II. Chemical Composition of a Soil.—Chemically considered, the soil is a body of great complexity. It is made up of a great variety of substances. The relations existing between these substances and the plant are not all of equal importance; some—and these form by far the largest proportion of the soil-substance—are concerned in acting simply as a mechanical support for the plant, and in helping to maintain those physical properties in the soil which, as we have just seen, exercise such important functions in the plant's development.
Fertilising Ingredients.
A small portion of the soil-substance, however, takes a very much more active part in promoting plant-growth, by acting as direct food of the plant. As we have already seen in the Introductory Chapter,[50] the substances which have been found in the ash of plants are the following: potash, lime, magnesia, oxide of iron, phosphoric acid, sulphuric acid, soda, silica, chlorine, oxide of manganese, lithia, rubidia, alumina, oxide of copper, bromine, and iodine. The general presence of some of these substances is doubtful; the presence of others, again, probably purely accidental; while some are only found in plants of a special nature, as, for instance, iodine and bromine, which are only found in the ash of marine plants.
Of these ash constituents, only the first six substances—those marked in italics—are absolutely necessary to plant-growth. In addition to these six ash constituents, the plant also derives its nitrogen, which is a necessary plant-food, chiefly from the soil.[51]
Importance of Nitrogen, Phosphoric Acid, and Potash.
But of these seven constituents of the soil which are necessary to plant-growth, some have come to be regarded by the agriculturist with very much greater interest than others. This is due to the fact that they are normally present in the soil in very much smaller quantities than is the case with the other equally necessary food ingredients; that, in short, they are nearly invariably present in the soil, in a readily available form, in lesser quantities than the plant is able to avail itself of, and often, as in impoverished or barren soils, in quantities too small for even normal growth. These ingredients are nitrogen, phosphoric acid, and potash.[52]
The importance of seeing that all the necessary plant ingredients are present in a soil in proper quantities will be at once properly estimated when it is stated that the absence or insufficiency in amount of one single ingredient is capable of preventing the growth of the plant, although the other necessary ingredients may be even abundantly present.
With lime, magnesia, iron, and sulphuric acid, most soils are abundantly supplied. The substances with which the farmer has to concern himself, then, are nitrogen, phosphates, and potash. It is these substances therefore, that, as a rule, are alone added as manures.
Chemical Condition of Fertilising Ingredients in Soil.
But in considering the chemical properties of a soil, a simple consideration of the quantity of the different ingredients present is not enough. A very important consideration is their chemical condition. Ere any plant-food can be assimilated by the plant's roots, it must first be rendered soluble. The quantity of soluble, or, as it is known, available, plant-food in a soil is very small. It is, of course, being steadily added to each day by the process of disintegration constantly going on in soils.
Amount of Soluble Fertilising Ingredients.
The exact nature and dissolving capacity of the soil-water, charged as it is, to a greater or less extent, with different acids and salts, as well as the dissolving power of the sap of the rootlets of the plant itself, render the exact estimation of the available fertilising constituents wellnigh impossible. An approximate estimate, however, may be obtained by treating the soil with pure water and dilute acid solutions. The treatment of the soil with dilute acid solutions is for the purpose of simulating, as nearly as may be done, the conditions it is submitted to in the soil. By treating a soil with water, we obtain a certain amount of plant-food dissolved in the water. This can only be regarded as indicating approximately the amount available at that moment to the plant. But every day, thanks to the numberless complicated reactions going on in the soil, this soluble plant-food is constantly being added to. Considerations such as the above, together with our ignorance as to the exact combinations in which the necessary minerals enter the plant, will serve to indicate the great difficulty of this part of the subject.[53]
Value of Chemical Analysis of Soils.
It is largely for these reasons that a chemical analysis of a soil is from one point of view of little value in giving evidence of its actual fertility. What it demonstrates more satisfactorily is its potential fertility. It is useful in revealing what there is present in it, not necessarily, however, in an available condition. Under certain circumstances it may be made of great value, as, for example, when we are anxious to know what will be the result of certain kinds of treatment, such as the application of lime, &c.
