilled water and after cooling to the temperature at which the calibration took place, the pyknometer is filled with distilled water at the given temperature and weighed. If the soil contain materials soluble in water, alcohol of definitely known specific gravity may be employed 99and the number thus obtained calculated to a water basis.

The calculations when water is used are made as follows:

Then specific gravity = 10.000 ÷ 4.9100 = 2.04.

98. Specific Gravity of Undried Soils.—It is often desirable to determine the specific gravity of an undried portion of the soil. For this purpose a portion of the sample is dried at 100° to determine its percentage of moisture. The specific gravity is then determined on a ten gram sample of the undried soil as just given. The actual weight of soil taken is calculated from the percentage of moisture obtained in the first instance. In the case given if the percentage of moisture at 100° be ten then the actual weight of dry soil taken is nine grams. This number is therefore used in making the calculations. In all statements of specific gravity taken in the manner described the temperature at which the pyknometer is calibrated should be stated and all weighings where water is involved made at that degree.

99. Volume of Soil.—If it be desired to calculate the volume occupied by a soil it is easily done by dividing the weight of water displaced by the weight of one cubic centimeter of water of the temperature at which the determination took place.

In the case given one cubic centimeter of water at 20° weighs 0.998259. Then 4.9100 ÷ 0.998259 = 4.9186 cubic centimeters = volume occupied by ten grams of dry soil excluding interstitial spaces between particles.

100. Volumetric Methods.—The water displaced by a given weight of soil may also be measured volumetrically by the method of Knop.^[72]

Place 200 grams of the soil in a flask of from three to five hundred 100cubic centimeters capacity. Add a measured quantity of water, and shake thoroughly to eliminate air, and fill up to the mark from a burette. The quantity of water required to complete the volume subtracted from the number expressing the volume of the flask will give the volume of water displaced by the earth.

Another method consists in thoroughly shaking about thirty grams of the soil in a graduated cylinder with fifty cubic centimeters of water containing a little ammonium chlorid and after twenty-four hours recording the volume occupied by the whole. The increase in volume over fifty cubic centimeters shows the quantity of water displaced. This method may also be used to determine the volume occupied by a soil when saturated with water. The above methods are only to be used when approximately correct results are all that are desired.

101. Apparent Specific Gravity.—The apparent specific gravity of a soil is obtained by dividing its volume, interstitial spaces included, by the weight of an equal volume of water.

The real and apparent specific gravities of six samples of soil are given below.^[73]

It is to be noted that in computing the apparent specific gravity of a soil dried at 125° the volume occupied by the water is assumed to occupy the same space as if it existed in a free state. The volume of this water is therefore to be subtracted from the contents of the flask before proceeding with the computations.

102. Determination of Apparent Specific Gravity.—Place in small quantity portions of the air-dried sample properly prepared, into an open glass cylinder, holding one liter, and about 170 millimeters high (if the height is exactly the mentioned one, the diameter of the cylinder will be 86.6 millimeters); pack the sample by striking the bottom of the cylinder hard against the palm of the hand after each new filling; close the cylinder thus 101filled by a glass plate and weigh on a balance sensitive to 0.1 gram; deduct the weight of the cylinder and glass plate, and the weight of one liter of soil in approximately similar conditions as it is found on the dry land prepared for cultivation, is thus ascertained. The weight of one liter of the soil in grams multiplied by 2000 will give in kilograms the weight of the surface soil from a hectare (2.47 acres) of the field from which the sample is taken when the depth of this is calculated at twenty centimeters.^[74]

103. Sources of Soil Heat.—The heat of the soil comes from three sources, viz.: solar heat, as the sun’s rays, heat of chemical and vital action within the soil, and the original heat of the earth’s interior. The latter is sensibly a constant quantity, and of great value to plants. The heat of chemical and vital action is not great in amount except in a few special cases but is often, as in germination, of the greatest importance to plant growth. The sun, therefore, remains the greatest source of heat of practical importance in relation to the production of crops. Dark-colored soils, absorbing most and radiating the fewest rays, must attain the highest temperature. Schübler’s classical researches on soil temperatures, show that there is at times a difference of over 7° in temperature between white and black soils, all other conditions being alike. Schübler’s researches, being made on dry soils in the laboratory, do not, however, apply wholly to conditions in the field.

104. Influence of Specific Heat.—The heat which a soil receives and retains is largely due to the specific heat of the soil. The specific heat of a body is expressed by a number which shows the amount of heat necessary to raise a given weight of the body 1° of temperature, as compared with the amount necessary to raise the same weight of water 1°. The specific heat of the soil is usually between 0.20 and 0.25, while that of water taken as the standard is unity.

105. Influence of Moisture.—The moisture of the soil possesses 102great influence on the soil temperature, so much so that a dry, light-colored soil may attain a greater degree of warmth than a moist, dark-colored one. The action of water in reducing soil temperature is easily explained. In our latitude, we see the water in all its forms, solid, liquid, and gaseous, and we know that these forms are the direct result of temperature. The changing of water from the solid to the liquid or gaseous form is performed at the expense of heat; the more water evaporated from the soil the more heat must be used for the evaporation. Therefore, the more water contained in the soil at any given time the lower must be its temperature during subsequent exposure to sun heat because of the greater evaporation. The experiments of Liebenberg, Pattner, Schübler and Dickenson have practically settled all the questions of soil temperatures. The radiation of heat from the soil, and the consequent cooling propensity of the latter, are directly proportional to the absorptive power of the soil. Two soils of like absorptive power towards heat possess, as a rule, equal radiating power.

In a general way, it can be said the greater the heating capacity and conductivity of a soil the more readily and rapidly does it give off its heat and become cooled.

106. Absorption of Solar Heat.—The quantity of heat absorbed from the sun by the earth is an important factor in the growth of vegetation. As has been established in the physics of heat, a black surface, other things being equal, will absorb a larger amount of heat than one of any other color; so, other things being equal in the physical and chemical composition of a soil, variations in the amount of organic matter producing greater or less black coloration will affect the heat absorption. Thus, black soils, in the conditions above mentioned, will absorb more heat than lighter colored soils. As a result, the vegetation in such soils gets an earlier start in the Spring and matures more rapidly. As an illustration of this it may be noted that the black prairie soils of Iowa produce uniformly crops of maize which are matured before the early frosts, while crops grown on lighter soils much farther South often suffer injury from that source.

107. General Principles.—The quantity of heat stored in any given weight of soil is capable of being measured and compared with the quantity stored in an equal weight of water at the same temperature. The ease, however, with which disturbing influences operate during the determination makes the manipulation somewhat difficult. The specific heat of the containing vessels must be carefully determined. Fortunately this has been done for most materials and the data thus obtained are recorded in standard works on physics. The material operated on must be protected from thermal influences from sources not controlled by the experiment and even the heat of the operator’s body may often disturb the conduct of the work. The general conditions which should control the experiment as well as the details thereof are given in the following method which, however, the ingenious analyst may profitably simplify.

108. Method of Pfaundler.—The process of estimating the specific heat of soils by the method of mixture, is essentially that of Regnault and is described as follows by Pfaundler^[75].

The apparatus used is illustrated in Fig. 13.

A and A′ show the heating apparatus. It consists of a vessel of sheet iron in which a test tube E is fixed by means of a cork. The test tube holds the soil whose specific heat is to be determined. The apparatus contains water, which is brought to the boiling point by means of a lamp, and the excess of steam is conducted away, as indicated in the figure, through one of the axes of the apparatus; the opposite axis is, of course, closed. It requires about thirty-five minutes boiling to bring the contents of the test tube to the temperature of the aqueous vapor. The exact temperature at which the water boils is determined by observing the barometer at the time and consulting a table of the boiling temperature of water at different barometric pressures.

The calorimeter is shown in the figures B and B′. It consists of a wooden box closed on one side by a glass plate G and on the other to the heighth F by a small board on which a calorimeter of ordinary construction is placed. The cylinder of the calorimeter is seventy millimeters high and forty-seven millimeters in diameter.

Figure 13.

Regnault’s Apparatus for Determining the Specific Heat of Soils.

104This part of the apparatus is supported by triangular pieces of cork. A delicate thermometer is fastened to the top of the box of the calorimeter and the value of the degrees is so arranged that about twelve of them correspond to about one degree C. The scale of the instrument can be arbitrarily fixed and the temperature of any part of it determined by comparison with a delicately graduated thermometer.

Near the thermometer in the calorimeter is a stirrer made of a very thin copper disk with a bent rim. This stirrer is operated by means of a silk cord moved by appropriate machinery.

The reading of the thermometer is made through a glass plate and this should be protected from the heat of the body of the observer by a paper screen.

The test tube E is first filled with the substance, whose specific heat is to be determined, and weighed. It is then placed in the water bath until constant weight is reached. After constant weight has been obtained the apparatus is again dried and the exact weight of the moisture lost thus determined. The test tube is then placed in the apparatus A closed with a well-fitted cork, the top covered with cotton and heated in the aqueous vapor for about one hour. The heating apparatus should be far removed from the calorimeter so that the temperature of the latter cannot be influenced thereby. Meanwhile the calorimeter is filled with water which has stood in the room for a long time until it has acquired, as nearly as possible, the room temperature.

The quantity of water is such that the water value of the whole of the calorimeter together with the immersed portions of the thermometer and stirrer shall amount to exactly 100 grams. A few minutes before bringing the substance into the calorimeter, the stirring apparatus is put in motion and the temperature observations are commenced. These should be at intervals of twenty seconds and should be continued until ten observations have been made. Meanwhile the height of the barometer is also read. A few seconds before the tenth interval the apparatus A is brought quickly to the calorimeter and its contents emptied 105into it at the moment of the tenth interval. The apparatus A should be removed as quickly as possible after its contents are emptied.

After the introduction of the substance and its thorough incorporation with the water of the calorimeter by the stirring apparatus, the thermometer is again read, at intervals of twenty seconds, until its maximum has been reached and as much longer thereafter as may be necessary to show that an appreciable fall of temperature has taken place. The test tube, in which the substance was heated is weighed and the exact quantity of the added substance thus determined.

In order that the sample of soil may be easily removed from the test tube in which it is heated, it is best to have it molded into appropriate forms before being placed in the heating tube. This is easily accomplished by pressing it into molds of convenient shape and of a size so that six or eight pieces (best of cylindrical shape) will be necessary to give the quantity sufficient for the experiment. Since some soils will not retain their shape after molding, the molds may be made of zinc foil whose water values in the calorimeter are previously determined and they can be placed with their contents in the calorimeter thus securing the total immersion of all the particles of soil in the water. With very dusty materials, it is necessary that these little cylinders should be closed with pieces of foil at the ends in order to prevent the particles of dust from escaping and rising to the surface of the water.

Another source of error consists in the solution of the soluble salts which the soil may contain. This is avoided by the use of turpentine instead of water. If the cylinder containing the soil be made water-tight, this danger from the solubility of the salts in water is avoided. Another method of correcting these errors is in making a blank experiment in which a quantity of the earth taken is kept at the temperature of the water in the calorimeter until both are of the same temperature. The earth is then mixed with the water and the change of temperature produced noted. In this way the corrections made necessary by the solution of the salts in water and other causes are determined.

106109. Method of Calculating Results.—Let t represent the mean temperature of the beginning period of the experiment, and v equal the loss in heat per interval. Let t′ and v′ represent the same values for the end period. Let θ₁, θ₂, θ₃, etc., represent the temperature at the end of the first, second and third intervals of the middle period and θ₀ the temperature at the beginning of the middle period and θₙ the end temperature of of the middle period. Let τ₁, τ₂, τ₃, ... τₙ, represent the mean temperature of the single intervals; then τ₁ = Θ₀ + Θ₁
2; τ₂ = Θ₁ + Θ₂
2, and τₙ = Θ_n–1 + Θₙ
2. The constant C represents the correction which must be applied in order to determine the true increase of temperature in the calorimetric system. The expression θₙ − θ₀ + C represents the true temperature increase of the calorimetric system which we may represent by Δθ and θₙ + C represents the true maximum, that is, the end temperature, which by exclusion of external influences is reached. The correction C, as already indicated, is to be added to θₙ − θ₀ when it is positive and is to be subtracted therefrom when it is negative. The numerical value of C is usually very small, and, in the experiments indicated, varied between zero and one division of the thermometer employed, that is it seldom exceeded one degree.

110. Illustration.—The method of determining value of specific heat is best illustrated by an example:

In one determination the water value of the calorimetric system, including stirrer and thermometer was 2.50 grams, the weight of water added was 97.50 grams and the total water value of the system 100 grams. The substance was dried at 100° and weighed in five envelopes:

The envelopes holding the soil were made of brass with zinc ends, the specific heat of which is 0.0939 and the water value of the whole of the envelopes was 1.0004 grams. Since, however, the ends were soldered on with zinc the true water value was somewhat smaller being equal to 0.8692 gram. The data of the observations were as follows:

From the twenty-second interval, the regular fall of temperature begins and 211°.4 is therefore taken as θₙ. The mean temperature of the beginning period is therefore 162°.6 + 162°.9
2 = 162°.75 = t. The value of v is 162°.6 − 162°.9
10 = –0°.03. For the end period the value of t′ is 211°.4 + 210°.5
2 = 210°.95 and the value of v′ is 211.4 − 210.5
8 = + 0.11. Then the sum of the observations from eleven to twenty-one inclusive = Σ′_n–1θ = 2280.1

108Then Δθ = θₙ − θ₀ + C = 211°.4 − 162°.9 + 1°.13 = 49°.63. The true end temperature = θₙ + C = 212°.53. The zero point of the thermometer = 24°.70, and the actual rise of temperature = 187°.83. The rise of temperature due to the proximity of the warming apparatus at the beginning was found by experiment to be equal to 0°.1 of the division of the scale. On comparing the thermometer used with a standard centigrade scale it was found that one division of the calorimetric thermometer was equal to 0°.0858. Converting these numbers into expressions of the centigrade scale we have the following summary:

From these data the specific heat is calculated according to the following formula:

From this formula the following rule for calculating specific heat is deduced:

Multiply the water value of the calorimetric system by the true rise in temperature in degrees Celsius and divide the product by the difference between the temperature of boiling water under the conditions of the experiment and the true end temperature. From the quotient subtract the water value of the envelopes holding the soil sample. Divide the remainder by the weight of soil taken.

111. Variations in Specific Heat.—Different soils deport themselves very differently in respect of specific heat. In a large number of soils examined by Pfaundler, the specific heats were found to vary from 0.19 to 0.51. The highest specific heat was observed in the case of a peaty soil. Next to peaty soils came those soils which were highest in humus, and in general it was found that the specific heat varied directly with the humus content.

112. General Principles.—The measurement of the temperature of the soil at stated depths is often of use in analytical processes connected with agricultural chemistry and physics. The general principles on which the process rests, depend on bringing the bulb of the thermometer into as intimate contact as possible with the particles of soil at the depth required, disturbing as little as possible the normal state of the soil particles.

In the thermometer chiefly used for this purpose in this country, the stem is strong and carries the degrees figured on the glass. The whole is inclosed in a wooden case which is cut away to expose the face of the scale. The scale is about eleven inches long. The part which enters the soil is of varying lengths, according to the depth at which the temperature is desired.

113. Method of Procedure.—An excellent method of determining soil temperatures and of recording results is well illustrated by Frear.^[76]

The thermometers are set in niches cut in a trench, the earth being afterwards carefully tamped about the bulbs to secure a good contact, the trench being filled at the same time. The surface of the soil is freed from vegetation and kept in good tilth.

The depths at which observations are made are at the surface and one, three, six, twelve, and twenty-four inches. The soil tested was moderately dark and loamy to a depth of seven inches and below that a stiff clay. Solid rock existed at from five to seven feet below the surface. Readings were made three times a day.

114. Method of Stating Results.—The individual readings of the thermometers should be entered at the time they are made. At the end of each month the mean of the readings should be determined, together with the maxima and minima, and a comparison made between the mean readings of the temperature of the air and maxima and minima. As a sample of the method of stating these mean results the data are given for the month of May, 1891, for the atmosphere, surface, and for the depths mentioned above:

Fig. 14. Soil Thermometer—Whitney and Marvin.

115. Method of Whitney and Marvin.^[77]—The thermometer devised by Whitney and Marvin is shown in Fig. 14. The principle on which this modification depends is as follows:

A mercurial thermometer of the ordinary construction is liable to give wrong indications of the temperature because it is difficult to determine the temperature of the column of mercury from the bulb to the surface of the ground. To avoid this source of error the thermometer figured was constructed.

The bulb of the thermometer is made quite small and a slender portion of the stem extends into its spherical portion. The top portion of the thermometer stem does not differ in any essential respect from the stem of an ordinary thermometer.

The bulb is almost wholly filled with alcohol, which acts as the principal thermometric fluid and has the advantages of a high coefficient of expansion. The thermometer bulb and the stem of the thermometer up to a point convenient for graduation, are filled with mercury. In the drawing the mercury is represented by the heavy black marking in and just above the small bulb. The peculiar construction at this point is for the purpose of retaining the mercury about the point of the slender capillary stem inside the bulb and preventing the entrance of alcohol into the stem when the thermometer is horizontal.

In order to register the maximum and minimum temperatures a short column of alcohol is placed in the upper portion of the stem, above the mercury, and within this are arranged two small steel indexes, so constructed that they will not slide in the tube of their own weight, but are easily pushed upward by the mercury column or pulled downward by the top meniscus 111of the alcohol column. The indexes are set by means of a small magnet, the one being drawn down upon the top of the mercurial column and the other raised up against the meniscus of the alcohol column.

The rise of the mercury carries its index upward, leaving it to register the highest point reached, while the alcohol meniscus withdraws the other index and leaves it at a point representing the minimum temperature. It remains only to mention that the graduations are fixed in the usual way, having reference only to the positions of the mercurial column. Beyond the highest point supposed to be reached by the mercury, say about 120°, the graduations are extended in an arbitrary manner. The scale numbers represent temperatures by the mercurial column and are continued in regular sequence beyond the 120°. On this plan the readings for minimum temperatures are on a purely arbitrary scale and are converted into true degrees of temperature by use of a table prepared for each thermometer, which table embodies as well all the corrections for instrumental error.

The arrangement of the alcohol columns above the mercurial column and the indexes are shown enlarged at one side of the illustration. The readings of the maximum temperature are made from the bottom end of the index next to the mercurial column. The minimum temperature is the reading of the top of the uppermost index. Thus in the figure the maximum temperature indicated is 76.5°, and the minimum 125.7°, which, by reference to the table of correction for this thermometer, No. 10, is found to be 53.3°.

The use of mercury in the stem of the thermometer not only admits of the use of the index for registering the maximum temperature, but possesses the additional advantage of reducing the error due to uncertain temperature of the stem to about one-sixth what it would be if alcohol were used. Moreover, if necessary, as in the case with thermometers for greater depths than that figured, the ungraduated portion of the stem can be made of very much finer bore than the graduated portion, the effect of which is to diminish the objectionable error to a comparatively unimportant quantity.

The chief objection to thermometers of this construction is the 112liability of alcohol getting from the bulb into the stem during the processes of construction, graduation and subsequent handling, and the difficulty of safely shipping them.

When once set up, however, there seems to be little or no possibility of derangement and the error common to mercurial thermometers due to rise of the freezing point with age does not apply owing to the high coefficient of expansion of the alcohol used in the bulb.

116. Estimation of the Absorption of Heat by Soils.—A cubical zinc box, six centimeters square, is filled with the sifted air dried soil. The box, one side of which is left open, is encased snugly in a wooden cover, exposing only the open end, and placed for a few hours in the direct rays of the sun. The temperature is then taken at a given depth. The box may be provided with thermometers at different depths, the bulbs thereof extending to the center. In this case the box should be covered with thick felt instead of wood. The temperature of the layers of soils of different depths can thus be read off directly. The air temperature directly above the box should be accurately noted while the experiment continues.

Any other kind of box well protected against all heat save the direct sunlight on the open surface of the soil will answer as well as the one described.

To determine the action of moist earth in similar conditions the soil may be previously moistened; the per cent of moisture being determined in a separate portion of the soil or the amount of water added to the air-dried soil being noted.

117. Estimation of the Conductivity of Soils for Heat.—The bulb of a thermometer is placed in the middle of a mass of fine earth which is then exposed, best in a metallic box painted with lamp black, in a warm place. The time required for the thermometer to reach a certain degree is noted. By reversing the experiment and placing the mass of earth heated to a given degree in a cool place the conductivity can be determined by 113the time required for the mercury in the thermometer to fall to any given point.

The experiment may also be made by packing the soil by gently jolting it in a glass tube six to eight centimeters in diameter. One end of the tube is closed with a piece of metal or fine wire gauze painted with lamp black and is exposed to the source of heat. The bulb of a thermometer is placed at a given distance from the end of the tube and the time for the mercury to be affected observed.

118. Behavior of Soil After Wetting.—The deportment of a soil when thoroughly wet in respect of its physical state on drying out is a matter of great practical concern to the agronomist. Some soils on becoming dry fall into a pulverulent state and are easily brought into proper tilth; others become hard and tenacious, breaking into clods and resisting ordinary methods of pulverization. The physical laws which determine these conditions depend largely on the principles of flocculation soon to be described. The present task is to describe briefly some of the methods of estimating the force of cohesion and adhesion.