It is hardly advisable, therefore, to place before the reader a number of soil analyses. That he may obtain an approximate idea of the composition of a soil, one or two representative analyses will be found in the Appendix,[54] along with a short account of the chief minerals out of which soils are formed.
A point of considerable interest is the quantity per acre different soils contain of nitrogen, phosphoric acid, and potash. Although the amount of these ingredients when stated in percentage seems very trifling, yet when calculated in lb. per acre, it is seen to be in large excess of the amount removed by the different crops. This question will be dealt with in succeeding chapters.
A point of further interest is the chemical form in which the necessary plant constituents are present in the soil. For information on this point the reader is referred to the Appendix.[55]
The third class of properties which affect the fertility of a soil are those which have been termed the biological.
III. Biological Properties of a Soil.—The important functions which modern discoveries have shown to be discharged by minute organic life in the terrestrial economy are nowhere more strikingly exemplified than in the important rôle they perform in the soil.
Bacteria of the Soil.
The soil of every cultivated field is teeming with bacteria whose function is to aid in supplying plants with their necessary food. The nature of, and the functions performed by, these organisms differ very widely. Regarding many of them we know very little; every day, however, our knowledge is being extended by the laborious researches of investigators in all parts of the world, and it is to be anticipated that ere long we shall be in possession of many facts regarding the nature and the method of the development of these most interesting agents in terrestrial economy. That they are present, however, in enormous numbers in all soils we have every reason to believe, one class of organism connected with the oxidation of carbonic acid gas being estimated to be present to the extent of over half a million in one gramme of soil[56] (Wollny and Adametz). One class—and their importance is very great in agriculture—prepare the food of plants by decomposing the organic matter in the soil into such simple substances as are easily assimilated by the plant. The so-called "ripening" of various organic fertilisers is effected, we now know, entirely through the agency of bacteria of this class. Plant-life is unable to live upon the complex nitrogenous compounds of the organic matter of the soil, and were it not for bacteria these substances would remain unavailable. Attention will be drawn in the Chapter on Farmyard Manure to this question more in detail. Of these bacteria, among the most important are those which are the active agents in the process known as "nitrification"—i.e., the process whereby organic nitrogen and ammonia salts are converted into nitrites and nitrates. The presence of these organisms, it would appear, is indispensable to the fertility of any soil. There are organisms, on the other hand, which have the power of reversing the work of the nitrification bacteria by converting nitrates into other forms of nitrogen. The reduction of nitrates in the soil is often the source of much loss of valuable nitrogen, which escapes in the free state, so that the action of bacteria is not altogether of a beneficial nature.
Three Classes of Organisms in the Soil.
So far as the subject has been at present studied, the micro-organisms in the soil may be divided into three classes.[57]
First Class of Organisms.
We have, first of all, those whose function it is to oxidise the soil ingredients. Organisms of this class may act in different ways. They may assimilate the organic matter of the soil and convert it into carbonic acid gas and water; or, on the other hand, they may oxidise it by giving off oxygen. Some of these organisms, whose action is of the first kind, choose most remarkable materials for assimilation. One has been found to require ferrous carbonate for its development, which it oxidises into the oxide (Winogradsky); while another,[58] the so-called sulphur organism, converts sulphur into sulphuretted hydrogen according to some, and according to others into sulphates. To this class of organism the nitrifying organisms belong. As will be seen more fully in a subsequent chapter, two distinct organisms connected with this process have already been isolated and studied—one of these effecting the formation of nitrites from organic nitrogen or ammonia salts, and the other the conversion of nitrites into nitrates. The second method in which these oxidising organisms act is by giving off oxygen. There is much interest attaching to this fact, as it was supposed till quite recently that all evolution of oxygen in vegetable physiology was dependent on the presence of light, and also intimately connected with chlorophyll, or the green colouring matter of plants. It would seem, however, that among the soil organisms these conditions are not necessary, and the evolution of oxygen may be carried on in the case of colourless organisms as well as in the case of light. With organisms of this kind every soil is probably teeming. A typical example is the organism which is the active agent in the oxidation of carbonic acid gas, and which has already been referred to as existing in the soil in such numbers.[59]
The Second Class of Organisms in the Soil.
The second class of organisms are those which reduce or destroy the soil constituents. The most important of these, from the agricultural point of view, are those which effect the liberation of nitrogen from its compounds. In the putrefaction of organic matter the organisms chiefly act, it is probable, in the entire absence of atmospheric oxygen; but it would seem, however, that they may also act in the presence of oxygen. It is through their agency that the soil may lose some of its nitrogen in the "free" form. To this class belong the denitrifying organisms already referred to which reduce the nitrates and nitrites in the soil.[60]
Third Class of Organisms.
The third class of organisms are those by whose agency the soil is enriched. Of this class those fixing the free nitrogen from the air are the most important. The nature of these organisms is still somewhat obscure, but that leguminous plants have the power of drawing upon this source of nitrogen is now a firmly established fact. Further reference to these interesting organisms may be delayed to another chapter.
The important point to be emphasised is, that for the healthy development of these organisms, which are so necessary in every fertile soil, certain conditions must exist. These necessary conditions will be treated more in detail later on. It is sufficient to notice that they have to do with the physical properties as well as the chemical composition of the soil. This furnishes a further reason for the necessity of having the mechanical condition of a soil satisfactory.
From what we have said, it will be seen that the question of soil-fertility is a very complicated one, and depends on numerous and varied conditions; that the properties which constitute fertility, while seemingly very widely different in their nature, in reality influence one another to a very great extent; that not merely is the presence in a soil of the necessary plant constituents necessary to fertility, but that the possession by the soil of certain physical or mechanical properties is equally necessary; while, lastly, we have seen that the presence of certain micro-organic life is bound up with the problem of fertility in a very direct and practical manner.
The importance of the conditions, other than those of a purely chemical nature, have been thus far somewhat prominently emphasised, for the reason that in what follows attention will be almost exclusively devoted to the purely chemical conditions of fertility. It is well, then, to realise that, while the latter conditions are by far the most important, so far as the farmer is practically concerned, inasmuch as they are most under his control, they are not the only conditions, and are not by themselves able to control fertility.
FOOTNOTES:
[33] This statement perhaps needs qualification. While the important rôle played by the physical qualities of the soil were in the early years of the science recognised, of more recent years the chemical composition of the soil has been engaging almost exclusive investigation. Physical properties of the soil have recently acquired a further importance in the eyes of the agricultural chemist, from the important influence they exert on what we have here called the biological properties of a soil—viz., the development of those fermentative processes whereby plant-food is prepared to a large extent.
[34] A good example of the absorptive capacity of a soil containing a large quantity of vegetable matter is furnished by peat-bogs, which, sponge-like, can absorb enormous quantities of water. (See Appendix, Note I., p. 98.)
[35] Jethro Tull, an early well-known agricultural writer, who lived about the middle of last century, propounded the theory, that as the food of plants consisted of the minute earthy particles of the soil, all that was required by the skilful farmer was to see that his soil was properly tilled. He accordingly published a work entitled 'Horse-hoeing Husbandry,' in which he advocated a system of thorough tillage. (See Historical Introduction, p. 10.)
[36] See Introduction, p. 55.
[37] See Introductory Chapter, p. 55.
[38] It is not exactly known why excess of water should prevent normal growth in the plant. Probably it is on account of the fact that free access of oxygen is hindered in such a case. The roots are thus not freely enough exposed to this necessary gas, and fermentative processes of the nature of nitrification are not promoted. It may be also due to the fact that the solution of plant-food is too dilute when such excess of water prevails.
[39] See Appendix, Note II., p. 98.
[40] Some experiments by E. Wollny show this. He found, when experimenting with summer rape, that the best results were obtained when the soil contained only 40 per cent of its total water-holding power; when the amount was either lessened or increased the results obtained fell off. The effect of either too little or too much water is seen in the development of the different organs of the plant as well as on its period of growth, much water seeming to retard the growth. The quality of the plant seems also to be influenced by this condition. Experiments on cereal grains by Wollny show that not merely is the texture of the grain influenced, but that much moisture lessens the percentage of nitrogen. Wollny is of the opinion that for crops generally, the best amount is from 40 to 75 per cent of the total water-holding capacity of the soil.