119. General Method.—The fine earth, air-dried, is mixed with enough water to make a paste and molded into forms suitable for trial in a machine for testing strength of cement, etc. The forms most used are cakes three to five centimeters in length and one to two centimeters thick. These are used for determining crushing power. For longitudinal adhesion the paste may be molded in prismatic or cylindrical shape.^[78] The prisms should show one to two centimeters in cross section or the cylinder be one to two centimeters in diameter. Before use they are to be exposed for several days until thoroughly air-dried. The force required to separate or crush these prepared pieces will measure the adhesive or cohesive property of the sample. A great number of trials should be made and the mean taken.

120. Method of Heinrich.^[79]—This process consists in mixing the air-dried earth with water until its aqueous content is fifty per 114cent of the highest water capacity determined by experiment. The sample is next placed between two pieces of sheet iron of ten centimeters square, each of which in its middle point is provided with a hook. The thickness of the layer between the two pieces of iron should be about five to ten centimeters. The exuding particles of soil are cut off with a knife. The upper piece of sheet iron is next suspended by a cord in such a way that the iron piece occupies a horizontal position. A small basket is attached to the lower surface and sand added thereto, little by little, until the column of earth is separated. The sand basket and iron plate are weighed, and the total weight gives the power necessary to separate a column of soil ten centimeters square in cross section. The iron plates may be roughened so that the adhesion thereto of the soil is greater than its cohesive force.

121. Adhesion of Soil to Wood, Iron, Etc.—The adhesive power of moist soil for wood, iron, etc., is measured by Heinrich^[80] in the following way: The soil is mixed with water, as above, until it contains just fifty per cent of its total water-holding content. It is then placed in a large vessel and the upper surface made as smooth as possible. A plate of wood, iron, etc., of ten centimeters square is then pressed on the surface until a complete contact is secured. This plate, by means of a hook and cord passing over a pulley, is then subjected to stress by weighting the cord which carries a basket for that purpose. The basket should be of the same weight as the plate in contact with the soil. The weight added to the basket necessary to separate the plate from the soil is taken to represent the cohesive force. The author of the method appears to take no account of the pressure of the air on the plate caused by the exclusion of the air from its under surface.

122. General Principles.^[81]—It is a fact of every-day observation that soils have a particular property of absorbing certain materials with which they come in contact. If it were not for this property all our wells would soon become unwholesome 115from the reception of decayed animal and vegetable matter carried to them in the drainage water from the surface. It is also a well-known fact that burying dead bodies prevents the gaseous products of decomposition from reaching and vitiating the atmosphere.

Besides this well-known power of soils to absorb the decomposition products of animal and vegetable matter, they also possess a property which is of far greater importance in plant economy; that is, the power of withdrawing and retaining certain mineral constituents from their solutions.

As far back as the sixteenth century mention is made by Lord Bacon of a process for obtaining pure water on the seashore by simply digging a hole in the sand and allowing it to fill with filtered sea water, which by this means is deprived of its salt. Although certain facts were observed by some of the earlier writers in regard to soil absorption, no systematic researches were conducted with a view of demonstrating the extent and cause of this power until within a comparatively few years.

In 1850 Prof. Way published in the Journal of the Royal Agricultural Society of England, the results of a thorough and most excellent investigation of the subject. Since then many distinguished chemists, such as Henneberg, Stohmann, Peters, Heiden, Knop, Ullik, Pillitz, Biedermann, Tuxen, and others have given their attention to this matter.

123. Summary of Data.—If a solution of a soluble sulfate, chloride or nitrate of an alkali or an alkaline-earth metal be placed in contact with a soil, the result is that the soil takes up a part of the base but none of the acid. This absorption of base is attended with the liberation of some other base from the soil which combines with the acid of the solution. Any alkali or alkaline earth base has the power of replacing any other such base. However, if soluble phosphates and silicates of these bases be placed in contact with the soil both the base and the acid are removed from the solution.

Peters^[82] has shown that the amount of absorption depends upon the concentration of the solution, the relation between the quantity of solution and the soil and the kind of salt used. He 116treated 100 grams of earth with 250 cubic centimeters of solutions of different potash salts with the following results:

Biedermann^[83] proves that, for phosphoric acid at least, the absorption increases with the temperature.

It has also been found that the amount of absorption depends upon the time of contact between the soil and solution. Way found that the absorption of ammonia was complete in half an hour, while Henneberg and Stohmann^[84] noticed that the phosphoric acid continued to be fixed after the expiration of twenty-four hours.

It is a very important fact that the absorption of a base is never complete; no matter how dilute the solution it will still carry a small portion of the base with it. Peters states that it requires about 28,000 parts of water to remove one part of absorbed potash and Stohmann found that it required about 10,000 parts of water to remove one part of absorbed ammonia. With phosphoric acid, the resulting compound seems to be much more insoluble.

According to Tuxen^[85] the presence of salts of soda and potash in solution decreases the power of a soil to absorb ammonia compounds and the presence of sodium salts decreases the power of a soil to absorb potash. On the other hand the presence of potassium compounds considerably increases the absorption of phosphoric acid. He further affirms that the compounds of potash, phosphoric acid, etc., formed in the soil, are decidedly more soluble in sodium salts than in pure water.

124. Cause of Absorption.—The withdrawing and fixing of phosphoric acid from solutions by the soil is not very difficult to understand as this acid forms insoluble compounds of iron, lime, and magnesium, some or all of which are present in all soils. As to the absorption of the alkalies, the explanation is far more 117difficult as nearly all of their ordinary compounds are readily soluble in water.

As lime is usually found combined with the acid part of an alkali salt, from which the base has been absorbed by the soil, it might naturally be supposed that the absorptive power of the soil would depend upon the amount of lime present. Way found, however, that the addition of chalk in no way influenced the absorption of ammonia by a soil which contained but a small amount of lime. This fact was also confirmed by Knop^[86] who found that chalk exerted no influence on the absorption of ammonia salts. These facts would seem to point to the conclusion that lime was present in sufficient quantity in these experiments, or that it is not essential to the phenomena of absorption. However, as any alkali or alkaline-earth base can replace any other such base, the presence of lime in the filtrate is probably more of an accidental occurrence, owing to the comparatively large amount of that substance in most soils, than a necessary condition, as any other base would doubtless answer in the absence of lime.

125. Warington^[87] has shown that hydrated oxides of iron and aluminum, and especially the former, are capable of absorbing potash and ammonia, and as more or less of these hydrates exist in nearly all soils, a part, at least, of absorptive phenomena is to be ascribed to them.

126. Way tried to determine which of the constituents of a soil exercised chiefly the absorptive power. He passed a solution of ammonia through tubes containing pure sand and found that it came through apparently unaltered from the first, while a soil treated in the same way removed the ammonia for a considerable time. He concluded from this that the absorptive power does not exist in the sand. He next oxidized the organic matter in a soil with nitric acid and then treated it with ammonia in the same way. The first portions of the filtrate showed no ammonia in any form, hence he concluded that organic matter is not essential to the act of absorption. He further showed that clay alone is capable of causing absorption phenomena, by treating powdered clay tobacco pipes with ammonia.

118Having shown that clay was the main constituent in a soil which caused the absorption of alkalies, he tried next to trace out the particular compound which caused the absorption. Having tried various natural silicates he at last succeeded in producing a hydrated silicate of aluminum and soda which exhibited displacement and absorptive properties very similar to those shown by the soil.

As Way had succeeded in producing an artificial hydrated silicate possessing absorptive properties, Eichorn^[88] thought of trying natural hydrated silicates or zeolites and found that they exhibited the same power as Way’s artificial preparation. It has also been shown by Biedermann,^[89] Rautlenberg,^[90] and Heiden^[91] that the absorptive power bears a close relation to the amount of soluble silicates present.

In view of these facts it is now generally accepted that the absorption of salts of the alkalies, accompanied by the change of base, is due chiefly to the presence of decomposed zeolite minerals in the soil.

Besides the purely chemical absorption of salts by the soil, we have a physical absorption of various substances similar to the action of charcoal when used as a filter.

127. Conclusions of Armsby.—The data connected with the absorption of bases by a soil have also been reviewed by Armsby.^[92] He shows that the absorption is accompanied by a chemical reaction between the salt whose base is absorbed and some constituent of the soil, and this change seems to be due particularly to certain zeolitic silicates, although Liebig and others were disposed to credit this absorption largely to physical causes.

Knop advances the idea that the soil has the power of disintegrating salts in the presence of some substances like calcium carbonate which can unite with the acid. In experiments made with hydrous silicates it was shown that the absorption resembled in all cases like phenomena in the soil; hence the supposition already advanced in regard to the influence of such silicates is doubtless true.

In respect of absorption in general, the following conclusions were reached:

1191. The absorption of combined bases by the soil consists in an exchange of bases between the salt and the hydrous silicates of the soil.

2. This exchange, which is primarily chemical, is only partial, its extent varying

(a) with the concentration of the solution, and

(b) with the ratio between the volume of the solution and the quality of soil used.

3. The cause of these variations is probably the action of mass or the tendency of resulting compounds to re-form the original bodies, the absorption actually found in any case marking the point where the two forces are in equilibrium.

128. Selective Absorption of Potash.—As a rule more potash is absorbed from the sulfate than from the chlorid. This fact would seem to point to the advisability of using sulfate as a fertilizer in preference to chlorid. However, as with the exception of nitrates, the absorptive power of a soil, for the salts used as fertilizers, is many times greater than it is ever called upon to exert in fixing applied fertilizers, we need not trouble ourselves in regard to the absorption of phosphoric acid, potash or ammonia, in so far as the practical side of the matter is concerned. For example, an acre of soil to the depth of nine inches weighs about 900 tons. Now it has been found by Huston,^[93] that 100 parts of a soil experimented upon absorbed over 0.25 part of P₂O₅, hence 900 parts would absorb over 2.25 parts of P₂O₅; or an acre of this soil to the depth of nine inches would absorb over two and one-fourth tons of phosphoric acid. 500 pounds per acre is a large dressing of a phosphatic fertilizer for field crops and 500 pounds of a high grade fertilizer would contain about 100 pounds of P₂O₅; hence the power of such a soil to absorb phosphoric acid is more than forty-five times as great as it is ever likely to be called upon to exert in fixing the phosphoric acid added to it as a fertilizer.

Huston has further shown that an acre of soil nine inches deep will absorb more than 2.7 tons of potash (K₂O) from potassium chlorid from which salt less potash is absorbed than from the sulfate. Now one-tenth ton of potassium chlorid 120per acre would be a large dressing of potash, hence this soil possesses the power of absorbing more than twenty-seven times as much potash as is ever likely to be applied as a fertilizer.

In like manner it may be shown that the power of an acre of soil nine inches deep to absorb ammonia from ammonium sulfate is more than thirty-two times as great as it would be called upon to exert in fixing the ammonia from a dressing of one-quarter ton of ammonium sulfate per acre.

With sodium nitrate, however, there is no absorption; hence great care is necessary in the application of nitrogen as a nitrate, for, if it be put on in large quantities, at a season when the plant is not prepared to assimilate it, or during a period of heavy rains, there must unavoidably result loss from drainage. The best time to apply a nitrate is evidently during the active growing season.

129. Whitney^[94] places great emphasis on the surface area of soil particles in respect to their power to absorb solutions of salts. The approximate surface area of a cubic foot of each of the different typical soils of Maryland is as follows:

It will be seen that there are about 24,000 square feet of surface area in a cubic foot of the subsoil of the pine barrens, no less then 100,000 square feet or two and three-tenths acres of surface area in a cubic foot of the subsoil of the river terrace, and 200,000 square feet of surface area in a cubic foot of the limestone subsoil.

These figures seem vast, but they are probably below rather than above the true values, on account of the wide range of the diameters of the clay group. This great extent of surface and of surface attraction, which has been described as potential, gives the soil great power to absorb moisture from the air, and to absorb and hold back mineral matters from solution. A smooth surface of glass will attract and hold, by this surface attraction, 121an appreciable amount of moisture from the surrounding air. A cubic foot of soil, having 100,000 square feet of surface, should be able to attract and hold a considerably larger amount of moisture.

It might have been added that if the potential of the surface, separating the solution from the soil, be greater than the potential in the interior of the liquid mass, there will be a tendency to concentrate the liquid on this surface of separation. It has been shown that certain fluids have greater density on a surface separating the fluid from a solid. On the other hand, if the potential were low there might be no tendency for this concentration, and even the reverse conditions would prevail and the soluble substance could be readily washed out of the soil.

130. Removal of Organic Matters.—It is probably largely due to this straining power that organic matters are removed from solutions in percolating through the soil. Whitney^[95] has observed that the organic matter may be coagulated and precipitated from solution by the soil constituents, and held in the soil in loose flocculent masses, while the liquid passes through nearly free of organic matter.

131. Importance of Soil Absorption.—The importance of the absorptive power of the soil can hardly be overestimated. By means of this power those mineral ingredients of plant food, of which most soils contain but little, are held too closely to allow of rapid loss by drainage, and still sufficiently available to answer the needs of vegetation, provided the store is large enough. The only important plant food liable to be deficient in the soil which does not come under the influence of absorption is nitrogen in the form of salts of nitric acid, and nature has made a wide provision for this element by binding it in the form of organic bodies which nitrify but slowly, and by supplying each year a small quantity from the atmosphere.

By means of the absorptive power of soils the farmer, if he puts on an excess of potash or phosphoric acid as a fertilizer, does not lose it but is able to reap some benefits from it in the next and even in succeeding crops. If it were not for this power the best method for applying fertilizers would be a much more 122complicated problem than it is at present; and it would be necessary to apply them at just the proper season and in nicely regulated amounts to insure against loss.

132. Method of Determining Absorption of Chemical Salts.—The soil which is to be used for this experiment should be treated as has been indicated and passed through a sieve the meshes of which do not exceed half a millimeter in size. From twenty-five to fifty grams of the fine earth may be used for each experiment.

The fine earth should be placed in a flask with 100 to 200 cubic centimeters of the one-tenth to one-hundredth normal solution of the substance to be absorbed. The flask should be well shaken and allowed to stand with frequent shaking twenty-four to forty-eight hours at ordinary temperatures. The whole is then to be thrown upon a folded filter and an aliquot part of the filtrate taken for the estimation. The methods of determining the quantities of the substances used will be found in other parts of this manual. It is recommended to conduct a blank experiment with water under the same conditions in order to determine the amount of the material under consideration abstracted from the soil by the water alone. The difference in the strength of the solution as filtered from the soil, corrected by the amount indicated by the blank experiment, and the original solution will give the absorptive power of the soil for the particular substance under consideration.

If it should be desired to determine the absorptive power of the soil for all the ordinary chemical fertilizing materials at the same time, a larger quantity of the sample should be taken corresponding to the increased amount of the standard solutions used. About 500 cubic centimeters of the mixed salt solution should be shaken with 125 grams of the earth and the process carried on in general as indicated above. The absorption coefficient of an earth for any given salt according to Fesca,^[96] is the quantity of the absorbed material expressed in milligrams calculated to a unit of 100 grams of the soil.

133. Method of Pillitz and Zalomanoff.—It is recommended by Pillitz and Zalomanoff^[25] to reject the old method, viz., shaking 123the soil with the solution in a flask, and substitute the filtration method both because it gives a more natural process and because the results are more constant. The apparatus is shown in Fig. 15.

Figure 15.

Zalomanoff’s Apparatus for Determining Absorption of Salts by Soils.

Two cylinders are placed vertically, one over the other. The lower cylinder is graduated in cubic centimeters, the upper cylinder is closed at each end by perforated rubber stoppers A and B through the openings of which the glass tubes c and d pass. Within the cylinder A the opening of the small tube d is closed with a disk of Swedish filter paper. The lower part of the small tube is d connected by means of a rubber tube carrying a pinch-cock C, with another small tube e which passes through the stopper f. In carrying out the process the weighed quantity of soil is placed in the upper cylinder and afterwards the measured quantity of the solution, the whole thoroughly mixed and the cylinder closed. The valve C is then opened, a given quantity of the solution, but not all, is made to drop into the lower cylinder and the valve C is then closed. The liquid which has passed into the lower cylinder as well as that which remains in the upper cylinder, is thoroughly stirred and the quantity of the material remaining in both liquids determined and the absorbing power of the soil estimated from their difference. It does not appear that this method of estimation of the absorption power possesses any special advantages over the old and far simpler method of shaking in a flask.

Figure 16.

Müller’s Apparatus to Show Absorption of Salts by Soils.

134. Method of Müller.—The method of Müller^[97] for illustrating absorption is carried out by means of the apparatus shown in Fig. 16. A glass cylinder A about 750 centimeters long and four to five centimeters wide is closed at each end with rubber stoppers with a single perforation. The cylinder A is for the reception of the soil with which the experiment is to be made. Before using, the lower part of it is filled with glass pearls or broken glass and above this a layer of glass wool is placed about one centimeter thick. The object of this is to prevent the soil from passing into the small tube below. As soon as the soil has all been placed in the cylinder A the upper part of the tube is also filled with glass wool. The cylinder A is connected with the pressure bottle B by means of a rubber tube and the small glass bulb tube shown in the figure. The bottle B should have a content of about two liters. It is filled with the standard solution of the material of which the absorption coefficient is to be determined. At c the rubber tube is connected with a glass T one arm of which is provided with a piece of rubber tubing which can be closed by means of a pinch-cock. At c a screw pinch-cock is placed which can be used to regulate the flow of the solution from B to A. By opening the pinch-cock at e on the short arm of the T piece, a sample of the original liquid can be taken and this can be compared with the part which runs to b. If it is desired for instance, to show that potassium carbonate has been absorbed by the soil the two bulbs shown on the small glass tubes connecting with A can be filled with red litmus paper. This paper will at once be turned blue in the lower bulb while in the upper one it will retain its original color because the liquid in passing through the soil will have lost its alkaline reaction. The solutions used should be very dilute. The apparatus 124is designed for lecture experiments and not for quantitative determinations.

135. Method of Knop.—For rapid determination of the absorption coefficient of the soil Knop’s method may be used.^[98]

The fine earth which is employed is that which passes a sieve with meshes of half a millimeter. From 50 to 100 grams of this soil are mixed with from five to ten grams of powdered chalk and with about twice the weight of ammonium chlorid solution of known strength, viz., from 100 to 200 cubic centimeters. The ammonia solution should be of such a concentration that the ammonia by its decomposition for each cubic centimeter of the liquid evolves exactly one cubic centimeter of nitrogen. This solution is prepared by dissolving in 208 cubic centimeters of water one gram of ammonium chlorid. With frequent shaking the solution is allowed to stand in contact with the soil for forty-eight hours. The whole is now allowed to settle and the supernatant clear liquid is poured through a dry filter. From the filtrate twenty to forty cubic centimeters are removed by a pipette, and evaporated to dryness in a small porcelain dish, with the addition of a drop of pure hydrochloric acid. The ammonium chlorid remaining in the porcelain dish is washed with ten cubic centimeters of water into one of the compartments of the evolution flask of the Knop-Wagner azotometer. It is decomposed with fifty cubic centimeters of bromin lye and the nitrogen estimated volumetrically. The difference between the amount of nitrogen in this material and that of the original material will give the amount of absorption exercised by the fine earth. This number, without any further calculation, can be taken as the coefficient of absorption.

136. Method of Huston.—The salt solutions recommended by Huston^[99] are sodium phosphate (Na₂HPO₄), potassium chlorid, potassium sulfate, ammonium sulfate and sodium nitrate.

The solutions should be approximately tenth normal, the actual strength in each case being determined by analysis. The phosphorus is determined as magnesium pyrophosphate in the usual way, the potash as potassium platinochlorid, the ammonia by collecting the distillate from soda in half normal hydrochloric 125acid and titrating with standard alkali, and the nitrate by Warington’s modification of Schlösing’s method for gas analysis. The details of these methods of determination will be given later. One hundred grams of the sifted, air-dried soil are placed in a rubber stopped bottle and treated with 250 cubic centimeters of the solution to be tested. The digestion is continued for forty-eight hours in each case, the bottles being thoroughly shaken at the end of twenty-four hours. At the end of the treatment the solutions are filtered and the salts determined in aliquot portions. The details of this method are essentially those already described.

137. Statement of Results.—Duplicate analyses should be made and the tabulation of the data is illustrated in the following analyses by Huston:

Upon comparing the figures it will be found that the absorption, passing from the greatest to the least, is as follows: phosphoric acid (P₂O₆), potassium sulfate, ammonium sulfate, potassium chlorid and sodium nitrate.

It will be seen that there was no absorption in the case of the nitrate, while with each of the other salts there was quite a marked absorption. It will also be noticed that the percentages of absorption are not very different, and especially is this true of the potassium and ammonium salts, the P₂O₅ being somewhat higher. Whether this fact is merely an accidental occurrence or is due to the law of combination by equivalents could hardly be predicted from the single soil experimented upon; but taking into consideration the possibility of difference in solubility of the resulting compounds in the saline solutions used, and of other varying conditions, the percentages are evidently not far enough apart to exclude the possibility of the bases uniting in equivalent proportions.

138. Preparation of Salts for Absorption.—The salts employed in the foregoing determinations are conveniently prepared, in fractional normal strength.

In grams per liter the following quantities in grams are recommended, viz., 5.35 g NH₄Cl; 10.11 g KNO₃; 16.40 g Ca(NO₃)₂; 24.60 g MgSO₄ + 7H₂O; 23.4 g CaH₄(PO₄)₂, etc.

The ammonium chlorid, potassium nitrate and magnesium sulfate can be weighed as chemically pure salts and the standard solution be directly made up. Calcium nitrate is so hygroscopic that a stronger solution must be made up, the calcium determined and the proper volume taken and diluted to one liter.

Monocalcium phosphate is prepared as follows:

127A solution of sodium phosphate is treated with glacial acetic acid and precipitated with a solution of calcium chlorid. It is then washed with water until all chlorin is removed. The fresh precipitate is saturated with pure, cold phosphoric acid of known strength. After filtering the solution is placed in a warm room and left for two or three weeks until crystallization takes place.

The crystals are pressed between blotting papers and finally dried over sulfuric acid and washed with water-free ether, and again dried. Since this salt is decomposed in strong solutions it should be used only in one hundredth normal strength, viz., 2.34 grams per liter.

139. Porosity.—The porosity of a soil depends upon the state of divisibility and arrangement of its particles, and upon the amount of interstitial space within the soil. If a soil be cemented together into a homogeneous mass, its porosity sinks to a minimum; if it be composed, however, of numerous fine particles, each preserving its own physical condition, the porosity of the soil will rise to a maximum. The porosity of a soil may be judged very closely by the percentage of fine particles it yields by the process of silt analysis to be described further on. In general, the more finely divided the particles of a soil, the greater its fertility. This arises from various causes; in the first place, such a soil has a high capacity for absorbing moisture and holding it; thus the dangers of excessive rain-falls are diminished, and the evil effects of prolonged drought mitigated. In the second place, a porous soil permits a freer circulation of the gases found in the soil. The influence of lime in securing the proper degree of porosity of a soil is very great, especially in alluvial deposits and other stiff soils. It prevents the impaction which will necessarily follow in a soil which is too finely divided. In general, the porosity of the soil may be said to depend on three factors, viz.: 1. Upon the state of divisibility or the number of particles per unit volume; 2. Upon the nature and arrangement 128of these particles; 3. Upon how much interstitial space there is in the soil.

140. Influence of Drainage.—Good underdrainage increases the porosity of a soil by removing the excess of water during wet seasons and rendering the soil more suitable to capillary attraction which will supply moisture during dry seasons. The influence of tile drainage on the production of floods has been carefully studied by Kedzie,^[100] who shows that surface ditching in conjunction with deforesting may increase floods and contribute to droughts, and that tile-draining may increase flood at the break-up in spring, when the water accumulated in the surface soil by the joint action of frost and soil capillarity during the winter, and the surface accumulations in the form of snow are suddenly set free by a rapid thaw.

He also points out that during the warm months tile-draining tends to prevent flood by enabling the soil to take up the excessive rain-fall and hold it in capillary form, keeping back the sudden flow that would pass over the surface of the soil if not absorbed by it, and it mitigates summer drought by increased capacity of the soil to hold water in capillary form and to draw upon the subsoil water supply.

141. Soil Moisture.—The capacity of a soil to absorb moisture and retain it depends on its porosity and is an important characteristic in relation to its agricultural value.

The following general principles relating to soil moisture are adapted from Stockbridge:^[101]

During dry weather plants require a soil which is absorptive and retentive of atmospheric moisture. The amount of this retention is generally in direct ratio to two factors, viz., the amount of organic matter and its state of division. The capillary water of the soil is very closely related to its percolating power, since all waters in the soil are governed in their movements by what is known as capillary force. Liebenberg has shown that this movement may be either upwards or downwards, according as the atmosphere is dry or supplies soil-saturating rain. The water absorbed by the roots passes into the plant circulation, and the greater part is evaporated from the leaves. Where the supply of 129water is insufficient, the plant wilts, and if the evaporation long continue in excess of the supply obtained from the soil, the plant must die. The experiments of Hellriegel have shown that any soil can supply plants with all the water they need, and as fast as they need it, so long as the moisture within the soil is not reduced below one-third of the whole amount that it can hold. The quantity of water required and evaporated by different agricultural plants during the period of growth has been found to be as follows:

Dietrich estimates the amount of water exhaled by the foliage of plants to be from 250 to 400 times the weight of dry organic matter formed during the same time. Cultivation conserves soil moisture. It must be remembered that this water contains soil ingredients in solution. Hoffmann has estimated that the quantity of matter dissolved from the soil by water varies from 0.242 to 0.0205 per cent of the dried earth. The experiments of Humphrey and Abbott have shown that about one-sixth of the total sediment of the Mississippi river is soluble in water.

142. Determination of the Porosity of the Soil.—The porosity of the soil is fixed by the relative volume of the solid particles as compared with the interstitial space. It is most easily determined by dividing the apparent by the real specific gravity.

Let the real specific gravity of a soil be 2.5445 and the apparent specific gravity of the same soil be 1.0990.

The porosity is then calculated according to the following ratios, viz.:

Whence X = 43.2 = per cent volume occupied by the solid particles of the soil.

The per cent volume occupied by the interstitial space is therefore 56.8.

130143. Method of Whitney.—The total volume of interstitial space within the soil, in which water and air can enter, is best determined by calculation from the specific gravity and the weight of a known volume of soil. To determine this in the soil in its natural position in the field, a sample is taken in the following way: A brass tube, about two inches in diameter and nine inches long, has a clock spring securely soldered into one end, and this end turned off in a lathe to give a good cutting edge of steel. The area enclosed by this steel edge is accurately determined, and a mark is placed on the side of the tube exactly six inches from the cutting edge. A steel cap fits on top of the brass cylinder to receive the blows of a heavy hammer or wooden mallet. The cylinder is driven into the ground until the six-inch mark is just level with the surface. The whole is then dug out, care being taken to slip a broad piece of steel under the cylinder before it is removed, so as to prevent the soil which it contains from falling out. The cylinder is then carefully laid over on its side, and the soil is cut off flush with the cutting edge of steel. The soil is then removed from the cylinder, carried to the laboratory and properly dried and weighed. The object of the steel inserted in one end of the cylinder is to reduce the friction on the inside of the tube to a minimum, and thus prevent the soil inside the cylinder being forced down below the level of the surrounding earth. The volume of the soil removed with this sampler can readily be determined by calculation, as the area of the end of the tube is known and the sample is six inches deep. In a sampler, such as described here, this volume is about 300 cubic centimeters. From the weight of soil and the volume of the sample, the volume of interstitial space may be found by the following formula:

S is the per cent by volume of interstitial space, V is the volume of the tube in cubic centimeters, W is the weight of soil in grams, and ω is the specific gravity of the soil. The specific gravity can be determined for each soil, or the factor 2.65 can be used, which is sufficiently accurate for most work.

131The per cent by volume of interstitial space in the undisturbed subsoil is found to range from about thirty-five for sandy land, to sixty-five or seventy for stiff clay lands.

For the determination of the amount of water an air-dried soil will hold, if all the space within it is completely filled with water, an eight-inch straight argand lamp chimney, with a diameter of about two inches, can be conveniently used. A mark is placed on the side of the tube, six inches from one end, and the volume of the tube up to this mark is found by covering the end with a piece of thin rubber cloth, or by pressing the chimney down firmly on a glass plate, and making a water-tight joint with paraffin or wax. Water is then poured into the tube up to the six-inch mark, and the weight or volume of water determined. The tube can then be dried, a piece of muslin tied tightly over the top and the whole then weighed. Soil is carefully poured in and the tube gently tapped on a soft support until the soil is six inches deep in the tube, and has the desired degree of compactness. The weight and volume of the soil can thus be determined, and the volume of the interstitial space from the formula already given. This can also be determined directly by introducing water from above, or by immersing the cylinder of soil up to the six-inch mark in water, and allowing the water to enter the soil from below. With such a short depth of soil, very little water will flow out when the cylinder is suspended in the air. The amount which will flow out when the cylinder is thus suspended, will depend both upon the texture and the depth of soil. It is impossible, however, by this method, to completely remove the air or to completely fill the space within the soil with water; for as the water enters the soil, a considerable amount of air becomes entangled in the capillary spaces, and this could not be removed except by boiling and vigorous stirring, which would altogether change the texture of the soil. The amount of water held by the soil, or the amount of space within the soil into which water and air can enter, will evidently depend upon the compactness of the soil, and this is best expressed in per cent by volume of space.

144. Capacity of the Fine Soil for Holding Moisture.—The 132soil, as it is taken from the field, may have quite a different water coefficient from the same soil after it has been passed through a fine sieve or been dried at air temperatures or at 100° or 110°. The method of determination which depends upon adding excess of water to a given weight of fine earth, and afterwards eliminating the excess by percolation or filtration, is apt to give misleading results. If, however, the results are obtained by working on the same weight of soil, and in the same conditions, they may have value in a comparative way. The comparison between soils must be made with equal weights, in like apparatus and with the same manipulation, to have any value. These determinations, however, cannot have the same practical value as those made in the samples in a natural condition as has just been described.

145. Method of Wolff Modified by Wahnschaffe.^[102]—A cylindrical zinc tube (Fig. 17), sixteen centimeters long and four centimeters internal diameter, is used, the cubical capacity of which is 200 cubic centimeters.

The cylinder is graduated by placing the moist linen disk on the gauze and tying a piece of rubber cloth over the bottom. Water is now poured in until the level is even with the gauze bottom. Add then exactly 200 cubic centimeters of water, mark its surface on the zinc, throw out the water, and file the zinc down to the mark.

The bottom of the tube is closed with a fine nickel-wire gauze. Below this a piece of zinc tubing, of the size of the main tube, is soldered; pierced laterally with a number of holes.

Before using, the gauze bottom of the cylinder is covered with a moist, close fitting linen disk, and the whole apparatus weighed. It is then filled with the fine earth, little by little, jolting the cylinder on a soft substance after each addition of soil to secure an even filling. When filled even full the whole is weighed, the increase in weight giving the weight of soil taken.

Figure 17.

Capacity of the Fine Soil for Holding Moisture. Method of Wolff Modified by Wahnschaffe.

133A large number of cylinders can be filled at once and placed in a large glass crystallizing dish containing water and covered with a bell jar (Fig. 17). The water should cover the gauze bottoms of the cylinders to the depth of five to ten millimeters. More water should be added from time to time as absorption takes place. The cylinders should be left in the water until when weighed at intervals of an hour no appreciable increase in weight takes place. The temperature and barometer reading should be noted in connection with each determination. With increasing temperature the water coefficient is diminished.

The method of Wolff, as practiced in the laboratory of the Chemical Division of the U. S. Department of Agriculture, has given very concordant results. Five determinations were made on a sample of vegetable soil with the Wolff cylinders, which were weighed at intervals of ten, twenty, and thirty days, with the following results:

The data obtained show that there was a very slight increase in the amount of moisture absorbed after the tenth day.

As will be seen, however, from the following data, the soil within the cylinder does not contain in all parts the same percentage of moisture, the lower portions of the cylinder containing notably larger proportions than the upper parts. The cylindrical soil column was divided into four equal parts and the moisture determined in each part. Beginning with the top quarter the percentages of moisture were as follows:

146. Method of Petermann.^[103]—The method of Wolff as practiced by the Belgian Experiment Station, at Gembloux, is essentially the same as described above.

Petermann recommends the use of tared cylinders twenty to twenty-five centimeters long and six to eight centimeters in diameter. The cylinder is to be filled with the fine earth, little by little, with gentle tapping after each addition. The bottom of the cylinder is closed with a perforated rubber stopper on which is spread a moistened disk of linen. The cylinder, thus prepared and filled, is weighed and afterwards placed in a vessel containing distilled water, to such a depth as to secure a water level about two centimeters above the lower surface of the soil in the cylinder. The level of the water is kept constant as the contents of the cylinder are moistened by capillarity. When the earth appears to be thoroughly moistened, as can be told by the appearance of the upper surface, maintain the contact with water for about five or six hours.

The cylinder is then removed, the upper surface covered to avoid evaporation, allowed to drain for a few hours, wiped and weighed. The cylinder is again placed in water to see if any increase in weight takes place. The weight of the fine earth and of the absorbed water being known, the percentage of absorption is easily calculated.

147. Method of A. Mayer.^[104]—A glass tube, one and seven-tenths centimeters in diameter, composed of two pieces, seventy-five centimeters and twenty-five centimeters in length, is united by a piece of rubber tubing. The lower free end of the seventy-five centimeter piece is closed with a piece of linen. The tube is filled, with gentle jolting, to the depth of one meter with fine earth, the earth column thus extending twenty-five centimeters above the point of union of the two pieces. Thus prepared, a quantity of water is poured into the upper tube sufficient to temporarily saturate the whole of the soil.

During the sinking of the water in the tube there is thus 135effected a moistening of the material before it is wholly filled with water. After waiting until the water poured on top has disappeared the tube is separated at the rubber tube connection and a sample of the moist soil taken at that point. This is at once weighed and then dried at 100°. The loss in weight gives the water absorbed.

The number thus obtained is calculated to the standard by volume, by use of the number representing the apparent specific gravity of the fine earth.

For sand of different degrees of fineness the following numbers were found:

The numbers thus obtained are taken to represent the absolute water capacity of a mineral substance in powder.

The full water capacity, i. e., the power of holding water when the powder is immersed in water, the excess of which is then allowed to flow away is much greater than the absolute number.

This difference is shown in the following data:

In general the absolute is markedly inferior to the full water capacity. Only in the finest dust do the two numbers approach each other.

148. Volumetric Determination.—A convenient apparatus for this determination has been devised by Mr. J. L. Fuelling, of the Chemical Division, Department of Agriculture. It is shown in Fig. 18.

It consists of an ordinary percolator the diameter of which decreases slightly towards the lower end, a thick-wall rubber tube and an ordinary burette, divided in tenths. A rubber stopper is fitted to the mouth of the percolator and perforated twice—in the middle and at the side, the former for a small tube provided with pinch-cock and the latter for the neck of a small funnel. The whole is supported on a convenient stand, the clamp holding the percolator being placed above that supporting the burette, both clamps arranged to slide on the stand-rod.

Figure 18.

Fuelling’s Apparatus.

The method is as follows:

A mark is placed upon the projecting tube at the lower end of the percolator, and the tube at this point may be drawn out sufficiently to decrease the width of meniscus to one-eighth inch. Into the percolator is first introduced a small disk of wire gauze or perforated porcelain, with heavy wire pendant in the tube. Through the rubber stopper a small glass tube is passed and its lower end pressed firmly upon the wire or porcelain disk, its upper end being curved and supplied with a pinch-cock. Into the percolator is now poured one inch of fine shot (No. 20) and then one inch of fine sand which has been previously digested with hydrochloric acid and well cleaned of dust by washing.

The zero.—After the shot and sand have been shaken even, the burette is filled with water and raised above the level of the sand, wetting the percolator for four inches of its length. The burette is lowered and the shot and sand bed allowed to drain by opening the pinch-cock of the inner tube. The burette is raised and the shot-sand flooded repeatedly until, by lowering the burette until the zero mark of the percolator tube is reached, a uniform reading on the 136burette is secured. Thus the shot-sand bed is completely charged with water. The water level is now made zero on the percolator stem, the burette filled to its zero mark and the apparatus is prepared for introduction of the soil.

The Determination.—From 100 to 200 grams of soil, previously dried free of moisture, are weighed, the burette raised until the water level is three inches above the sand, and the soil gently dropped through a funnel into the water. When the soil has been introduced and wetted completely the water level is raised above the soil and allowed to remain thus two hours. The burette is then lowered and the water allowed to drain from the wetted soil. Four to six hours are usually given the draining, the reading taken on the burette after establishing the zero on the percolator stem, the volume of absorbed water thus ascertained and divided by the weight of soil multiplied by 100; the result expresses the water absorbed per hundred of soil.

Example:

For general analytical work the correction for variations in the weight of water for different temperatures is of no practical importance.

149. Accuracy of Results.—A sample of soil from the beet sugar station, in Nebraska, gave the following duplicate results:

Muck soils from Florida, containing varying proportions of sand, gave the following numbers:

Soil number one, 144.85 per cent, and 145.43 per cent; soil number two, 109.13 per cent, and 107.93 per cent; soil number three (very sandy), 46.86 per cent, and 46.51 per cent.

137150. Method of Wollny.^[105]—A zinc tube, ninety centimeters long and four centimeters internal diameter, carries at each end, at right angles to the axis, a flattened rim 1.5 centimeters broad. The lower end of the tube is closed with a strong piece of coarse linen. The soil to be examined is then filled in little by little, with gentle tamping.

On the upper end two glass tubes are placed, each ten centimeters long and four centimeters internal diameter. These tubes are furnished at each end with cemented brass cylinders which are expanded to a circular, evenly ground rim, 1.5 centimeters wide, also at right angles to the axis of the main tube. These rims are greased and placed together, one on the other, and held together by wooden clamps.

The glass tube in immediate connection with the zinc tube is also firmly filled with the soil sample, while the second tube is only partly filled, so that any settling which may take place in the soil on the addition of water may still find the first glass tube full of the sample.

The empty part of the upper glass tube is now filled with water and additional quantities of water are added from time to time until the soil is saturated. In order to be able to observe when this takes place there is a slit at the lower end of the zinc tube which is closed with a piece of glass. This slit should be about two centimeters broad and ten centimeters long. The lower end of the zinc tube is set on a glass plate to prevent evaporation.

As soon as the water shows itself at the lower end of the zinc tube, the excess of water in the upper glass tube is at once removed by a pipette and a stopper inserted through which a glass tube passes drawn out into a fine point above. The object of this is to avoid evaporation on the upper surface. The apparatus is then left at rest for thirty-six hours.

At the end of this time the clamps are removed and the column of moist earth cut with a piece of platinum foil, and the two ends of the glass tube, next to the zinc tube, covered with glass plates. It is then weighed and the weight of moist earth determined by deducting the weight of the tube and its glass covers. The moist earth is carefully removed to a large porcelain dish and dried at 100°. Before weighing it is allowed to stand twenty-four 138hours in the air. The data obtained are used to calculate the water content to volume per cent.

The volume of the glass tubes should be determined by careful calibration.

151. Method of Heinrich.^[106]—The soil to the depth turned by the plow is dug out and in the hole a lead vessel without bottom, twenty centimeters in diameter and forty centimeters high, is placed. The soil is then thrown back around and outside the lead vessel until the latter appears buried in the fragments.

The rest of the soil is passed into the lead vessel, through a sieve having four meshes to the centimeter, using for this purpose enough water to thoroughly moisten it. Care should be taken not to use enough water to cause any separation of the fine from the coarse particles.

By this process all coarse stones, sticks, etc., are separated. In sandy soils the flask is left for a few hours while in clay soils a much longer time is necessary. When the excess of water has disappeared the lead cylinder is removed, and a piece cut out of the center of it placed in a weighed drying flask and dried at 100°.

152. Effect of Pressure on Water Capacity.^[107]—The increasing capacity of soil to hold water developed by shaking or pressure, is determined by Henrici in the following way:

Into a glass cylinder of twenty millimeters internal diameter are poured twenty cubic centimeters of water. A given quantity of soil is next added, and after standing until thoroughly saturated, the residual water is measured by pouring off, or better, by graduations on the side of the tube. The increase in the volume of the clear water is also measured, after shaking, in the same way. The data of a determination made as above described follow:

By repeated shaking the volume of e + w, the content of w therein, and the relative values of e
w were found to be as follows:

If e′ represent the volume of the saturated soil then e′ = e + w, and this gives the relation to the volume of dry earth represented by the equation e′
e = 1 + w
e. This indicates that the relative volume of the saturated soil is equal to unity increased by the relative content of water.

153. Coefficient of Evaporation.—At an ordinary room temperature in the shade, samples of soil, if they are subjected to experiment in tolerably thin layers have nearly an equal coefficient of evaporation. That is, the absolute quantity of water evaporated in a given time is almost entirely conditioned upon the magnitude of the surface exposed and the temperature of the surrounding air. Only when exposed in conditions as nearly as possible natural in thin layers to the action of the sunlight and shade do the soils show their peculiarities in respect of the evaporation of moisture. In order to see these peculiarities, samples of soil which have been previously examined must be subjected to examination at the same time with the soil whose properties are to be determined.

The zinc box, before described, should be protected with a well fitting cover of thick paper, and the different samples of soil which are to be tested placed therein. This should now be placed in a wooden box, the top of which is exactly even with the top of the zinc vessel. This box containing the vessel should be exposed to the sunlight. After twenty-four hours the zinc boxes can be taken away from position and their loss in moisture determined, and these weighings, according to the condition of the atmosphere, can be continued from fourteen days to three weeks, the temperature of the air of course being carefully determined at each time. At first, all the different soils being saturated with moisture, it will be observed that the loss of moisture is proportionately the same for all. Soon, however, the rapidity of the evaporation in the samples of soil rich in humus and clay will be decreased as compared with the sandy 140soils, and in general, those which possess a high capillary power capable of bringing the moisture rapidly from the deeper layers to the surface. There soon comes a point when the difference in evaporation is at its greatest; and then there will be a gradual diminution until the samples lose no further moisture. This point, for the different soils, can be determined by frequent weighings of the vessel.