[41] See Appendix, Note III., p. 99.
[42] See p. 55.
[43] The effect of the temperature of the soil on the development of the plant is most important. This is especially marked at the period of germination, but is felt at subsequent periods of growth. Up to a certain temperature the warmer the soil the more rapid the plant's development. In this country the temperature most favourable to growth is rarely exceeded, or indeed reached.
[44] See Chapter on Farmyard Manure.
[45] As will be seen further on, the fermentation of organic substances is caused by the action of micro-organic life.
[46] See Appendix, Note IV., p. 100.
[47] Of course it must be remembered that a large amount of carbonic acid in soils comes from the decay of vegetable matter. Soils are twenty to one hundred times richer in carbonic acid than the air.
[48] See Chapter III., p. 119.
[49] See Introduction, p. 40.
[50] See Introductory Chapter, p. 54.
[51] See pp. 44 and 135.
[52] Occasionally also lime.
[53] See Appendix, Notes V. and VI., pp. 100, 101.
[54] Note VI., p. 101.
[55] Note VII., p. 107.
[56] Even larger estimates of the number of germs in a gramme of soil have been made—from three-quarters to one million (Koch, Fülles, and others).
[57] These organisms consist of molds, yeast, and bacteria, the last-named being most abundant. In the surface-soil, among the bacteria, bacilli are most abundant. Micrococei are not abundant.
[58] Investigated by Winogradsky, Olivier, De Rey Pailhade, and others.
[59] Organisms of this kind have been investigated among others by Heraüs, Hueppe, and E. Wollny. According to the two first-mentioned investigators, certain colourless bacteria effect the formation in the absence of light from humus and carbonates a body resembling in its nature cellulose.
[60] Investigated by Springer, Gayon and Dupetit, Dehérain, and Marguenne.
APPENDIX TO CHAPTER I.
NOTE I. (p. 68).
The following determinations by Schübler show the absorptive power of different kinds of soil-substances. These were obtained by soaking weighed quantities of the soil in water, and allowing the excess of liquid to drain away, and weighing the wet earth.
| Per cent of water absorbed by 100 parts of earth. |
|
| Siliceous sand | 25 |
| Gypsum | 27 |
| Calcareous sand | 29 |
| Sandy clay | 40 |
| Strong clay | 50 |
| Arable soil | 52 |
| Fine calcareous | 85 |
| Garden-earth | 89 |
| Humus | 190 |
It has been calculated that the absorptive power of a mixture of different substances is not simply equal to the sum of their separate ingredients.
NOTE II. (p. 74).
Evaporation.
The retentive property of a soil for water tends to retard evaporation. The following table by Schübler shows the rate at which evaporation proceeds in different soils. The experiment was conducted in the following way. The soil experimented upon was saturated with water and spread over a disc, and allowed to evaporate for four hours, when it was weighed. The amount of time required for the evaporation of 90 per cent of the water was also estimated. Of 100 parts of water in the wet soil there evaporated, at 60° Fahr.—
| In four hours— | Time required to evaporate 90 per cent. |
||
| From— | per cent. | Hours. | Minutes |
| Quartz | 88 | 4 | 4 |
| Limestone | 76 | 4 | 44 |
| Sandy clay | 52 | 5 | 1 |
| Stiffish clay | 46 | 6 | 55 |
| Loamy clay | 46 | 7 | 52 |
| Pure grey clay | 32 | 11 | 17 |
| Loam | 32 | 11 | 15 |
| Fine calcium carbonate | 28 | 12 | 51 |
| Humus | 21 | 17 | 33 |
| Magnesium carbonate | 11 | 33 | 20 |
NOTE III. (p. 76).
Hygroscopic Power of Soils.
Davy found the hygroscopic power of soils to be as follows. He found that 100 parts by weight of three samples of different sands absorbed 3, 8, and 11 parts of water, respectively, in one hour; while three loams absorbed similarly 1.3, 1.6, and 1.8 parts.