154. Determination of Capillary Attraction.—Long glass tubes graduated in centimeters may be used for this determination, or plain tubes so arranged as to admit of easy measurements with a rule. The tubes may be from one to two centimeters internal diameter and about one meter long. The fine earth should be evenly filled in little by little, with gentle jolting. The lower end of each tube, before filling, is closed with a piece of linen.

The tubes, after filling, are supported in an upright position by a frame AE, Fig. 19, in a vessel B containing water in which the linen covered ends D dip to the depth of two centimeters. The height of the water in the several tubes should be read or measured at stated intervals. The water contained in the supply vessel should be kept at a constant height by a Mariotte bottle.

Figure 19.

Apparatus to show Capillary Attraction of Soils for Water.

The observations may be discontinued after one hundred and twenty hours, but even then the water will not have reached its maximum height.

It is recommended by some experimenters to cut the tubes, 141after the above determination is completed, into pieces ten centimeters in length, and to determine the per cent of water in each portion.

155. Statement of Results.—The following table illustrates a convenient method of tabulating the observed data as given by König.^[108]

156. Inverse Capillarity.—In tubes filled with fine earth, as described in paragraph 154, water is quickly poured, the same quantity into each tube of the same diameter, or such quantities in tubes of different diameters as would form a water column of the same depth over the surface of the sample. The rate at which the water column descends in each tube, the time of the disappearance of the water at the surface and the final depth to which it reaches, are the data to be entered.

157. Statement of Results.—The points to be observed in the determination of inverse capillarity are the number of hours required for the total absorption of a column of water of a given height, the depth of the moisture column at that moment, and the total depth to which the moisture column finally reaches. The data of observations with six samples with a water column four centimeters high are given by König^[109] as follows:

158. Determination of the Coefficient of Evaporation.—The coefficient of evaporation is the number of milligrams of water evaporated from a square centimeter of soil surface in a 142given unit of time. It is evident that this number will vary with the physical state of the soil, the velocity of the wind, the saturation of the air with aqueous vapor and the temperature. In all statements of analyses these factors should appear.

The process may be carried on first (a) with soil samples kept continually saturated with water and (b) with samples in which the water is allowed to gradually dry out.

Method a.—The determination may be made in the shade or sunlight.

In the Shade.—A zinc cylinder (Z Fig. 20), fifteen centimeters in diameter and 7.5 centimeters high, with a rim one centimeter wide and one centimeter from top, is covered at one end with linen or cotton cloth and filled with fine earth, with gentle jolting, until even with the top. It is then placed in a zinc holder H, into the circular opening of which it snugly fits as in A. This holder is twenty centimeters in diameter and 7.5 centimeters deep. It has an opening at O through which water can be added until it is filled so as to wet the bottom of Z when in place. As the water is absorbed by the soil more is added and, the top being covered, the apparatus is allowed to stand for twenty-four hours. At the end of this time the soil in the zinc cylinder is saturated with water to the fullest capillary extent.

The whole apparatus, after putting a stopper in O, is now weighed on a large analytical balance and placed in an open room, with free-air circulation, for twenty-four hours. At the end of this time it is again weighed and the loss of weight calculated to milligrams per square centimeter. Where large and delicate balances can not be had, the apparatus can be constructed on a smaller scale suitable for use with a balance of the ordinary size.

In the Sunlight.—The apparatus described above is enclosed in a wooden box having a circular opening the size of the soil-zinc cylinder. In the determination of the rate of evaporation, the apparatus, charged and weighed as above described, is exposed to the sun for a given period of time, say one hour. On the second weighing the loss represents the water evaporated. The time of year, time of day, velocity of wind and temperature, 143and degree of saturation of the air with aqueous vapor, should be noted. The data obtained can then be calculated to milligrams of water per square centimeter of surface for the unit of time.

Method b.—As in method a the determination may be made in the shade or in the sunlight. The rate of evaporation is, in this method, a diminishing one and depends largely on the reserve store of water in the sample at any given moment.

The same piece of apparatus may be used as in the determinations just described. After charging the sample with moisture all excess of water in the outer zinc vessel is removed and the rate of evaporation determined by exposure in an open room or in the sunlight, as is done in the operations already described.

Alternate Method.—The zinc cylinders used in determining saturation coefficient, paragraph 145, may also be employed in determining the rate of evaporation. Each cylinder should be wrapped with heavy paper or placed in a thick cardboard receptacle, and all placed in a wooden box, the cover of which is provided with circular perforations, just admitting the tops of the cylinders, which should be flush with the upper surface of the cover.

Arranged in this way the cylinders previously weighed are exposed in the shade or to direct sunlight and reweighed after a stated interval. On account of the small surface here exposed in comparison with the total quantity of soil and moisture it is recommended to weigh the cylinders once only in twenty-four hours. The weighings may be continued for a fortnight or even a month.

In soils fully saturated with water the rate of evaporation is at first nearly the same on account of the surface being practically that of water alone. As the evaporation continues, however, the rate changes markedly with the character of the soil.

159. Rapid Method of Wolff.—In order to expose a larger surface to evaporation and to secure the results in a shorter period of time, Wolff^[110] fills square boxes, having wire-gauze bottoms, with fine earth, and after saturating with moisture weighs and suspends them in the open air. The wire-gauze bottoms are previously covered with filter paper to prevent loss of soil.

Figure 20.

Apparatus for Determining Coefficient of Evaporation.

144160. Estimation of Water Given up in a Water-Free Atmosphere.—The air-dried sample, in quantities of from five to ten grams in a thin layer on glass, is placed over a vessel containing strong sulfuric acid. It is then placed on a ground glass plate and covered with a bell jar. The sample is weighed at intervals of five days until the weight is practically constant.

This method is valuable in giving the actual hygroscopic power of a soil depending on its structure alone.

161. Estimation of the Porosity of the Soil for the Passage of Gases.—Some further notion of the physical state of the soil known as porosity, may also be derived by a study of the rate at which it will admit of the transmission of gases. A method for estimating this has been devised by Ammon.^[111]

Air is compressed in two gas holders by means of a column of water of proper height to give the pressure required.

The tubes through which the air passes out of the gas holders are each furnished with a stop-cock and united with a glass tube having a side tube set in at right angles for carrying off the air.

The use of two holders makes it possible to carry on the experiment as long as may be desired, one holder being filled with air while the other is emptying.

The common conducting tube is joined with a meter which is capable of measuring, to 0.01, the volume of air passing through it. The pressure is regulated by means of the stop-cocks. The air passing from the meter is received in a drying tube filled with calcium chlorid.

From the drying tube the air enters a drying flask filled below with concentrated sulfuric acid and above with pumice stone saturated therewith. Next the dried air passes through a worm, eight meters long, surrounded with water at a given temperature. The dried air of known temperature next enters the experimental tube. This tube is made of sheet zinc 125 centimeters in length and five centimeters in internal diameter. It is placed in an upright position, and about six centimeters from its upper end carries a small tube at right angles to the main one for connection with a water-filled manometer.

The upper and lower ends of the tube are closed with perforated rubber stoppers carrying tubes for the entrance and exit of the air.

145In the inside of the zinc tube are found two close-fitting but movable disks, of the finest brass wire gauze, between which the material to be experimented upon is held.

The layer of fine soil is held between these disks and may be of such a depth as is required for the proper progress of the experiment. With soils of firm texture opposing a great resistance to the passage of the air the column of earth tested should be shorter than with light and very permeable soils. The experimental tube is surrounded with a water jacket, which may also be made of sheet zinc, carrying small tubes directed upwards for holding thermometers. The water jacket should be kept at the same temperature as the air which is used in the experiment.

The process of filling the tube, the amount of pressure to be used and the air and soil temperature, will naturally vary in different determinations.

The volume of air at a given pressure and temperature which passes a column of soil of a given length in a unit of time will give the coefficient of permeability.

162. Determination of Permeability in the Open Field.—A method for determining the rate of transmission of a gas through the soil in the field has been devised by Heinrich.^[112]

A box C (Fig. 21) is made of strong sheet iron and has an opening below, ten centimeters square, and a height of about twenty centimeters. At exactly ten centimeters from the bottom, the box has a rim at right angles to its length so that it can be placed only ten centimeters deep in the soil. The box holds a volume of earth equal to 1,000 cubic centimeters.

Figure 21.

Method of Heinrich.

The part of the box above ground is connected with the bottle B by a glass tube as indicated in the figure. The 146bottle B should have a capacity of about ten liters. The air in B is forced out through C by water running in from the supply A and the pressure in B is recorded by the manometer D. The experiment should be tried on a soil thoroughly moist.

In measuring the pressure in B the water pressure should be cut off by the pinch-cock between A and B, and the pressure on the manometer observed after the lapse of one to two minutes.

163. Porosity in Relation to Water Movement.—The intimate relation which water movement in a soil bears to fertility makes highly important the analytical study of this feature of porosity. A soil deficient in plant food, in so far as chemical analysis is concerned, will produce far better crops when the flow of moisture is favorable than a highly fertile soil in which the water may be in deficiency or excess.

Aside from the actual rain-fall the texture of the soil, in other words its porosity, is the most important factor in determining the proper supply of moisture to the rootlets of plants. Even where the rain-fall is little, a properly porous soil in contact with a moist subsoil will furnish the moisture necessary to plant growth. This fact is well illustrated by the beet fields in Chino Valley, California. In this locality most excellent crops of sugar beets are produced without irrigation and almost without rain.

164. Methods of Water Movement—The translocation of soil water is occasioned in at least two ways; namely,

1. By changing the porosity of a given stratum of soil.

2. By changing the amount of water a given stratum contains.

The following experiment by King^[113] illustrates a convenient method of studying this movement of water:

On a rich fallow ground of light clay soil, underlaid at a depth of eighteen inches by a medium-grained sand, water, to the amount of two pounds per square foot on an area of eight by eight feet, was slowly added with a sprinkler, samples of soil having been previously taken in six-inch sections down to a depth of 147three feet. The samples were taken along a diagonal of the square under experiment and one foot apart. The middle sample of the line being from the center of the area. The sampling and wetting occurred between one and three P. M., on July 22, and on the evening of the 23 a corresponding series of samples was taken along a line parallel to the first but eight inches distant. The changes in the percentages of water in the soil are given in the following table, showing the translocation of water in soil due to wetting the surface:

The figures given in the last column of the table are computed from the absolute dry weights of the upper three feet of soil as determined in a locality some rods from the place of experiment, and are therefore only approximations, but the error due to this cause is certainly small. It will be seen that while only two pounds of water to the square foot were added to the surface, the upper six inches contained 2.87 pounds per square foot more than before the water was added, and the second six inches contained 0.199 pound more, and this too in the face of the fact that the evaporation per square foot from a tray sitting on a pair of scales close by, was 0.428 pound during the interval under consideration. Similar experiments were made by taking the samples of soil at 5.30 P. M. in one-foot sections down to four feet, at four equally distant places along the diagonal of a square, six by six feet, and having the ground sprinkled. At the same time four similar sets of samples were taken on lines vertical to each of the sides of the square but four feet distant from them. The amount of water the soil contained was then determined, and at 11.30 A. M., nineteen hours later, another series of samples was taken at points about four inches distant from the last and the amount of water determined with the result given below.

The above data show sufficiently well the method of investigation to be pursued in studies of this kind.

165. Capillary Movement of Water.—The method of investigation proposed by King^[114] consists in taking samples of soil at intervals of one, two, three, or four feet in depth, and determining the amount of moisture in each in connection with the amount of rain-fall during the period. The quantity of water contained in a given soil, at various depths and on different dates, is shown in the following table:

The rain-fall during the interval was 4.18 inches, equal to 21.77 pounds per square foot.

166. Lateral Capillary Flow.—To determine the lateral capillary flow of water in a soil the following method, used by King^[115] may be employed:

A zinc lined tray, six by six feet in area and eight inches deep, 149is filled with a soil well packed. In one corner of this tray a section of five inches of unglazed drainage tile, having its lower end broken and jagged, is set and the dirt well filled in round it. By means of a Mariotte bottle water is constantly maintained in the bottom of this tile, three-quarters of an inch deep, so that it will flow laterally by capillary action into the adjacent soil, the object being to determine the extent and rate of capillary flow laterally.

The water content of the soil is determined at the time of starting the experiment, on the circumferences of circles described with the tile as a center, the distance between the circles being one foot. At stated periods, usually at intervals of one day, the content of moisture is again determined at the same points. The investigations show that the lateral movement of water in the soil is not rapid enough to extend much beyond three feet in thirty-one days, for beyond that distance the soil was found to be drier than at the beginning of the experiment. A record is to be kept of the amount of water delivered to the soil by weighing the supply bottle at intervals, and the rates given at which the soil takes up the water in grams per hour and pounds per day. Also the amount of flow per square foot of soil section together with the mean daily evaporation should be noted. The mean flow per foot of soil section is computed on the assumption that the outer face of the zone of completely saturated soil is the delivering surface. In King’s work this point, as nearly as could be determined, was twelve inches from the corner of the tray and hence the figures at best can only be regarded as approximations. The method of stating results is shown in the following table:

150From this table it will be seen that the flow of water in the soil varied in rate, being slower during the first five days than in the succeeding fifteen days. After twenty days the flow dropped again to the beginning rate and then fell below, but remained quite constant during the following nineteen days. For the sake of uniformity in units of measure the daily quantity of flow should be given in kilograms when the hourly flow is given in grams.

167. Causes of Water Movement in the Soil.—The movement of water in a soil as explained by Whitney^[116] is due to two forces, viz., gravitation and surface tension.

The force of gravitation in a given locality is always uniform, both in direction and magnitude per unit volume of water.

Surface tension is the tendency of any exposed water surface to pull itself together. It may act in any direction, according to circumstances, and may thus sometimes help and sometimes antagonize the force of gravitation.

According to the law of surface tension any particle of moisture tends to assume the smallest possible area. This tendency is a constant definite force per unit of surface at a given temperature. In the soil this constant strain on the free surface of water particles serves, in a high degree, to move them from place to place, in harmony with the requirements of the different portions of the field.

When a soil is only slightly moist the water clings to its grains in the form of a thin film. When these soil particles are brought together the films of water surrounding them unite, one surface being in contact with the soil particles and the other exposed to the air. If more water enter the soil the film thickens until finally, when the point of saturation is reached, all the space between the soil particles becomes filled with water, and surface tension within the soil is thus reduced to zero. Gravity then alone acts on the water and with a maximum force.

In a cubic foot of ordinary soil the total surface of the soil particles will be at least 50,000 square feet. It follows that when the soil is only slightly moist the exposed water surface of the films surrounding the soil particles approximates that of the particles themselves. If such a mass of 151slightly moist soil be brought in contact with a like mass saturated with water, the films of water at the point of contact will begin to thicken in the nearly dry soil at the expense of the water content of the saturated mass. The water will thus be moved in any direction.

During evaporation the surface tension near the surface of the soil is increased, and water is thus drawn from below. In like manner, when rain falls on a somewhat dry soil, the surface tension is diminished and the greater surface tension below pulls the moisture down even when gravitation would not be sufficient for that purpose.

Certain fertilizers have the faculty of modifying surface tension and thus change the power of the soil in its attraction for moisture. In this way such fertilizers act favorably on plant growth, both by providing plant food and by supplying needed moisture.

168. Surface Tension of Fertilizers.—Whitney gives the following data in respect of the surface tension of aqueous solutions of some of the more common fertilizing materials. It is expressed in gram meters per square meter, i. e., on a square meter of liquid surface there is sufficient energy to lift the given number of grams to the height of one meter.

169. Method of Estimating Surface Tension.—The determination of surface tension is made by measuring the rise of the liquid in a capillary tube. A short piece of thermometer tubing is used, the diameter of the bore being determined by careful microscopic measurements with a micrometer eyepiece. The diameter of the tube should be about 0.5578 millimeter. The tube is very thoroughly cleaned after each observation, or set of observations, with a strong caustic potash solution, and, after washing, is allowed to stand for some time in a saturated solution of potassium bichromate in strong sulfuric acid. The height of the rise in the capillary tube is measured with a cathetometer.

The following formula is used for the calculation of the results:

Where T is the surface tension, d is the diameter of the tube in centimeters; h the height to which the liquid rises in the capillary tube in centimeters; ω is the specific gravity of the solution; and 4 cos. a refers to the angle of the liquid with the sides of the glass tube. For a tube of the size given above, 5° 24′ is the value of this edge angle. In regard to saline solutions, Quincke^[117] says, that the edge angle appears to increase a little with augmenting concentration of the saline solution, but otherwise 153to differ only inconsiderably from the edge angle of pure water.

170. Effect of the Solutions on Surface Tension.—The mineral fertilizers, as a rule, increase the surface tension of water, while organic matters in solution decrease it. But it must not be forgotten in this connection that but little of the organic matter in the fertilizers employed for the experiment passes into solution. Moreover, with these substances, the accuracy of the work is impaired somewhat by the increased viscosity. In general, the results of the experiment are in harmony with the well-known effect of magnesium, sodium, and potassium chlorids, and sodium nitrate, to make the soil more moist in dry weather, and the opposite effect produced by the application of organic matter.

171. Method of Preparing Soil Extracts.—The soil extracts used in determining the surface tension, as given in the above table, are prepared as follows:

Ten grams of the soil are rubbed up with fifteen to twenty cubic centimeters of distilled water and allowed to stand for twenty-four hours with frequent stirring. Any fine particles not removable by a filter are neglected, although they may give a turbid appearance to the solution.

172. Lysimetry.—The process of measuring the capacity of a soil to permit the passage of water and of collecting and determining the amount of flow and determining soluble matters therein is known as lysimetry. In general, the rate at which water will pass through a soil depends on the fineness and approximation of its particles. Water will pass through coarse sand almost as rapidly as through a tube, while a fine clay may be almost impervious. The study of the phenomena of filtration through soil, and the methods of quantitatively estimating them, are therefore closely related to porosity.

Two cases are to be considered, viz.: First, percolation through samples of soil prepared for analysis, and second, the passage of the water through soil in situ, whether it be virgin or cultivated.

The determination of the rate of flow through a soil in laboratory 154samples, gives valuable information in respect of its physical properties, while the same determination made on the soil in situ, has practical relations to the supply of moisture, to growing plants, and the waste of valuable plant food in the drainage waters. The determination of the rate of flow of water through a small sample, disturbed as little as possible in its natural condition, is classed with the first divisions of the work, inasmuch as the removal of a sample of soil from a field, and its transfer to the laboratory, subjects it to artificial conditions, even if its texture be but little disturbed by the removal.

173. Calculation of the Relative Rate of Flow of Water Through Soils.—There will evidently be one space, or opening, into the soil for every surface grain, as pointed out by Whitney,^[118] and the approximate number of grains, or of openings, on a unit area of surface may be found by the following formula:

where N is the number of grains, or openings, on one square centimeter of surface, M is the approximate number of grains in one gram of soil, W is the weight of soil, V is the total volume of the soil grains and the empty space.

If the grains are assumed to be symmetrically arranged and the spaces between them cylindrical in form, the radii of the spaces can be found by the following formula:

where r is the radius of a single space, V is the total volume of the empty space, N is the number of grains or spaces on one square centimeter of surface, and L is the depth of the soil.

If the space within the soil is completely filled with water the relative rate of flow of water through the soil will be according to the fourth power of the radius of a single space multiplied by the number of spaces on the unit area of surface, as shown by the following formula:

where N-N₁ are the numbers of spaces, and r-r₁ are the radii 155of single spaces in the respective soils, and T-T₁ the times required for a unit volume of water to flow through the soils under the same head or pressure.

The space within the soil is rarely filled with water in agricultural lands, and the most favorable amount of water for the soil to hold, as Hellriegel and others have shown, is from thirty to fifty per cent of the total amount of water the soil can hold if all the space within it were filled.

If the space within the soil be only partly filled with water, as in most arable lands, the water will move in a thin film surrounding the soil grains and according to the fourth power of the thickness of the film. The mean thickness of the film surrounding the soil grains may be theoretically determined by the following formula, which is based on the conception that the film is cylindrical and of uniform size throughout:

where s is the per cent by weight of water which the soil will hold when the empty space is filled with water, p the per cent of water actually contained in the soil, r the radius of a single space, and t the mean thickness of the film surrounding the soil grains.

The relative rate of flow of water through the soils will then be according to the following formula:

It must be remembered that these formulæ give only approximate and comparative values for comparing one soil with another. The structure of the soil is altogether too intricate to expect ever to obtain absolute values.

If the observed rate of flow varies widely from the relative rate calculated from the mechanical analysis, it will indicate a difference in the arrangement of the soil grains, or in the amount or condition of the organic matter in the soils. In the older agricultural regions of the United States, south of the influence of the glacial action, the great soil areas appear to have sensibly similar arrangements of the soil grains, and sensibly uniform 156conditions of organic matter, save where these have been modified by local conditions.

174. Measurement of Rate of Percolation in a Soil Sample.—In order to measure the power of the soil for permitting the passage of water, a box, about twenty-five centimeters high and having a cross section of about three centimeters square, is used. Below, this box has a funnel-shaped end with a narrow outlet tube, which at its lower end is closed with cotton, in such a way that a portion of the cotton extends through the stem of the funnel. A little coarse quartz sand is scattered over the cotton and afterwards the funnel part of the apparatus filled with it. The sand and the cotton are saturated with water and the apparatus weighed. The box is then filled with the fine sample of earth, with light tapping, until the depth of earth has reached about sixteen centimeters. The apparatus, after the addition of the air-dried earth, is again weighed to determine the amount of earth added, and the soil is then saturated by the careful addition of water. After the excess of water has run down the funnel, the total quantity of absorbed water is determined by reweighing the apparatus and the total water-holding power of the soil is determined. There is carefully added, without stirring up the surface of the soil, a column of water eight centimeters high, making in all from sixty to seventy grams. The time is observed until the water ceases to drip from the funnel. The dripping begins immediately after the water is poured on and ceases as soon as the liquid on the surface of the soil has completely disappeared. On the repetition of this operation a longer time for the passage of the water is almost always required than at the first time. The experiment, therefore, must be tried three or four times and the mean taken.

175. Method of Welitschowsky.^[119]—The soil is placed in the vessel a, Fig. 22, which is cylindrical in shape and five centimeters in diameter. The lower end of the cylinder is closed with a fine wire-gauze disk and the upper end is provided with an enlargement for the reception of the tube b, which is connected to a with a wide rubber band. The lower end of the tube b is also closed with a wire-gauze disk. These tubes may be conveniently made of sheet zinc. The tube b carries on the side, 157at distances of ten centimeters, small tubes of fifteen millimeters diameter. On the opposite side it is provided with a glass tube set into a side tube near the bottom for the purpose of showing the height of the water. The side tube carrying the water meter is provided with a stop-cock as shown in the figure.

Figure 22.

Method of Welitschowsky.

In conducting the experiment, after the apparatus has been arranged as described, the small lateral tubes are, with one exception, closed with stoppers. On the open one, d, a rubber tube is fixed for the purpose of removing the water. The required water pressure is secured by taking the lateral opening corresponding to the pressure required. Water is introduced into the apparatus slowly through the glass tube f.

The water rises to d and then any excess flows off through e. By a proper regulation of the water supply the pressure is kept constant at d. The water flowing off through a is collected by the funnel and delivered to graduated flasks where its quantity can be measured for any given unit of time. Since the rate of flow at first shows variations, the measurement should not be commenced until after the flow becomes constant.

In general, the experiments should last ten hours, and, beginning with a water pressure of 100 centimeters, be repeated successively with pressures of eighty, sixty, forty, and twenty, centimeters, etc. In coarse soils, or with sand, one hour is long enough for the experiment.

176. Statement Of Results.—In the following tables the results for ninety centimeters, seventy centimeters, etc., are calculated from the analytical data obtained for 100 centimeters, eighty centimeters, etc.

Similar sets of data have been collected with powdered limestone, clay and humus.

The general conclusions from the experiments are as follows:

1. Clay (kaolin) and humus (peat) are almost impermeable for water, and fine quartz and limestone dust are also very impermeable.

2. The permeability of a soil for water increases as the particles of the soil increase in size, and when particles of different sizes are mixed together the permeability approaches that of the finer particles.

3. The quantity of water passing through a given thickness of soil increases with the water pressure but is not proportional thereto, increasing less rapidly than the pressure.

4. The quantity of water passing under a given pressure is inversely proportional to the thickness of the soil layer when the particles are very fine and the pressure high.

177. Method of Whitney.—To determine the permeability 159of the soil or subsoil to water or air, in its natural position in the field, the following method, due to Whitney, can be recommended:

A hole should be dug, and the soil and subsoil on one side removed to the depth at which the observation is to be made. A column of the soil or subsoil, two inches or more square, and four or five inches deep, is then to be carved out with a broad bladed knife, or a small saw can be conveniently used for cutting this out. A glass or metal frame, a little larger than the sample and three or four inches deep, is slipped over the column of soil, and melted paraffin is run in slowly to fill up the space between the soil and the frame. The soil is then struck off even with the top and bottom of the frame, preferably with a saw, or at any rate taking care not to smooth it over with a knife, which would disturb the surface and affect the rate of flow. The frame is then placed upon some coarse sand or gravel, contained in a funnel, to prevent the soil from falling out and to provide good drainage for the water to pass through. Another similar frame can then be placed on top and secured by a wide rubber band. A little coarse sand, which has been thoroughly washed and dried, is then placed on the soil, and water carefully poured on until it is level with the top of the frame. When the water begins to drop from the funnel more water must be added to the top, so as to have the initial depth of water over the soil the same in all the experiments. A graduated glass is then pushed under the funnel, and the time noted which is required for a quantity of water to pass through the soil. The quantity usually taken for measurement is equivalent to one inch in depth over the soil surface. In taking the sample, root and worm-holes are to be avoided, and these are particularly troublesome in clay lands.

178. Measurement of Percolation through the Soil in Situ.—If lateral translocation could be prevented, the measurement of the quantity of water descending in the soil through a given area would be a matter of simplicity. But to secure accurate results all lateral communication of a given body of soil with adjacent portions must be cut off. Various devices have been adopted to secure this result. An elaborate system 160of lysimetric measurements is illustrated by the apparatus erected by the Agricultural Experiment Station, of Indiana.

The plan and section of the apparatus are shown in Fig. 23.

Each lysimeter box, when finished, resembles somewhat a hogshead with one head out. The sides, however, are perfectly straight inside, having a slight thickening in the center, on the outside, for making them stronger. The sides and bottom of the apparatus are constructed of oak and lined with sheet copper carefully soldered so as to be water-tight. Six inches above, and parallel to the bottom of each of the boxes, is a perforated copper tube, which extends entirely across the lysimeter, and passing through one of the sides connects the box with an underground vault in which the observations are taken. These tubes give an outlet to the drainage water, as described further on. The lysimeters are made of any required depth, the two which are shown in section being three and two-thirds and six and two-thirds feet deep, respectively.

The following method is employed for filling them with soil: There are first placed in the bottom of each lysimeter six inches of fine sand, sifted and washed, which fills them up to the level of the drainage tubes. The lysimeters are then filled with fine, sifted surface soil, to the depth of three and six feet, respectively, making a complete pair of lysimeters, and leaving two inches of the lysimeter boxes projecting above the surface of the soil so that each one will receive exactly its proper share of the rain-fall.

The lysimeters of the other pair, which are the same size as the first, are filled in a different way. The lysimeters are first constructed and placed over vertical columns of soil in situ, which are obtained by digging away all the surrounding soil and leaving the columns standing. The shorter lysimeter is sunk in this way to within two inches of its entire length. It is then tipped over carrying the column of soil with it. Six inches of the subsoil are then removed, when the drainage tube and sand are put in, as in the first pair, and the bottom of the tube soldered in place. The lysimeter is thus filled with the natural soil in place. The longer box is in the same way filled, as far as possible, with the soil in place, but a gravelly nature of the soil may render it impossible to do the filling with a single column unbroken, so the gravel and sand from the lower portion of the soil are to be filled in separately. The drainage tube and bottom of sand are placed in the longer lysimeter in the same way as in the shorter.

Figure 23.

Ground Plan and Vertical Section of Lysimeters and Vaults Showing Position of the Apparatus.

1, 1, 1, 1, Lysimeters.
2, 2, 2, 2, Receiving bottles.
3, 3, Supplying apparatus.
4, 4, Skylights.
5, 5, 5, 5, Wall of vault.
6, 6, Brick walls.
7, Entrance Steps.
8, Vault.

161The purpose of placing sand at the bottom of each lysimeter is to offer a porous stratum in which free water may collect and rise to the level of the perforated copper tube, which would prevent any further rise by conveying the surplus above into the vault as drainage water. The soil above the tube will therefore be constantly drained and the sand below constantly saturated, unless the water be drawn up by the capillary action of the soil as the result of evaporation from the surface.

By means of a proper arrangement within the vault, of a kind of Mariotte’s bottle, the water may be caused to flow back through the drainage tube into the lysimeter to take the place of that lost by evaporation, and thus maintain the level of free water just below the drainage tube. The water flowing back to the lysimeter, and the amount of drainage water, are carefully measured by a system of graduated tubes.

The lysimeters thus constructed represent tile-drained land; in one case the tile being three feet below the surface and in the other six feet below. The drainage waters collected in the receiving bottles can be measured and analyzed from time to time, as occasion may require, to determine the amount of plant food which is removed.

179. Improved Method of Deherain.^[120]—Deherain’s earlier experiments were made in pots containing about sixty kilos of soil. These vases serve very well for some kinds of plants, but there are other kinds which do not grow at all normally when their roots are imprisoned. For instance, in pots, even of the largest size, wheat is always poor, beets irregular, maize never acquires its full development, and the conclusions which can be drawn from the experiments can not be predicated of the action of the plant under conditions entirely normal.

It is necessary therefore to carry on the work in an entirely different way, and to construct boxes so large as to make the conditions of growth entirely normal. The arrangement of these boxes is shown in Fig. 24.

162They are placed in a large trench, two meters wide, one meter deep, and forty meters long. There are twenty boxes in this trench, the upper surface of each containing four square meters area. The boxes are one meter deep, and therefore can contain four cubic meters of soil. The sides and bottoms of the boxes are made of iron lattice work, covered with a cement which renders them impervious to water.

The bottom inclines from the sides towards the middle, and from the back to the front, thus forming a gutter which permits of the easy collection of the drainage. The drainage water is conveyed, by means of a pipe and a funnel, into a demijohn placed in the ditch in front of the apparatus, as shown in the figure. These receptacles stand in niches under the front of the cases, and are separated by the brick foundations. Access to them is gained by means of the inclined plane shown in the figure, and this plane permits the demijohns in which the drainage water is collected, to be removed with a wheelbarrow for the purpose of weighing. This apparatus is especially suitable for a study of the distribution of the nitrogen to the crop, the soil and the drainage waters. The loss in drainage waters of potash and phosphoric acid is insignificant in comparison with the loss in nitrogen.

The cases having been placed in position they are filled with the natural soil, which is taken to the depth of one meter, in such a way that the relative positions of the soil and subsoil are not changed.

While the soil is transferring to the cases it is carefully sampled in order to have a portion representing accurately the composition of both the soil and subsoil. These samples are subjected to analysis and the quantities of nitrogen, phosphoric acid, and potash contained therein carefully noted.

One or two cases should be left without crop or fertilizer to determine the relations of the soil and subsoil to the rain-fall. Three or four cases should be kept free of vegetation and receive treatment with different fertilizer, in order to determine the influences of these on the deportment of the soil to rain-fall. The rest of the cases should be seeded with plants representing the predominant field culture of the locality, and some of them should be fertilized with the usual manures used in farm culture.

Figure 24.

Deherain’s Apparatus for Collecting Drainage Water.

PART FOURTH. MECHANICAL ANALYSIS OF SOILS. — THE FLOCCULATION OF SOIL PARTICLES. — SEPARATION OF THE EARTH PARTICLES BY A LIQUID. — SEPARATION OF THE SOIL PARTICLES BY A LIQUID IN MOTION. — AUTHORITIES CITED IN PART FOURTH.

180. Relation of Flocculation to Mechanical Analysis.—The tendency of the fine particles of silt to form aggregates, which act as distinct particles of matter, is the chief difficulty connected with the separation of the soil into portions of equal hydraulic value by the silt method of analysis. This tendency has been discussed fully by Johnson^[121] and Hilgard.^[122]

181. Illustration of Flocculation.—A sediment, consisting of particles of a hydraulic value, equal to one millimeter per second, is introduced into an ordinary conical elutriating tube placed vertically, in which the current of water entering below performs all the stirring which the particles receive.

A current of water corresponding to a velocity below one millimeter per second will, of course, not carry any of the particles out at the top of the cylindrical tube, but will keep them moving through the conical portion of the tube. If now the current be increased until its velocity is greater than one millimeter per second after having run at the slower velocity for fifteen or twenty minutes, very little of the sediment will pass over, although theoretically the whole of it should. Even at a velocity of five millimeters per second, much of the sediment will remain in the tube. This, of course, is due to the coagulation of the particles into molecular aggregates having a higher hydraulic value even than five millimeters per second. These aggregates can be broken up by violent stirring or moderate boiling, and the sediment reduced again to its proper value. The conclusions which Hilgard derives from a study of the above phenomena are as follows:

1. The tendency to coagulation is, roughly, in an inverse ratio to the size of the particles. With quartz grains it practically 166ceases when their diameter exceeds about two-tenths of a millimeter having a hydraulic value of eight millimeters per second. The size of the aggregates formed follows practically the same law as above. Sediment of 0.25 millimeter hydraulic value will sometimes form large masses like snow-flakes on the sides of the elutriator tube.

2. The degree of agitation which will resolve the aggregates into single grains is inversely as the size of the particles; or, more properly perhaps, inversely as their hydraulic value.

3. The tendency to flocculation varies inversely as the temperature. So much so is this the case that Hilgard at one time contemplated the use of water at the boiling point in the mechanical analysis of soils, in place of mechanical stirring.

4. The presence of alcohol, ether, and of caustic or carbonated alkalies, diminishes the tendency to flocculation, while the presence of acids and neutral salts increases it.

5. As between sediments of equal hydraulic value, but different densities, the tendency to flocculation seems to be greater with the less dense particles.

In regard to the mechanical actions which take place between the particles, Hilgard considers them as irregular spheroids, each of which can at best come in contact at three points with any other particle. The cause of aggregation cannot therefore be mere surface adhesion independent of the liquid, and the particles being submerged there is no meniscus to create an adhesive tension.

Since experiment shows that the flocculative tendency is measurably increased by the cohesion coefficient of the liquid, it seems necessary to assume that capillary films of the latter interposed between the surfaces of solids create a considerable adhesive tension even in the absence of a meniscus.

182. Effect of Potential of Surface Particles.—Whitney suggests that this is due to the potential of the surface particles of solids and liquids.^[123] The potential of a single water particle is the work which would be required to pull it away from the surrounding water particles and remove it beyond their sphere of attraction. For simplicity, it may be described as the total force of attraction between a single particle and all other 167particles which surround it. With this definition, it will be seen that the potential of a particle on an exposed surface of water is only one-half of the potential in the interior of the mass, as half of the particles which formerly surrounded and attracted it were removed when the other exposed surface of water was separated from it. A particle on an exposed surface of water, being under a low potential, will therefore tend to move toward the center of the mass where the potential, i. e., the total attraction, is greater, and the surface will tend to contract so as to leave the fewest possible number of particles on the surface. This is surface tension.

If, instead of air, there is a solid substance in contact with the water, the potential will be greater than on an exposed surface of the liquid, for the much greater number of solid particles will have a greater attraction for the water particles than the air particles had. They may have so great an attraction that the water particle on this surface, separating the solid and liquid, may be under greater potential than prevails in the interior of the liquid mass. Then the surface will tend to expand as much as possible, for the particles in the interior of the mass of liquid will try to get out on the surface. This is the reverse of surface tension. It is surface pressure, which may exist on a surface separating a solid and liquid.

Muddy water may remain turbid for an indefinite time, but if a trace of lime or salt be added to the water the grains of clay flocculate, that is, they come together in loose, light flocks, like curdled milk, and settle quickly to the bottom, leaving the liquid above them clear. Ammonia and some other substances tend to prevent this and to keep the grains apart if flocculation has already taken place.

If two small grains of clay, suspended in water, come close together they may be attracted to each other or not, according to the potential of the water particles on the surface of the clay. If the potential of the surface particle of water is less than that of the particle in the interior of the mass of liquid, there will be surface tension, and the two grains will come together and be held with some force, as their close contact will diminish the number of surface particles in the liquid. If, on the other hand, 168the potential of the particle on the surface of the liquid is greater than of the particle in the interior of the mass, the water surface around the grains will tend to enlarge, as there will be greater attraction for the water particles there than in the interior of the mass of liquid, and the grains of clay will not come close together and will even be held apart, as their close contact would diminish the number of surface particles in the liquid around them.

183. Influence of Surface Tension.—Hilgard supposes that the surface tension which is assumed to exist between two liquid surfaces must exert a corresponding influence between the surfaces of solids and liquids, apart from any meniscal action.

It is then to be expected that the adhesion of the particles constituting one of these floccules will be very materially increased whenever the formation of menisci between them becomes possible by the removal of the general liquid mass. Suppose one of the floccules to be stranded, it will, in the first place, remain immersed in a sensibly spherical drop of liquid. As this liquid evaporates, the spherical surface will become pitted with menisci forming between the single projecting particles, and as these menisci diminish their radius by still further evaporation, the force with which they hold the particles together will increase until it reaches a maximum. As the evaporation progresses beyond this point of maximum, the adhesion of the constituent particles must diminish by reason of the disappearance of the smaller menisci, and when finally the point is reached when liquid water ceases to exist between the surfaces, the slightest touch, or sometimes even the weight of the particles themselves, will cause a complete dissolution of the floccule, which then flattens down into a pile of single granules.

In regard to natural deposits from water, Hilgard supposes that they are always precipitated in a flocculated state. The particles of less than two-tenths millimeter diameter are carried down with those of a larger diameter having much higher hydraulic value. Thus the deposition of a pure clay can take place under only very exceptionable circumstances.

Whitney, on the other hand, suggests that grains of sand and clay carry down mechanically the particles of fine silt and clay as they settle in a turbid liquid in a beaker; and it is often difficult 169to wash out a trace of fine material from a large amount of coarse particles, for this reason, although there may be no trace whatever of flocculation.

184. Destruction of Floccules.—The destruction of the natural floccules is seen in the ordinary process of puddling earth or clay. It is also the result of violent agitation of water or of kneading or boiling, or, finally, to a certain extent, of freezing. All these agencies are employed by the workers in clay for the purpose of increasing the plasticity which depends essentially upon the finest possible condition of the material to be worked. As an illustration of this, Hilgard cites the fact that any clay or soil which is worked into a plastic paste with water, and dried, will form a mass of almost stony hardness. If, however, to such a substance one-half per cent of caustic lime be added, a substance which possesses in an eminent degree the property of coagulating clay, the diminution of plasticity will be obvious at once, even when in a wet condition. If now the mass be dried, as in the previous case, it is easily pulverized. This is an illustration of the effect of lime upon stiff lands, rendering them more readily pulverulent and tillable. The conversion of the lime into a carbonate in the above experiment by passing bubbles of carbonic acid through the mass while still suspended in water does not restore the original plasticity, thus illustrating experimentally the fact known to all farmers that the effect of lime on stiff soil lasts for many years, although the whole of the lime in that time has been converted into carbonate.

185. Practical Applications.—The practical application of this is, according to Hilgard, that the loosely flocculated aggregation of the soil particles is what constitutes good tilth. For this reason the perfect rest of a soil, if it is protected from the tamping influence of rains and the tramping of cattle, may produce a condition of tilth which cannot be secured by any mechanical cultivation. As an illustration of this, the pulverulent condition of virgin soils protected in a forest by the heavy coating of leaves may be cited. On the contrary, as pointed out by Hilgard, there are some kinds of soil in which a condition of rest may produce the same effect as tamping. These are soils which consist of siliceous silt without enough clay to maintain 170them in position after drying. In such a case, the masses of floccules collapse by their own weight or by the least shaking, and fall closely together, producing an impaction of the soil. This takes place in some river sediment soils in which the curious phenomenon is presented of injurious effects produced by plowing when too dry, which is the direct opposite of soils containing a sufficient amount of clay and which are injured by plowing too wet.

It is further observed that the longer a soil has been maintained in good tilth, the less it is injured by wet plowing. This is doubtless, according to Hilgard, due to the gradual cementation of the floccules by the soil water which fixes them more or less permanently.

Whitney believes that the arrangement of the grains, or the condition of flocculation in the soil, or the distance apart of the soil grains, is determined, to a large extent, by the potential on the surface of the grains; and he suggests that by changing this the exceedingly fine grains of silt and clay can be pulled together or can be pushed further apart, and so alter the whole texture of the land.

The action of alkaline carbonates in preventing flocculation, and thus rendering tillage difficult or impossible, is pointed out by Hilgard in the case of certain alkali soils of California. The soils which are impregnated with alkaline carbonates are recognized by their extreme compactness. The suggestion of Hilgard to use gypsum on such soils has been followed by the happiest results. This gypsum renders any phosphates present insoluble, and thus prevents loss by drainage, and yet leaves the plant food in a sufficiently fine state as to be perfectly available for vegetation.

186. Suspension of Clay in Water.—The suspension of clay in water and the methods of producing or retarding flocculation and precipitation have also been studied by Durham.^[124] His experiment is made as follows:

In a number of tall glass jars fine clay is stirred with water, and the results of precipitation watched. In all cases it will be noticed that the clay rapidly separates into two portions, the greater part quickly settling down to the bottom of the jars, and the smaller part remaining suspended for a greater or less length of time.

171The power which water possesses of sustaining clay is gradually destroyed by the addition of an acid or salt; a very small quantity, for instance, of sulfuric acid, is sufficient to precipitate the clay with great rapidity. In solutions of sulfuric acid and sodium chlorid of varying strengths, suspended clay is precipitated in the order of the specific gravity of the solutions, the densest solutions being the last to clear up. This may be due to the greater viscosity of the denser liquids.