The following samples of soil were dried at 212° Fahr., and exposed to an atmosphere saturated with water and a temperature of 62° Fahr., when it was found they absorbed the following amounts in twelve hours' time:—
| Quartz sand | 0.0 |
| Limestone sand | 0.3 |
| Lean clay | 2.1 |
| Fat clay | 2.5 |
| Clay soil | 3.0 |
| Pure clay | 3.7 |
| Garden-loam | 3.5 |
| Humus | 8.0 |
Gases present in Soils.
The air which we find enclosed in the pores of the soil is distinctly poorer in oxygen than ordinary air. Boussingault found the percentage of oxygen in a sandy soil, freshly manured and wet with rain, to be as low as 10.35 per cent; while the air in forest-soil contained 19.5 per cent of oxygen, and .93 per cent of carbonic acid. The percentage of oxygen in soils depends on the rate of decay of the organic portions. The depth of the soil-layer also determines the quantity. This is owing to the fact that diffusion takes place more slowly deep down than near the surface.
NOTE V. (p. 90).
Amount of Soluble Plant-food in the Soil.
Two of the most reliable methods of ascertaining an approximation of the quantity of soluble soil constituents are (1) by treating the soil with distilled water, and (2) by analysing the drainage-water. With regard to the former of these two methods, it has been found that even the amount of fertilising matter dissolved out by pure distilled water varies. This variation depends on the amount of distilled water used, as well as the length of time the soil is left in contact with the solvent. By washing the soil with different quantities of water, different amounts of soluble soil ingredients will be found to have been washed out; for although the first washings contain by far the greater portion of the soluble matter, each subsequent washing will be found to contain further quantities.
A number of experiments have shown that 1000 parts of distilled water dissolved out from different soils from one half to one and a half parts of soluble constituents; or from .05 to .15 per cent. Of this soluble matter from 30 to 67 per cent is mineral in its nature, and from 33 to 70 per cent organic. Poor sandy soils yield the minimum quantity, while peaty soils yield the maximum. The quantity of soluble matter in a regular peaty soil may vary from .4 to 1.4 per cent; this consists chiefly, however, of organic matter. (See Johnson's 'How Crops Feed,' p. 312.)
Perhaps a more satisfactory method is by analysing the drainage-water of a soil. This has been found to vary very considerably in composition. The average of a large number of analyses are .04 to .05 per cent of dissolved matter. Of this dissolved matter the largest proportion is made up of organic matter, nitric acid, lime, and soda salts. It must be borne in mind, however, that even the drainage-water does not furnish an exact indication of the amount of dissolved matter in a soil. Much, perhaps the largest proportion of dissolved matter, never finds its way into the drainage-water. That contained by the drainage-water really represents the surplus quantity of dissolved matter which the soil is unable to retain, and which is thus washed by the rain into the drains. The composition of drainage-water is interesting, as it shows that, practically speaking, all the necessary plant ingredients are in a state of solution in the soil.
NOTE VI. (p. 90).
Chemical Composition of the Soil.
The most important substances present in soils are as follows: silica, alumina, lime, magnesia, potash, soda, ferric oxide, manganese oxide, sulphuric acid, phosphoric acid, and chlorine. Of these substances the presence of alumina, silica, lime, and, in certain cases, magnesia, along with the organic portion of the soil—the humus—has the chief influence in determining the nature and the physical properties of a soil.
In order to clearly understand to what it is soils owe the nature of their chemical composition, it is necessary to consider the composition of some of the chief minerals out of the disintegration of which soils are formed.
While we know of some seventy elements present in the earth's crust, it is practically made up of only some sixteen. These sixteen are—oxygen, silicon, carbon, sulphur, hydrogen, chlorine, phosphorus, iron, aluminium, calcium, magnesium, sodium, potassium, fluorine, manganese, and barium.[61] Of these, oxygen is by far the largest constituent, forming, roughly speaking, about 50 per cent.
The main mass of the rocks consists of silica, and this is generally combined with alumina, as in clay, forming aluminium silicate, and with the commoner alkalies and alkaline earths. Another extremely abundant compound is carbonate of lime, which, as limestone, chalk, and marl, forms one-sixth of the earth's total rocks.