The power which water possesses of sustaining clay is gradually decreased by the addition of small quantities of certain salts and of lime.

187. Effect of Chemical Action—Brewer^[125] emphasizes the importance of chemical action in the flocculation of clays. As expressed by him the chemical aspects of the phenomena of sedimentation have either been lightly considered or entirely ignored. Brewer is led to believe that the action of clay thus suspended is analogous to that of a colloidal body. Like a colloid, when diffused in water, the bulk of the mass is very great, shrinking enormously on drying. He therefore concludes that clays probably exist in suspension as a series of hydrous silicates feebly holding different proportions of water in combination and having different properties so far as their behavior to water is concerned.

Some of them he supposes swell up in water much as boiled starch does, and are diffusible in it with different degrees of facility, and that the strata observed on long standing of jars of suspended clay represent different members of this series of chemical compounds which hold their different proportions of combined water very feebly and are stable under a very limited range of conditions.

These compounds are probably destroyed or changed in the presence of acids, salts and various other substances, and are stable only under certain conditions of temperature, those which exist at one temperature being destroyed or changed to other compounds at a different temperature.

188. Theory of Barus.—Brewer’s hypothesis, however, is not in harmony with the demonstration of Barus, who proves that a given particle of clay has the same density in ether as in water.

172The physical and mathematical aspects of sedimentation have also been carefully studied by Barus.^[126] The mathematical conditions of a fine particle suspended in a liquid and free from the influences of flocculation are described by Barus in the following equations.

If P be the resistance encountered by a solid spherule of radius r, moving through a viscous liquid at the rate x, and if k be the frictional coefficient, then P = 6πkrx. Again, the effective part of the weight of the particle is P´ = ⁴⁄₃πr³ (ρ-ρ´)g, where g is the acceleration of gravity and ρ and ρ´ the density of solid particle and liquid, respectively. In case of uniform motion P = P´. Hence x = 2
9kr² (ρ-ρ´)g ... (1).

In any given case of thoroughly triturated material the particles vary in size from a very small to a relatively large value; but by far the greater number approach a certain mean figure and dimension. An example of this condition of things may be formulated. To avoid mathematical entanglement let y = Ax^³⁄₂e^-x² ... (2) where y is the probable occurrence of the rate of subsidence x. If now the turbidity of the liquid (avoiding optical considerations) be defined as proportional to the mass of solid material particles suspended in unit of volume of liquid, then the degree of turbidity which the given ydx particles add to the liquid is, caeteris paribus, proportional to r³ydx, where r is the mean radius. Hence the turbidity, T, at the outset of the experiment (immediately after shaking), is T = T₀∫₀^∞r³ydx = T₀, where equations (1) and (2) have been incorporated.

If the plane at a depth d below the surface of the liquid be regarded, then at a time after shaking the residual turbidity is

The equation describes the observed occurrences fairly well.

The phenomena of stratification observed by Brewer are explained by Barus from the above formula: In proportion as the time of subsidence is greater, the tube shows opacity at the bottom, shading off gradually upward, through translucency, into clearness at the top. If, instead of equation (2), there be 173introduced the condition of a more abrupt maximum, if, in other words, the particles be very nearly of the same size, then subsidence must take place in unbroken column capped by a plane surface which at the time zero coincided with the free surface of the liquid. Again, suppose one-half of the particles of this column differ in some way uniformly from the other half. Then at the outset there are two continuous columns coinciding, or, as it were, interpenetrating throughout their extent. But the rate of subsidence of these two columns is necessarily different, since the particles, each for each, differ in density, radius and frictional qualities, by given fixed amounts. Hence the two surfaces of demarcation at the time zero coincided with the free surface. In general, if there be n groups of particles uniformly distributed, then at the time zero n continuous columns interpenetrate and coincide throughout their extent. At the time t, the free surface will be represented by n consecutive surfaces of demarcation below it, each of which caps a column, the particles of which form a distinct group.

From a further discussion of the mathematical condition under which the subsidence of the particles takes place, Barus is of the opinion that Durham’s theory of suspension being only a lower limit of solution is rapidly gaining ground, yet without being attended with concise experimental evidence which will account for the differences in the rate of subsidence. On the contrary, Brewer’s hypothesis of colloidal hydrates is more easily subjected to experimental proof. The test shows that the particles retain their normal density, no matter how they are suspended or circumstanced.

Further, in the explanation of the phenomenon of sedimentation, the following principle may be regarded as determined; namely, if particles of a comminuted solid are shaken up in a liquid, the distribution of parts after shaking will tend to take place in such a way that the potential energy of the system of solid particles and liquid, at every stage of subsidence, is the minimum compatible with the given conditions.

According to Barus it is necessary, in order to pass judgment on the validity of any of the given hypotheses, to have in hand better statistics of the size of the particles relatively to the water 174molecule, than are now available. Inasmuch as the particles in pure water are individualized and granular, it is apparently at once permissible to infer the size of the particles from the observed rates of subsidence. His observations show that the said rate decreases in marked degree with the turbidity of the mixture. Hence the known formulæ for single particles are not rigorously applicable, though it cannot be asserted whether the cause of discrepancy is physical or mathematical in kind. It follows that special deductions must be made for the subsidence of stated groups of particles before an estimate of their mean size can fairly be obtained.

Rowland^[127] reaches a closer approximation for the fall of a single particle by showing that the liquid, even at a large distance from the particle, is not at rest.

In the case of water, however, it is noticed that despite the large surface energy of the liquid, subsidence takes place in such a way that for a given mass of suspended sediment the surfaces of separation are a maximum. On the other hand, in case of subsidence in ether or in salt solutions, the solid particles behave much like the capillary spherules of a heavy liquid shaken up in a lighter liquid with which it does not mix. In other words, the tendency here is to reduce surfaces of separation to the least possible value, large particles growing in mass and bulk mechanically at the expense of smaller particles; in other words, exhibiting the phenomenon of flocculation.

189. Physical Explanation of Subsidence.—Whitney^[128] thinks that the phenomena of the suspension of clay in water may be explained on purely physical principles, and that neither the partial solution nor hydration hypotheses are necessary, or will explain the suspension of clay in water, for the solution, or hydrated substance, would still have a higher specific gravity than the surrounding liquid. He calls attention in the first place to the fact, that in a turbid liquid, which has been standing for weeks and which is only faintly opalescent, the grains in suspension are still of measurable size, if properly stained as in bacteriological examinations and viewed through an oil emersion objective. He gives a value of 0.0001 millimeter, as the lower limit of the diameters of these particles of “clay,” which are 175usually met with in agricultural soils. He refers to the fact that fine dust and ashes, and even filings of metals, may remain in suspension in the air for days and even months in very apparent clouds, or haze, although they may be a thousand times heavier than the surrounding air. Particles of clay, no smaller than the limits which have been assigned, should remain in suspension in the much heavier fluid, water, for an indefinite time, for the volume or weight of the particles (⁴⁄₃)(πr³) decreases so much more rapidly in proportion than the surface (4πr²), that there is, relatively, a larger amount of surface area in these fine clay particles, and a great deal of surface friction in their movement through a medium, and they would settle very slowly. Under ordinary conditions, however, the mean daily range of temperature is about twenty degrees, the mean monthly range is fifty degrees, and the yearly range 100° F., and the ordinary convection currents, induced by the normal change of temperature, would be sufficient of itself to keep these fine particles in suspension in the liquid for an indefinite time, as it is known that currents of air keep fine particles of dust and ashes in suspension. If the volume or weight of a fine gravel, having a diameter of one and five-tenth millimeters, be taken as unity, then for a particle, having a diameter of 0.00255 millimeter, which is the mean diameter for Whitney’s clay group, the volume decreases in the ratio 1:0.000000004853, and the surface decreases only in the ratio 1:0.000286.

190. Practical Applications.—The action of mineral substances in promoting flocculence has been taken advantage of in later times in the construction of filters for purifying waters holding silt in solution. In these filters the introduction of a small quantity of alum, or some similar substance, into the water usually precedes the mechanical separation of the flocculent material. In the same way the action of iron and other salts on sewage waters has been made use of in their purification and in the collection of the sewage material for fertilizing purposes.

191. Separation of the Soil Into Particles of Standard Size.—The agronomic value of a soil depends largely on the relative size of the particles composing it. The finer the particles, 176within a certain limit, the better the soil. The size of the particles may be estimated in three ways: (1) by passing through sieves of different degrees of fineness; (2) by allowing them to subside for a given time in water at rest; (3) by separating them in water moving at a given rate of speed. The first method is a crude one and is used to prepare in a rough way, the material for the second and third processes.

192. Separation in a Sieve.—The soil should be dry enough to avoid sticking to the fingers or to prevent agglutination into masses when subjected to pressure. It should not, however, be too dry to prevent the easy separation of any agglutinated particles under the pressure of the thumb or of a rubber pestle.

The sieve should have circular holes punched in a sheet of metal of convenient thickness to give it the requisite degree of strength. Sieves made of wire gauze are not so desirable but it is difficult to get the finer meshes as circular perforations. Such sieves cannot give a uniform product on account of the greater diagonal diameter of the meshes and the ease with which the separating wires can be displaced. It is convenient to have the sieves arranged en batterie; say in sets of three. Such a set should have the holes in the three sieves of the following dimensions; viz.,

Coarser single sieves may be used to separate the fragments above two millimeters diameter if such a further classification be desired. Each sieve fits into the next finer one and the separation of a sample into three classes of particles may be effected by a single operation. In most cases, however, it is better to conduct each operation separately in order to promote the passage of agglutinated particles by gentle pressure with the thumb or with a rubber pestle. In no case should a hard pestle be used and the pressure should never be violent enough to disintegrate mineral particles.

There is much difference of opinion concerning the smallest size of particles which should be obtained by the sieve.

Most analytical processes prescribe particles passing a sieve of 177one millimeter mesh (¹⁄₂₅ inch). There is little doubt, however, of the fact that a finer particle would be better fitted for subsequent analysis by the hydraulic method.

For this purpose a sieve of 0.5 millimeter circular mesh is preferred.

193. Sifting with Water.—In soils where the particles adhere firmly the sifting should be done with the help of water. In such cases the soil is gently rubbed with a soft steple or the finger in water. It is then transferred to the sieve or battery of sieves which are held in the water, and rubbed through each of the sieves successively until the separation is complete. After the filtrate has stood for a few minutes the supernatant muddy liquor is poured off, the part remaining on the sieve is added to it and the process repeated until only clean particles larger than 0.5 millimeter are left on the sieve. These particles can be dried and weighed and entered on the note book as sand. The filtrate should be evaporated to dryness at a gentle temperature and when sufficiently dry be rubbed up into a homogeneous mass by a rubber pestle.

The sieve recommended by the Association of Official Agricultural Chemists^[129] for the preparation of fine earth for chemical analysis has circular openings ¹⁄₂₅ inch (one millimeter) in diameter.

Wahnschaffe^[130] directs that a sieve of two millimeters mesh be used in preparing the sample for silt analysis and that the residue after the silt analysis is finished, which has not been carried over by a velocity of twenty-five millimeters per second, be separated in sieves of one millimeter and 0.5 millimeter meshes respectively.

Hilgard objects to leaving this coarse material in the sample during the process of churn elutriation on account of the attrition which it exerts and therefore directs that it be separated by sieve analysis before the elutriation begins.

194. Method of the German Experiment Stations.^[131]—In the method recommended for the German Agricultural Stations an attempt is made to secure even a finer sieve separation than that already mentioned.

178Sieves having the following dimensions are employed; sieve No. 1, square meshes 0.09 millimeter in size, diagonal measure 0.11 millimeter; sieve No. 2, square meshes 0.14 to 0.17 millimeter in diameter, diagonal measure 0.22 to 0.24 millimeter; sieve No. 3, square meshes 0.35 to 0.39 millimeter in diameter, diagonal measure 0.45 to 0.50 millimeter; finally a series of sieves one, two and three millimeters circular perforations.

Five hundred grams of the soil (in the Halle Station only 250) are placed in a porcelain dish with about one liter of water and allowed to stand for some time with frequent stirring, on a water bath. After about two hours, when the soil is sufficiently softened so that with the help of a pestle it can be washed through the sieves, the process of sifting is undertaken in the following manner: Sieve No. 3 is placed over a dish containing water, the moistened soil placed therein and the sieve depressed a few centimeters under the water and the soil stirred by means of a pestle until particles no longer pass through. After the operation is ended the residue in the sieve is washed with pure water and dried. The part passing the sieve is thoroughly stirred and then washed with water into sieve No. 2 and treated as before. The product obtained in this way is brought into sieve No. 1 and carefully washed. All the products remaining on each of the sieves are dried at 100° and weighed. The portion passing sieve No. 1 is either dried with its wash water or estimated by loss by deducting from the total weight taken, the sum of the other weights obtained. If a more perfect separation of the first sieve residue be desired it can be obtained by passing it through sieves of the last series which may have meshes varying in size, viz.: one, two, or three millimeters in diameter. Each sieve of the same class should have holes uniformly of the same size.

The sieve products are characterized as follows: The part passing a three millimeter sieve is called fine earth, while the part remaining is called gravel. The fine earth is separated into the following products: The part that passes through the three millimeters opening and is left by the two millimeters opening is called steinkies. The product from the two millimeters opening and the residue from the one millimeter opening is called 179grobkies. The product from the one millimeter opening and the residue on the sieve No. 3 is called feinkies. The product from the sieve No. 3 and the residue from the sieve No. 2 is called coarse sand. The product from sieve No. 2 and the residue from sieve No. 1 is called fine sand. The product from sieve No. 1 is called dust. The dust can be further separated into sand, dust, and clay. For the examination of the clay the Kühn silt cylinder as modified by Wagner, is recommended. The cylinder has a diameter of eight centimeters and a height of thirty centimeters, and is furnished with a movable exit tube reaching to its bottom.

195. General Classification of the Soil by Sieve Analysis.—The classification recommended by the German chemists is satisfactory but the following one is more simple. All pebbles, pieces of rock, etc., should first be separated by a two millimeters circular mesh sieve, dried at 105° and weighed. The result should be entered as pebbles and coarse sand.

The finer sand may be separated with a sieve of one millimeter circular openings.

The still finer sand is next separated with the sieve of 0.5 millimeter circular openings as indicated above.

The sample may now be classified as follows:

1.

Coarse pebbles, sticks, roots, etc., separated by hand.

2.

Pebbles and coarse sand not passing a two millimeters sieve.

3.

Sand not passing a one millimeter sieve.

4.

Fine sand not passing a 0.5 millimeter sieve.

5.

Fine earth passing a 0.5 millimeter sieve.

196. Classification of Orth.^[132]—As fine silt are reckoned those particles which range from 0.02 to 0.05 millimeter; as fine sand the groups from 0.05 to 0.2 millimeter; as medium sized sand those ranging from 0.2 to 0.5 millimeter and for large grained sand those particles ranging from 0.5 to 2 millimeters in diameter. Particles over two millimeters form the last classification.

197. Methods of Silt Analysis.—The further classification of the particles of a soil passing a fine sieve can best be effected by separation in water. The velocity with which the current 180moves or with which the particles subside will cause a separation of the particles into varying sizes. The slower the velocity the smaller the particles which are separated. There is, however, a large and important constituent of a soil which remains suspended in water, or in a state of seeming solution. This suspended matter would still be carried over by a current of water moving at a rate so slow as to make a subclassification of it impossible. This suspended matter passing off at a given velocity may be classed as clay, and it consists in fact chiefly of the hydrated silicate of alumina, or other particles of equal fineness. The laws which govern its deposit have already been discussed.

The apparatus which have been used for silt analysis may be grouped into four classes.

(1) Apparatus depending on the rate of descent of the particles of a soil through water at rest. The apparatus for decanting from a cylinder or a beaker belong to this class.

(2) Apparatus which determine the rate of flow by passing the liquid through a vessel of conical shape. The system of Nöbel is a good illustration of this kind of apparatus.

(3) Apparatus in which the elutriating vessel is cylindrical and the rate of flow determined by a stop-cock or pressure feed apparatus. The system of Schöne represents this type.

(4) Apparatus in which the above system is combined with a device for mechanically separating the particles and bringing them in a free state into the elutriating current. The system of Hilgard is the type of this kind of apparatus.

In practice the use of cylindrical apparatus with or without mechanical stirring and the method by decantation have proved to be the most reliable and satisfactory procedures. Between the beaker and churn methods, of separation there is little choice in regard to accuracy. Which is the superior method, is a question on which the opinions of experienced analysts are divided. The various processes will be described in the order already mentioned.

198. Methods Depending on Subsidence of Soil Particles.—The simplest method of effecting the further separation of the soil particles is without doubt that process which permits them to fall freely in a liquid sensibly at rest. The practical difficulties 181of this method consist in the trouble of securing a perfect separation of the particles, in preventing flocculation after division and in avoiding currents in the liquid of separation.

For the separation of the soil particles for this method boiling and wet pestling are the only means employed. The flocculation of the separated particles may be partially prevented by adding a little ammonia to the water employed. The author has also tried dilute alcohol as the separating liquid but the results of this method are not yet sufficiently definite to find a place in this manual. Evidently the practical impossibility of avoiding convection currents prevents the use of water at a high temperature for this separation, although the tendency to flocculation almost disappears as the temperature approaches 100°. The general method of avoiding the errors due to flocculation in the subsidence method consists in repestling the deposited particles and thus subjecting them as often as may be necessary to resedimentation. These principles are well set forth by Osborne,^[133] who states that when a soil is completely suspended in water by vigorous agitation, particles of all the sizes present are to be found throughout the entire mass of liquid. When subsidence takes place, the larger particles will go down more rapidly than the smaller ones, but some of the small particles that are near the bottom will be deposited sooner than some of the larger ones which have a much greater distance to travel. Thus, independently of the fact that the larger particles in their descent are somewhat impeded by the smaller, the smaller being at the same time somewhat hastened by the larger, the sediment that reaches the bottom at any moment is a more or less complex mixture of all the mechanical elements of the soil. The liquid, however, above this sediment at the same moment will have completely deposited all particles exceeding certain dimensions of hydraulic value, determined mainly by the time of subsidence.

If now the aforesaid first sediment be suspended in pure water, and allowed to subside for the same time as before, the larger part of it will be again deposited, but some will remain in suspension, consisting of a considerable part of the finer matter of the first sediment. By pouring off these suspended particles 182with the water and agitating the sediment again with clear water as before, another portion of fine particles will be suspended and may be decanted from it. On continuing this process of repeated decantations it will soon be found that the soil has been separated into two grades.

It is evident that in this way a separation can be made, but it is perhaps not so clear that such a separation would be sharp enough for the purposes of a mechanical soil analysis. If, for instance, the separation is to be made at 0.05 millimeter diameter, it is evident that by repeated decantations all below 0.01 millimeter can be washed out of that above 0.05 millimeter, but it may not appear so probable that all below 0.045 millimeter can be removed without removing some above 0.055 millimeter.

Such a result may be easily attained, however, if the following principle be adhered to:

Make the duration of the subsidence such that the liquid decanted the first few times shall contain nothing larger than the desired diameter. Then decant into another vessel, timing the subsidence so that the sediment shall contain nothing smaller than the chosen diameter. This can not be done without decanting much that is larger than the chosen diameter, but the greater part of the particles greater and less than the chosen diameter can be removed and an intermediate product obtained, the diameters of whose particles are not very far from that desired.

If this intermediate portion be again subjected to the same process, two fractions may be separated from it, one containing particles larger than the chosen diameter and another containing particles smaller than this diameter, while a new intermediate product will remain which is less in amount than that resulting from the first operation. By frequent repetitions of this process this intermediate product can be reduced to a very small amount of substance the particles of which have diameters lying close to the chosen limit and may then be divided between the two fractions.

The principles of the separation described by Osborne set forth with sufficient clearness the purposes to be achieved by the analysis. The chief methods of manipulation practiced will be found below.

183199. Kühn’s Silt Cylinder.—A simple form of apparatus for the determination of silt by the sedimentation process is the one described by Kühn.^[134]

The cylinder should be about twenty-eight centimeters high with a diameter of 8.5 centimeters. At the lower end of the cylinder five centimeters from the bottom it carries a tube 1.5 centimeter in diameter furnished with a pinch-cock and held in position by a rubber stopper.

In carrying out the process thirty grams of sifted soil (two millimeters mesh sieve) are boiled with water for an hour and after cooling the soil and water are washed into the separating cylinder. The cylinder is then filled with water with constant shaking.

After standing for ten minutes the stop-cock is opened and the water with its suspended matter allowed to flow into a porcelain dish.

The cylinder is then again filled with water and the process is continued until the water drawn off is practically clear.

The fine particles having been separated in this way the next coarser grade of particles is separated by repeating the process at intervals of five minutes.

By these two operations it is considered that the clay is entirely removed. The residue remaining in the cylinder is dried and weighed. The relative proportions of clay and residue in the sample are thus determined.

The residue is then separated into two portions by sieves of one millimeter and 0.5 millimeter mesh.

The soil is thus separated into the following parts:

1.

By the first sifting coarse quartz larger than two millimeters diameter.

2.

Fine quartz two millimeters, to one millimeter diameter.

3.

Coarse sand one millimeter, to 0.5 millimeter diameter.

4.

Fine sand finer than 0.5 millimeter diameter.

5.

Silt, clay, humus, etc., separated by the water.

200. Knop’s Silt Cylinder.—The cylinder recommended by Knop^[135] is essentially that of Kühn being furnished with four lateral tubes instead of one (Fig. 25).

Figure 25.

Knop’s Silt Cylinder.

The sample of soil, twenty-five to thirty grams, after passing a two millimeters mesh sieve, and long boiling, is washed through a series of sieves of the following diameters of mesh respectively; viz., one millimeter, 0.5 millimeter, 0.25 millimeter, and 0.1 millimeter. The part which passes the finest sieve is placed in a Knop’s cylinder, the cylinder filled with water one decimeter above upper tube and well shaken. The cylinder is allowed to rest for five minutes when the upper cock is opened and the water drawn off. After five minutes more the next tube is opened and so on with equal intervals for the three upper tubes. The operation is repeated with fresh water until the water drawn off is clear. Finally the lowest tube is opened and all the water poured off of the sandy residue. The space between each tube is one decimeter. The dust remaining is dried and weighed and the weight of material carried over as silt determined by difference.

201. Siphon Silt Cylinder.—Instead of the tubulated cylinder one furnished with a siphon can be employed^[136] (Fig. 26).

It should be about forty centimeters high and six centimeters internal diameter. The cylinder first receives twenty-five to thirty grams of the well boiled fine earth and then water until there is but a small space between it and the stopper when the latter is inserted.

The cylinder is marked exactly 200 millimeters below the surface of the water with a narrow strip of paper at a, stoppered, inverted and well shaken. The cylinder being again placed in normal position the soil particles under the influence of gravity tend to sink with greater or less rapidity according to their size. The siphon a b c is filled with water, the cock at c closed and the opened end a placed in the cylinder 184A just at the mark 200 millimeters below the surface of the water, and the water thus transferred to B when desired. If the suspended matter is allowed to stand for 100 seconds the particles of more than two millimeters hydraulic value will have fallen below the open end of the siphon. If allowed to stand 1,000 seconds the silt value of the particles will be 0.2 millimeter per second. Whatever the number of seconds may be, the operation is continued until the water removed is practically clear. The open end of the siphon a should be bent upwards so that no disturbing current may bring the particles below the line into the liquid discharged into B.

Figure 26.

Siphon Cylinder for Silt Analysis.

While the results obtained by this method are satisfactory as compared with other similar processes it cannot be highly recommended because of the time and trouble required to get a 185complete separation and by reason of the difficulty of collecting the separated silt.

202. Wolff’s Method.^[137]—As modified by Wolff the Knop process is conducted as follows: Fifty grams of fine earth are boiled with water and then the entire mixture is passed through three sieves with openings of one millimeter, 0.5 millimeter, and 0.25 millimeter in diameter, respectively. The finest part is mixed with water to a height of eighteen centimeters in a bottle twenty centimeters high and having a capacity of one liter and thoroughly agitated, after which it is left to rest, and finally the turbid liquid is drawn off with a siphon, the bottle refilled with water, agitated, and left to rest, and the process repeated as long as the water carries any suspended matter after a definite time.

Wolff proposes for the first three periods of rest one hour, for the second three, a half an hour, for the third three, a quarter of an hour, and for the fourth three, five minutes.

203. Moore’s Modification of Knop’s Method.^[138]—The sample of soil is first passed through a sieve having round perforations three millimeters in diameter. The weight of the particles remaining on the sieve is then determined, and likewise that of the portion passing through, which is known as fine earth. The last named portion constitutes the material for all subsequent operations of mechanical and chemical analysis.

Thirty grams of the fine earth are boiled rapidly with water until the lumps are disintegrated and clayey portions separated from the sand. The material is then successively washed through perforated metal sieves, the holes of which are respectively 1, 0.5, and 0.25 millimeter in diameter. The portions retained on the sieves are severally dried, ignited and weighed, and the finest portion, or that passing through the 0.25 millimeter sieve, is then submitted to the following process of separation:

The sediment and water passing through the 0.25 millimeter sieve are placed in a glass cylinder fifty centimeters long and thirty-seven millimeters in internal diameter. The cylinder is closed at the bottom and is provided with a lateral tube inserted 186six centimeters above the bottom. Three other lateral tubes are inserted at intervals of ten centimeters above the first tube, and a ring is etched into the cylinder ten centimeters above the uppermost tube. The lateral tubes are closed with rubber tubes compressed by spring clips. The sediment being placed in the cylinder, water is added to the mark or ring, the cylinder closed with a rubber stopper, and vigorously shaken until the contents are thoroughly mixed. It is then placed upright, the stopper removed, and after standing undisturbed for five minutes the clip on the uppermost tube is opened and the water allowed to flow into a beaker. After five minutes further standing, the second clip is opened and the water drawn off into the same beaker; in the same manner the water is drawn off from the other tubes at intervals of five minutes until the level of the lowest tube is reached. The cylinder is then refilled with water to the mark, thoroughly shaken after inserting the stopper and the water again drawn off at intervals of five minutes, as before; the operation being repeated until the water drawn off is almost free from turbidity. The sediment remaining in the cylinder from this process of washing by subsidence is termed by Knop, fine sand, the material flowing off in suspension in the wash waters, dust, and the process of separation by Knop’s original method ends here.

In order to remedy the imperfect separation into definite particles secured by the above method, Moore proposes the following device:

The fine sand from the first series of subsidences is placed in a separate vessel, the washings are allowed to remain undisturbed for twelve hours, the turbid liquid decanted and the sediment returned to the cylinder. Water is then added to the mark, the whole shaken, and the liquid drawn off at intervals of five minutes, as in the first series. The sediment from this operation is placed in a separate beaker, the washings returned to the cylinder, and again allowed to subside as before; the sediment from this second subsidence is added to that from the preceding operation, and the washings again returned to the cylinder, the operation being repeated as long as any sediment can be obtained from renewed treatment of the washings; the final 187washings are then placed in a separate vessel for subsequent microscopic measurements.

The collective sediments from the last series of operations are then returned to the cylinder and allowed to subside with fresh additions of water, as in the case of the first series; the fine sand thus obtained being added to that from the first series, and the washings being collected in a large beaker. The latter are left at rest for twelve hours, and the sediment returned to the cylinder and treated as before until no further separation can be effected. The fine sand resulting from all of these operations is then dried, ignited and weighed; the weight of the portion removed by the washing being determined by difference, as it is, owing to its excessively slow rate of subsidence, found impracticable to collect it for direct weighing. The size of the particles of fine sand is then determined by micrometric measurement. Similar measurements are made on the material obtained by long subsidence from the washings from the foregoing operations. The average diameter of the largest particles should not exceed 0.01 millimeter.

204. Statement of Results.—The results of the analyses on three soils from the localities indicated in the table, and the method of stating them, are given in the following table:

205. Method of Bennigsen.—The silt flasks recommended by Bennigsen^[139] are shown in Fig. 27.

188The glass flask b carries a long cylindrical neck a the upper part of which is graduated in cubic centimeters. Ten grams of the fine soil are shaken with water in the flask, the neck of which is closed with a rubber stopper. The flask is then inverted bringing the soil and water into the neck. The flask is hung up and sedimentation is assisted by imparting a pendulous motion to the neck for ten minutes. After an hour the soil particles have separated into a coarse layer below and a fine layer above. The relative volumes of the two layers are then read off in cubic centimeters. While this method may be useful in helping to form a speedy judgment concerning the character of a soil it can lay no claim to being an accurate method of silt separation.

Figure 27.

Bennigsen’s Silt Flasks.

206. Method of Gasparin.—The method of Gasparin only gives a very primitive separation of the various components of the earth according to their fineness. It is conducted as follows:

Ten grams of sifted earth are put into a beaker, water is added and strongly agitated; after five minutes the water is decanted into another vessel, the first vessel is filled anew with water, agitated, decanted, and this process is repeated until the liquid remains perfectly clear. Only two portions are weighed, i. e., the pebbles which remain in the sieve and the coarse sand which remains in the beaker; while the argillaceous portion drawn off with the water is determined by the difference.

207. The Italian Method.—The following modification of 189Gasparin’s process is practiced by the Italian chemists:^[140]

Twenty grams of earth are passed through a sieve having openings of one millimeter in diameter, then the sifted part is mixed with 100 cubic centimeters of water in a 200 cubic centimeters beaker and left to rest for some hours, then strongly agitated and after ten seconds the turbid liquid is poured into another vessel of half a liter capacity. This manipulation is repeated until the liquid is clear.

The decanted liquid is thoroughly agitated, then left to stand until the movement shall be completely arrested, after which the supernatant liquid is poured into another vessel holding two liters. To the residuum is added more water; it is agitated, decanted, and this process is repeated until the water is no longer turbid.

208. Method of Osborne.—In the foregoing paragraphs the methods of silt separation by subsidence as practiced in different countries have been outlined. The good points of the various methods are combined in the process as carried out by Osborne.^[141] The details of this method will be given with sufficient minuteness to make its practice possible by all analysts.

Selecting the Sample.—Several pounds of air-dried, fine earth are secured by passing the soil through a sieve, the holes of which are three millimeters in diameter.

Sifting.—Thirty grams of the above fine earth are stirred with 300 to 400 cubic centimeters of water and then thrown successively upon sieves with circular holes of 1, 0.5, and 0.25 millimeter diameter respectively. By means of successive additions of water and the use of a camel’s hair brush, all the fine material is made to pass through the sieves and these at the last are agitated under water in a shallow dish in such a way that the soil is immersed. The finest sieve should be well wet with water on its lower surface just before using. The finest particles which render the water turbid are easily washed through. The turbid water is kept separated from the clear water which comes off with the last portions that pass the sieves. The turbid water usually does not amount to more than one liter.

Elutriation.—The elutriation should be carried on so as to secure three grades of silt; the diameters of the particles ranging 190in the first grades from 0.25 to 0.05 millimeter, in the second grade from 0.05 to 0.01 millimeter, and in the third grade from 0.01 millimeter to the impalpable powder. The term sand is applied to the first grade, silt to the second, and dust to the third. After the turbid liquid from the sifting has stood a short time it is decanted from the sediment and after standing until a slight deposit is formed, is again decanted and the sediment examined with a microscope. If sand be present, the subsidence of the turbid liquid is continued until no more sand is deposited. As the sand subsides rapidly there is no difficulty in altogether freeing the liquid first decanted from this grade of particles. The sediment thus obtained contains all the sand, a part of the dust and much silt. As only dust and the finest silt render the water turbid the sediment is stirred a few times with a fresh quantity of water and decanted after standing long enough to let all the sand settle. When the water decanted is free from turbidity, the last portions of the soil passing through the sieve with clear water are added to the sediment and the decantations continued so as to remove most of the silt. When no more silt can be easily removed from the sediment without decanting sand, the decantations are made into a different vessel and the subsidences so timed as to remove as much of the silt as possible. By using a little care, at least three-quarters of the sand are thus obtained free from silt. The rest of the sand is mixed with the greater part of the silt which has been decanted into the second vessel. The size of the smallest particles in this vessel is determined with the microscope, to make sure that its contents are free from dust as they usually will be if, after settling for a few moments, they leave the water free from turbidity.

The soil is thus separated into three portions, one containing sand, one sand and silt, and the other silt, dust, and clay. The sand and silt are separated from each other by repeating the subsidences and decantations in the manner just described.

In this way there is removed from the sediment, on the one hand, a portion of silt free from sand and dust, and on the other hand a portion of sand free from silt. Thus is obtained a second intermediate portion consisting of sand and silt, but less in amount than the first and containing particles of diameters much 191more nearly approaching 0.05 millimeter. By repeating this process a few times, this intermediate portion will be reduced to particles whose diameters are very near 0.05 millimeter and which may be divided between sand and silt, according to judgment. The amount of this is usually very small. As soon as portions are separated, which the microscope shows to be pure sand or pure silt, they are added to the chief portions of these grades already obtained.

The same process is applied to the separation of silt from dust. When all the silt has been removed from the dust and clay, the turbid water containing the dust and clay is set aside and allowed to settle in a cylindrical vessel for twenty-four hours. The vessel is filled to a height of 200 millimeters. According to Hilgard, the separation of the dust from clay during a subsidence of twenty-four hours, will give results of sufficient accuracy, although the clay then remaining suspended will not be entirely free from measurable fine particles up to 0.001 or 0.002 millimeter diameter.

Small beakers and small quantities of distilled water are used at first for the decantations, as thus the duration of subsidence is less and more decantations can be made in a given time than when larger quantities of water are employed. Beakers of about 100 cubic centimeters capacity are convenient for the coarser grades, but it is necessary to use larger vessels for the fine sediments from which turbid water accumulates that cannot be thrown away, as may be done with the clear water, from which the coarse sediments settle out completely in a short time.

It is best to keep the amount of water as small as possible in working out the dust since loss is incurred in using too large quantities.

It is also necessary in most cases to subject the various fractions obtained during elutriation, to careful kneading with a soft rubber pestle so that the fine lumps of clay may be broken up and caused to remain suspended in the water. This treatment with the pestle should be done in such a way as to avoid as far as possible all grinding of the particles, the object being merely to pulverize the minute aggregations of clay and extremely fine particles which always form on drying a sample of soil after removing it from the ground.

192Measurement of the Particles.—To determine the size of particles in suspension, a small glass tube is applied to the surface of the liquid in such a way as to take up a single drop which is transferred to a glass slide. This drop will contain the smallest particles in the liquid.

To obtain a sample of the coarsest particles the liquid is allowed to stand long enough to form a very slight sediment and a portion of this sediment is collected with a glass tube.

To determine the diameter of the particles in a sediment it is stirred vigorously with a little water and the pipette at once applied to the surface of the water. On decanting the greater part of the sediment, the large particles remain at the bottom of the beaker and may be easily examined.

Time.—The time required to make the separations, above described, is about two hours for each, so that an analysis including the sittings, is made in five or six hours, exclusive of the time necessary for collecting the dust and separating the clay, for which a subsidence of twenty-four hours is allowed.

Weighing the Sediments.—The sediments are prepared for weighing by allowing them to subside completely, decanting the clear water as far as possible, rinsing them into a weighed platinum dish and igniting. The dish is cooled in a desiccator.

Effect of Boiling.—The analyses show a very decided increase in the particles smaller than 0.01 millimeter diameter at the expense of coarser particles as the result of boiling. The surfaces of the coarser particles are seen to be polished and of a lighter color than those not boiled. The surfaces of the unboiled particles are coated with a film of fine material probably cemented to them by clay. When these coarse particles which have not been boiled, are violently stirred with water for a short time, no fine particles are detached from them; and a careful examination under the microscope fails to reveal in any of the sediments more than an occasional grain exceeding the 0.05 millimeter limit by so much as 0.01 millimeter, or the 0.01 limit by as much as 0.005 millimeter. It would, therefore, appear that these small particles thus set free by long boiling are really a part of the larger ones and should be treated as such in a mechanical analysis of these soils.

193209. The French Method.—The Schloesing method^[142] as practiced by the French agricultural chemists^[143] differs essentially from those already described in attempting to first free the silt from carbonates and organic matter. It is conducted as follows:

One kilogram of the soil previously dried in the air, is taken and passed through a sieve of which the meshes are five millimeters. The agglomerated particles of earth are broken up by the hand. The pebbles are also taken out and weighed. The pebbles are then treated with hydrochloric acid until all effervescence is over. The insoluble part is dried and again weighed. The difference in weight gives the quantity of calcium carbonate contained on the external surface of the pebbles. The earth which passes the sieve of five millimeters mesh is next passed through a sieve having ten meshes to the centimeter. The masses on the sieve are broken up with the hand or with a pestle, in such a manner as to separate the fine agglomerated particles. The material which remains upon the sieve after being dried at 100°, is weighed. This gives the coarse sand. This is treated with hydrochloric acid as were the pebbles before, washed and the residue dried and weighed. The difference in weight gives the quantity of calcium carbonate adhering to the surface of the coarse sand.

The mechanical analysis is continued with the matter which has passed the sieve with ten meshes to the centimeter and which consists of the soil, properly so-called. Ten grams of this are taken, dried at 100° until no further loss takes place and the moisture thus determined. Another ten grams are taken and placed in a capsule with a flat bottom, and from nine to ten centimeters in diameter. This is moistened with a small quantity of water in such a way as to make a paste. This paste is rubbed with the finger in fifteen cubic centimeters of water. Ten seconds after the stirring is completed the supernatant liquid is poured into a precipitating jar of about 250 cubic centimeters capacity, taking great care not to allow any particles to pass over which have been deposited during that time. This operation is repeated in the same way waiting about ten seconds each time before decanting, until the decanted liquor is almost perfectly clear. In this way the particles of different fineness 194are separated. The decanted portions contain the fine sand and clay. The remaining portion contains the sand and particles of medium fineness. This last part is dried, being kept at 100° until it has a constant weight. It is afterwards treated with dilute nitric acid to dissolve the calcium carbonate. When the carbonate is abundant, it is sufficient to determine it by difference which is done by washing the material, drying and weighing. But when the proportion of carbonate is very small and in consequence when its exact determination acquires a greater importance, it is better to determine the lime directly. For this purpose the part soluble in dilute nitric acid is collected, treated with ammonia and acetic acid and precipitated with ammonium oxalate. Details of this operation will be given in another part of this manual.

In regard to the matter which is insoluble in nitric acid, it is composed chiefly of silica or silicates, and sometimes also of vegetable débris. The vegetable matter is determined by the incineration of the material which has been previously dried. The loss of weight gives the proportion of vegetable or organic débris contained in the soil and of combined water.

The portion which has been decanted, the volume of which should not exceed 500 cubic centimeters, is treated with nitric acid until effervescence ceases. It is then left to digest for some time, in order to permit the whole of the carbonate to dissolve. It is next thrown upon a smooth filter about one decimeter in diameter. After filtration it is washed to secure the complete elimination of the soluble lime salts. The lime is determined in the filtered liquid.

The insoluble portion contains the fine sand, the clay and humus bodies. In order to separate the three elements the precipitate which was received upon the filter, is rubbed with water, the filter is broken and all its contents washed through. The volume of wash water is made up to 200 cubic centimeters; two or three cubic centimeters of ammonia are added and the whole left to digest for two or three hours. The volume of the liquid is then made up to one liter with distilled water, vigorously shaking in such a way as to put all the matter in suspension. It is then left to settle for twenty-four hours. At the 195end of this time the supernatant liquid is decanted by the aid of a siphon. To the residue are added two cubic centimeters of ammonia and one liter of water. The matter is again brought into suspension and allowed to settle for twenty-four hours. The supernatant liquid is again decanted with a siphon, and added to the liquid previously removed. For ordinary soils two decantations are generally sufficient but when the soils contain a large quantity of clay it is convenient to decant three or four times. By an examination of the supernatant liquid it is easy to tell if the washings have been sufficiently prolonged. The decanted liquors contain the organic matter and that which it is convenient to call clay, which is constituted of very fine particles of sand and colloidal clay which play, in arable soil, a rôle somewhat like that of cement.

These matters are estimated in the following manner: The liquor is first treated with nitric acid and the clay and the humic matters are precipitated together. They are collected upon a smooth filter one decimeter in diameter and washed with water. By means of a washing bottle all the solid matters which have stuck to the sides of the filter are finally collected in the bottom of it. Since the last washings pass the filter very slowly, they can be removed after the complete deposition of the matter they contain, by means of a pipette. When all the liquid is removed the filter is placed upon blotting paper, great care being taken to avoid desiccation, having in view only the elimination of the excess of humidity. The folds in the filter are then carefully smoothed out with the finger. The matter which has collected upon the filter is then removed completely with a washing bottle, placed in a dish and dried at 100° and weighed. After weighing, the mass is incinerated in a muffle in order to destroy the humic bodies. The difference in weight before and after incineration, gives the total weight of the humic bodies and since the diminution in weight comprises not only the weight of the humic bodies, but also the weight of the combined water which is lost during the process of incineration, there should be subtracted from the total loss of weight ten per cent of the weight of the residual mineral matter, which represents the water of composition of the hydrated silicate.

196210. Statement of the Analysis.—Schloesing in his original paper^[144] recommends that the analysis be commenced with 1,000 grams of soil. The data of the analysis and the method of arrangement are illustrated by the following example.

The physical examination of the earth having been completed as above, the results can be tabulated as follows: taken, 1,000 grams of dry earth, digested in water, thoroughly worked by hand, sifted, and passed through the meshes of the sieve by a stream of water, the meshes having a diameter of one millimeter.

Humidity of the homogeneous paste, twenty-seven per cent. Then 945 grams of the dry sifted earth correspond to 945
1.00 − .27 = 1294.5 of paste.

The analysis, therefore, should be carried on upon this weight or some aliquot part say 0.01 thereof; viz., 12.945 grams.