The word "mineral" means a definite chemical compound of natural occurrence. The number of minerals is very great, and it is impossible to go into the subject here. Reference can only be made to a few of the more prominent ones, which are chiefly concerned in the formation of soils.
Those formed out of silicates are, from the agricultural point of view, the most important, as they form a very large group; and it is by their disintegration that soils are chiefly formed. They consist of silica and alumina, along with various other substances, chiefly alkalies and alkaline earths. It is important to note one peculiarity about the solubility of silicates. We have two classes of silicates: the one, which is called "acid," and contains an excess of silica; the other, "basic," and which contains an excess of base. Now, while the former of these is more or less insoluble, the second is soluble. This fact has an important signification in the process of the disintegration of the silicate minerals we are about to consider.
The first and most important class are the Felspars. Felspar is not really a definite mineral, with a definite chemical composition, but rather the name of a class of minerals of which there are several different kinds. The felspars are composed of silica and alumina, along with potash, soda, and lime, with traces of iron and magnesia. Their principal constituents, however, are silica and alumina, along with either potash, soda, or lime. According as the base potash, soda, or lime predominates, the felspar is known as Orthoclase, Albite, and Oligoclase, respectively.
The following are the analyses of the three minerals (by the late Dr Anderson):—
| Orthoclase. | Albite. | Oligoclase. | ||||
| 1. | 2. | 1. | 2. | 1. | 2. | |
| Silica | 65.72 | 65.00 | 67.99 | 68.23 | 62.70 | 63.51 |
| Alumina | 18.57 | 18.64 | 19.61 | 18.30 | 23.80 | 23.09 |
| Peroxide of iron | traces | 0.83 | 0.70 | 1.01 | 0.62 | none |
| Oxide of manganese | traces | 0.13 | none | none | none | none |
| Lime | 0.34 | 1.23 | 0.66 | 1.26 | 4.60 | 2.44 |
| Magnesia | 0.10 | 1.03 | none | 0.51 | 0.02 | 0.77 |
| Potash | 14.02 | 9.12 | none | 2.53 | 1.05 | 2.19 |
| Soda | 1.25 | 3.49 | 11.12 | 7.99 | 8.00 | 9.37 |
| 100.00 | 99.47 | 100.08 | 99.83 | 100.79 | 101.37 | |
According as these various felspars are present in a soil, so will the quality of the soil be. It stands to reason that as the presence of potash in a soil is one of the distinguishing features of its fertility, much will depend on the extent to which the orthoclase felspar is present; and also, not only on the extent, but on the state and degree of its disintegration. It is important to note the method of this disintegration. It is effected by the absorption of water. This water is not merely absorbed mechanically, but actually enters into the composition of the mineral. It is not present as moisture merely, capable of being expelled at ordinary boiling temperature, but it forms what is known as water of composition. In this process of hydration, the mineral loses its lustre and crystalline appearance, crumbles away into a more or less—according to its state of disintegration—powdery mass. A very great change is also effected in its chemical composition; it loses nearly all its base. This is effected in the following way. As water enters into the mineral's composition, it sets free a certain portion of the base; there is thus formed a basic silicate, which, being soluble in water, is washed away in solution. This change may be illustrated by quoting the analysis of a kaolin clay formed by the disintegration of orthoclase felspar.
| Kaolin Clay formed by disintegration of Orthoclase. | |
| Silica | 46.80 |
| Alumina | 36.83 |
| Peroxide of iron | 3.11 |
| Carbonate of lime | 0.55 |
| Potash | 0.27 |
| Water | 12.44 |
| 100.00 | |
The chief difference here is the almost total loss of potash and a portion of the silica, and the gain of water. The other constituents practically remain insoluble.
Another important mineral is Mica. Its composition is not unlike felspar. It contains silica, alumina, and iron, in considerable quantities, also magnesia and potash. There are two kinds of mica—that containing potash, and that containing magnesia, in excess. The analyses of these two kinds are as follows (by the late Dr Anderson):—