Treatment of the Fine Elements.—Treated by nitric acid until a complete decomposition of the calcareous matter is secured, filtered, washed, the residual matter collected upon a filter, and the liquid received in a two-liter flask, a little ammonia added, allowed to digest, the flask filled with distilled water, left for twenty-four hours at repose, and decanted:

197Calculating these results to the original quantity of 1,000 grams the following data are obtained:

One thousand grams of dry earth contain:

There are counted as clay all the elements which have remained in suspension in the water after a period of repose of twenty-four hours. In fact, these elements comprise a notable proportion of very fine sand which is not deposited during that time. In order that the liquid should become entirely freed from this sand it would be necessary to wait several weeks and even several months. Such a prolongation of the analysis is evidently inadmissible. The period of twenty-four hours of repose therefore has been adopted. This is merely conventional, in the same way that the period of ten seconds adopted for the precipitation of the gravel is conventional. But this convention is justified by the fact that the substance which is called clay presents, when it has a proper degree of humidity and cohesion, a plasticity entirely analogous to that property of natural clay. Moreover, as has already been said, that which is chiefly important in these analyses is the employment of processes always comparable among themselves in their results and generally followed.

211. The Belgian Method.—The method of estimating the percentage of sand and clay practiced at the Gembloux Station^[145] is essentially that recommended by Schloesing with a few minor modifications.

With the ball of the thumb or with the finger, 100 grams of 198fine earth are rubbed with water in a porcelain capsule or mortar with a capacity of about 250 cubic centimeters. The suspended particles are poured off with the wash water and the process repeated five or six times, using in all about 200 cubic centimeters of water.

The water containing the sediment is rendered slightly acid (hydrochloric acid) adding the acid in minute particles with constant stirring for about an hour in order to dissolve all the carbonate and to separate the organic acids from the bases with which they are combined.

The liquid is allowed to remain at rest for five or six hours and a part of the liquor decanted to remove any supernatant particles of organic matter which may have passed the sieve in the original preparations of the sample. Filter through a smooth filter about twelve centimeters in diameter, wash until the chlorin has disappeared, and throw the filtrate away.

Break the filter paper over the vessel in which the soil was treated with hydrochloric acid and wash all the contents of the filter into this vessel with as little water as possible (about 100 cubic centimeters), add five cubic centimeters of strong ammonia water, allow to stand for three hours, shaking from time to time and with distilled water make the volume up to 250 cubic centimeters. Stir vigorously with a glass rod or spatula, take this out and wash any adhering particles back, leave at rest for twenty-four hours, siphon the turbid liquid into a two-liter vessel. Make the volume up again to 250 cubic centimeters and treat as above described and repeat the operation until the water becomes clear after standing for twenty-four hours. Usually eight or ten washings are necessary. Wash the residual sand into a weighed dish, evaporate to dryness, ignite and weigh. The weight obtained divided by the weight of the original sample gives the per cent of sand. The sand is separated by sieves of varying fineness into coarse, fine, and pulverulent sand.

Add to the ammoniacal liquor collected in the two-liter flask some powdered potassium chlorid (five grams per liter) to hasten the coagulation and rapid deposit of the clay.

After twenty-four hours siphon the clear liquor, collect the deposited clay in a smaller vessel, allow to remain at rest and 199decant as much of the clear liquor as possible. Pass through a plain tared filter about nine centimeters in diameter, dry at 150° and weigh the clay.

212. The Italian Method.—Schloesing’s method as carried out by the Italian chemists^[146] is as follows:

A kilo of earth dried in the air is passed through a sieve the threads of which are separated a distance of five millimeters; and with this the small pebbles are separated.

With another sieve having spaces of one millimeter, the coarse sand is separated. The pebbles and sand are dried, weighed, treated with hydrochloric acid and again weighed in order to find the quantity of calcareous matter contained in them. In ten grams of this fine earth the humidity is determined by drying at 100°.

Ten grams are mixed in a capsule with fifteen to twenty cubic centimeters of water and after eight to ten seconds the supernatant liquid is poured into a beaker having a capacity of 250 cubic centimeters. The same operation is repeated until there are contained in the beaker the fine sand and the clay, while the coarser sand remains in the capsule.

This last is then dried and weighed and the quantity of calcium carbonate determined by treating it with diluted nitric acid. By means of calcination the organic matter is determined. The liquid decanted in the beaker, the volume of which must not surpass 200 to 250 cubic centimeters, is treated with nitric acid, filtered after some time, washed and the calcium is directly determined by precipitating the solution with ammonium oxalate.

The part in the filter which contains the fine sand, the clay, and the humus material is mixed with water to a volume of about 200 cubic centimeters; there are then added to it two to three cubic centimeters of ammonia and after two or three hours it is diluted to a liter and strongly agitated.

After twenty-four hours of rest it is decanted and the residuum is treated a second time with diluted ammonia, decanting after twenty-four hours. Ordinarily these two treatments suffice, if, however, the earth is very argillaceous, this operation should be repeated three and even four times.

200The clay which is found in the liquid suspended in colloidal form coagulates and is precipitated by adding thirty to forty cubic centimeters of a saturated solution of potassium chlorid, while the humus substance, under the influence of the ammonia remains dissolved.

Sestini found that the method of Schloesing was the only one which indicated exactly the quantity of clay in the soil. He modified this method by reducing the time of rest from twenty-four hours, as proposed by Schloesing, to only twelve hours, a reduction which in his opinion does not in the least impair the exactness of the method.

Sestini also proposes twelve treatments instead of six.

213. General Principles.—The laws, already discussed, applying to the subsidence of a solid particle in a liquid, are equally applicable to the separation of the particle by imparting a motion to the liquid at a given rate. If a solid particle subside in a given liquid at the rate of one millimeter per second it follows that this particle will remain at rest if the liquid be set in motion upward with a like velocity. If the velocity be greater the particle will be carried upward and eventually out of the containing vessel. Such a particle is said to have a hydraulic value of one millimeter per second. If there be a perfect separation of a soil into its constituent particles and no subsequent flocculation, all the particles of one millimeter hydraulic value and less will be separated by a current of the velocity mentioned.

The general principles on which the separation rests, therefore, are the securing of the proper granulation of the sample and the maintenance of a fixed velocity of the current until the separation is finished. The separation must be commenced with a period of subsidence so as to remove first of all the suspended clay or impalpable particles. The velocity can then be increased in a certain fixed ratio to secure a separation into particles of any required hydraulic value.

214. Nöbel’s Apparatus.—One of the earliest methods of 201separating the soil particles by a moving liquid is that of Nöbel.^[147] The apparatus is shown in Fig. 28. The four separating vessels 1, 2, 3, 4 are of glass, pear shaped, and have a relative capacity of 1³, 2³, 3³, 4³, or 1 : 8 : 27 : 64. No. 4 has an outlet tube leading to the beaker B, of such a capacity as to allow the passage of just nine liters of water in forty minutes, constant pressure being maintained by means of a Mariotte’s bottle or of the constant level apparatus A, a, b, which is connected with the main water supply through the tube a by means of a rubber hose. The reservoir C should hold about ten liters. The sample of soil to be separated should be previously boiled and passed through a sieve having circular openings one millimeter in diameter. The flask in which the sample is boiled is allowed to stand for some time when the muddy supernatant liquid is poured into elutriator No. 2 and the remaining sediment washed into No. 1. No. 1 is filled with water by connecting it with the water supply and opening the pinch-cock p. The water is carefully admitted until the air is all driven out and Nos. 1 and 2 connected. The cock p is then opened and the vessels all filled, and the water allowed to run into B for forty minutes, the level being maintained uniformly at A.

Figure 28.

Nöbel’s Elutriator.

Of the water used, four liters are found in the elutriating vessels and nine liters in the receiving vessel No. 5. The apparatus is left standing for an hour until the liquid in the elutriators is clear and the portions in each vessel are received on weighed filters dried at 125°, and the weight of each portion determined.

It is recommended that the loss on ignition of each part be 202also determined. The separated particles thus secured are classified as follows:

No. 1. Débris and gravel. No. 2. Coarse sand. No. 3. Fine sand. No. 4. Clayey sand. No. 5. Finest parts or clay.

Although the method of Nöbel has been much used, the results which it gives are entirely misleading. The convection currents produced in the conical vessels by the passing water and the flocculation of the soil particles prevent any sharp separation into classes of distinct hydraulic value. The process may be useful for a qualitative test, but its chief claim to a place in this manual is in its historic interest arising from its use in the first attempts at silt analysis.

215. Method of Dietrich.^[148]—The difficulties attending the silt separation by the Nöbel method, led Dietrich to construct an apparatus in which the sides of the elutriating vessels were parallel, but these vessels, with the exception of the first, were not set in an upright position.

Figure 29.

Dietrich’s Elutriator.

The apparatus (Fig. 29) consists of a series of cylindrical vessels connected by rubber tubing.

203The elutriators are of the following dimensions:

No. 1. Seventeen centimeters long, 2.8 centimeters in diameter, position upright.

No. 2. Thirty-four centimeters long, four centimeters in diameter, inclined 67°.5.

No. 3. Fifty-one centimeters long, 5.2 centimeters in diameter, inclined 45°.

No. 4. Sixty-eight centimeters long, 6.4 centimeters in diameter, inclined 22°.5.

The rubber tubes passing from one vessel to the other are furnished with pinch-cocks so that each one of the elutriating vessels can be shut off from the others and independently removed from the circuit.

The stream of water is made to pass through the apparatus under a constant pressure of one meter.

Only the fine earth, boiled with water or hydrochloric acid, is to be placed in the apparatus. The part coming through a sieve with a mesh 0.67 millimeters is to be used and placed in No. 1. About thirty grams of soil, are employed for each elutriation. Before adding the soil, the air is completely removed from all parts of the apparatus by connecting it with the water supply and allowing it to be filled with water.

The rate of flow is controlled by the orifice of the last effluent tube and the analyst is directed to continue the operation until the effluent water collected in the beaker glass (5) is clear. The particles then remaining in each of the vessels are collected separately.

The author of the method claims that in respect of likeness of particles the results are especially gratifying and that duplicate analyses give results fully comparable. The process, however, has not commended itself to analysts, but it marks a distinct progress toward the principles of later investigators. Had each of the elutriating vessels been placed upright and the rate of flow determined, the apparatus of Dietrich would have served, to a certain extent, for the more rigid investigations of his successors.

216. Method of Masure.—The sifted earth, from ten to fifteen grams, is carefully mixed with 200 cubic centimeters of water. 204It is then introduced into a doubly conical elutriator B, Fig. 30, of about 250 cubic centimeters capacity. A current of distilled water is allowed to flow from a Mariotte’s bottle, A, which secures a regular and constant flow. The bottle A is joined to the elutriator B by means of a rubber tube and the vertical glass tube D, the top of which is expanded into a funnel for the purpose of receiving the water from the Mariotte flask. The current of water flowing upward through the elutriator B carries in suspension the most finely divided particles of clay, and these are collected with the emergent water in the receiver C. The sand and coarser particles of clay remain in the elutriator. The water flows out by the tube F, the diameter of which should be less than that of D. When the emergent water becomes limpid the operation is terminated. After the apparatus is disconnected, the water is decanted from the sand in the elutriator, and the whole residue is weighed after drying for two hours at 110°.

Figure 30.

Masure’s Silt Apparatus.

The fine soil collected in C may also be separated and weighed, for control, after drying as above.

The pebbles and coarse sand separated by the sieves should also be weighed. By this process the soil is separated into four portions; viz.,

(1) Pebbles. (2) Coarse sand. (3) Fine sand and other materials not carried off by the current of water. (4) Fine soil, carried into the receiver C.

205217. Method of Schöne.—The method of Schöne^[149] is based on the combination of a cylindrical and conical separatory tube through which the flow of water is regulated by a piezometer.

If, in the process of silt separation, the water move perpendicularly upward with a given velocity, e.g. = v the separation is dependent:

(1) On the volume of the silt particles, (2) On their specific gravity, and, (3) On their state of disintegration.

If it be assumed that the silt particle is a sphere with a diameter = d, then according to Newton’s law of gravity, the following formula would be applied: d = v² (3Z
4g (S − 1)).

Figure 31.

Schöne’s Elutriator.

In the above formula Z = a coefficient which depends on the condition of the surface against which the hydraulic pressure or resistance works, in this case a sphere; g = the acceleration of gravity equivalent to 9.81 meter; and S = the specific gravity of the particle.

This expression signifies that in a given case, the velocity of the current in the apparatus is just sufficient to counteract the tendency of a given particle to sink. All particles of a smaller diameter, in such a case, will be carried on by the current, while all of a greater diameter would separate by sedimentation. These theoretical conditions are not met with in practice where silt particles of all shapes and degrees of aggregation abound. These particles, whatever their shape, may be said to have the same hydraulic value when carried by the same current. It is necessary, therefore, to secure some uniform standard of expression to assume a normal form of particle and a normal specific gravity. For the form, a sphere is evidently the normal which must be considered and for specific gravity that of quartz is taken; viz., 2.65. The mean coefficient for Z may also be placed at 0.55, although slightly different values are ascribed to it. Substituting these values in the formula, it is reduced to the expression; d = v² × 0.0000255 millimeters. It can, therefore, be said that by this or that velocity of the current, silt particles will be 206removed of this or that diameter, it being understood that all particles of equal hydraulic value to spherules of quartz of the given diameter are included in each class. In order to have the theoretical formula agree with the results of analysis it is necessary to modify it empirically to read d = v^⁷⁄₁₁ × 0.0314 millimeters.

This formula is found to agree well with the results obtained for all velocities between 0.1 millimeter and twelve millimeters per second, the ordinary limits of silt separation.

The Apparatus.—The conic-cylindrical elutriating vessel A, B, C, D, E, F, G, Fig. 31, is of glass. The part B, C, is cylindrical, ten centimeters in length and as nearly as possible five centimeters in diameter.

The conical part C, D, is fifty centimeters in length. Its inner diameter at D must not be greater than five centimeters nor smaller than four centimeters.

The bend, D, E, F, should have the same diameter; viz., four to five centimeters.

The part A, B, C, D, and D, E, F, G, may be made of separate parts and joined by a rubber tube.

Outflow Tube and Piezometer.—The outflow tube and piezometer, H, J, K, L, is constructed as shown in Fig. 32. It should be made of barometer tubing having an internal diameter of about three millimeters. The tube is bent at J at an angle of forty to forty-five degrees. The knee J must be as acute as possible not to interfere with the inner diameter. The form and especially the magnitude of the outlet are of great importance. It must be circular and nearly 1.5 millimeter in diameter. It must not be larger than 1.67 millimeter nor smaller than 1.5 millimeter. The opening should be so made as to direct the stream of outflowing liquid in the direction shown by the arrow.

Figure 32.

Schöne’s Elutriator, Outflow Tube.

The piezometer L, K is parallel to the arm H, J, of the delivery tube. Its graduation has its zero point in the center of the outlet K. It commences with the one centimeter mark. From 207one to five centimeters it is divided into millimeters, from five to ten centimeters into one-fourth centimeter, from ten to fifty centimeters into one-half centimeter, and from fifty to 100 centimeters into centimeters. The dimensions given are those required for ordinary soils and for velocities ranging from two-tenths millimeter to four millimeters per second.

For greater velocities, a delivery tube with a larger outlet must be used and the piezometer must be of greater internal diameter than indicated.

Figure 33.

Schöne’s Elutriator, Arrangement of Apparatus.

Arrangement of the Apparatus.—The apparatus is conveniently mounted as shown in Fig. 33, giving front and side views 208of all parts of apparatus in position ready for use. When numerous analyses are to be made much time is saved by having a number of apparatus arranged en batterie.

The Sieve.—The soil, before being subjected to elutriation, should be passed through a sieve of which the meshes are 0.2 millimeter square.

The Process.—To measure the diameter of the cylinder, two marks are made with a diamond upon the glass which are distant from each other a certain space, for instance, h centimeters. The space between these two marks is filled with water exactly measured. Suppose that a cubic centimeters were used, then the diameter is determined by the formula:

In order to determine that the elutriating cylinder is strictly comparable in all its parts this measurement should be made upon several parts thereof.

The apparatus should now be tested in regard to the quantity of liquid which it will deliver under a given pressure in the piezometer. By means of the stop-cock H the flow of water is so regulated that the outflow at c can be measured at a given height of the water in the piezometer. Suppose that a cubic centimeters of water flow in t seconds, then the quantity which would flow in one second is determined by the formula, Q = a
t cubic centimeters. Since according to the law of hydraulic outflow the quantities are proportional to the square root of the height of the column it is easy to compute from any given height the quantity which will flow from any other one desired. For the retardation due to capillary attraction, it is sufficient, in general, to take it in a constant quantity; if this constant quantity be represented by C, the observed height of the water in the piezometer by h, and the quantity of water flowing out by Q, the data required for any given velocity can be calculated from the following proportion:

It is necessary to compute the magnitude of this constant C 209which is to be subtracted. This is accomplished by measuring the quantity of water which flows out at two different heights of the column in the piezometer. From the foregoing proportion, the value of C is as follows:

The value of C can be the more exactly determined as h₁ is greater and h₂ smaller. It is best to choose the lowest height from which an exact reading can be made; that is, by which the regular rise and fall of the level of the water in the piezometer (in consequence of the formation of drops) just begins to disappear. This usually takes place when h₂ = 1.5 centimeter to 1.7 centimeter. For the higher value h₁ it is best to take about 100 centimeters. Suppose, for example, the following results are obtained:

The same quantity of water which flows out in a unit of time passes also at the same time over a cross section of the elutriating cylinder. The diameter of this cylinder being D the equation is derived

Since the velocity in the elutriating cylinder v is directly as the quantity of water overflowing so is

210is obtained from the means of a number of estimations; for example as illustrated in the following data:

Then are obtained the following values of hₙ and vₙ:

In order to be able easily and rapidly to judge under what pressure the outflow has taken place in any particular instance, a larger number of values are computed with the help of the formula given and placed together in tabular form. As an example the following table may serve which was computed for one of the apparatus used. Usually it will be sufficient to test the apparatus for four different heights and then to interpolate the values for all the others. The numbers marked with a star in the table are those which were determined by experiment; the others were calculated.

Suppose the problem is by means of the apparatus tested above, to separate into a number of groups a mixture of silt particles, whose hydraulic values are found between the following diameters: 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07 millimeters. The table will show at once under what pressure of water the piezometer must be placed in order to give the values; viz., 1.4, 2.8, 7.0, 15.0, 29.0, 53.0, and 83.0 centimeters respectively.

The apparatus described above, is adapted for velocities in the elutriating cylinder varying from two-tenths millimeter to four millimeters per second. The largest silt particles which can be separated by the velocities given above, have approximately a diameter of 0.08 millimeter. For the separation of larger particles a sieve can take the place of the silt apparatus. If, however, it be desired to subject larger particles to silt analysis, the dimensions of the elutriating cylinder and of the outlet of the delivery tube must be changed accordingly.

212Preparation of Sample.—The conduct of silt analysis of natural soils must, in certain cases, be preceded by a special treatment of the sample. If the latter be rich in humus the organic substance must previously be separated as completely as possible. With sandy soils this can be accomplished by ignition. With clayey soils, on the contrary, it is to be performed by boiling the soils at least one hour with water which contains from one to two per cent of free alkali. Soils which contain lime must also be subjected to treatment with dilute hydrochloric acid, and the hydrochloric acid must be as carefully removed, as possible before the sample is subjected to elutriation; afterward follows the boiling of the sample in the ordinary way with water. This, of course, can be omitted when it has already been treated with boiling dilute alkali. It is also important to remove the larger particles by a sieve before the elutriation begins. It is well to pass a sample through a sieve after it has been boiled, by which all particles of a larger diameter than 0.2 millimeter are removed. This will usually require about one liter of water and this water should be allowed to rest from one to two hours and poured off with the suspended material which it contains. Only what subsides should be brought into the apparatus. In rinsing the sample as much water must be used as will fill the apparatus up to its cylindrical portion.

After the sample has been placed in the apparatus, the water is allowed slowly to enter, being careful to avoid reaching more than the lowest required velocity, until the outflow begins. The water then is so regulated by the stop-cock as to bring it to the desired height in the piezometer. This being accomplished, the different velocities which have been decided upon for separating the particles of silt are used one after the other, as soon as all the silt which can be removed at each given velocity, has been secured. From three to five liters of water will be required for the separation of each class of particles. Sometimes the reading of the height of the water in the piezometer is difficult; as, for instance, when foam or bubbles accumulate therein. 213These bubbles can be removed by simply blowing into the tube, or dropping into it a little ether. The outflow of water can be received in vessels, beaker glasses, or cylinders, in which it is allowed to subside. The finest particles which remain in suspension in the water are best determined by difference. If it be desired to weigh them directly, the water can be treated with ammonium bicarbonate until it contains from one to two per cent thereof. The precipitation then takes place in a few hours.

The collection and weighing of silt particles are accomplished in the usual way. That which finally remains in the elutriating vessel is taken out after the end of the operation by closing the stop-cock, removing the stoppers with the piezometer tube, pouring the contents of the elutriating vessel into a beaker glass and rinsing out carefully all adhering particles. Examples of the working of the apparatus follow:

The soil was taken from the Imperial Russian Agricultural Experimental Institute at Gorki. It was a fine clay sand and was carefully treated with hydrochloric acid. The results of the analysis are given in the following table: