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.

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.—

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:—

NOTE IV. (p. 81).

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):—

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.

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):—

The decomposition of mica is very slow, however, as it is a peculiarly hard mineral.

Other important minerals are Hornblende and Augite. These are composed of silica, alumina, iron oxide, manganese oxide, lime and magnesia. These are the chief minerals out of which soils are formed. It is scarcely necessary to say that few soils are made up out of any of these three minerals alone. Nearly all rocks are formed out of a mixture of these minerals. Where, however, any one mineral predominates over the rest, the nature of the soil will be thereby affected. In order to illustrate this, it may be well to mention the composition of one or two of the commoner rocks.

1. Granite, which is so abundant in certain parts of the north of Scotland, and which gives rise to the soils in the neighbourhood of Aberdeen, is made up of a mixture of quartz, felspar, and mica. It depends on the felspar present—i.e., whether it is orthoclase, oligoclase, or albite—whether the soil will be rich in potash or not. Granite containing orthoclase felspar produces a fairly fertile soil. An important consideration, which is apt to complicate this question, is the situation of such soils. They are generally so high above sea-level, that their fertility is seriously impaired on these grounds.2. Gneiss, another common rock, is similar in composition, only that it contains very little felspar, and a correspondingly greater amount of mica.

3. Syenite contains quartz, felspar, and hornblende.

The rocks of which greenstone and trap are types, are found very largely scattered over the country. They are of two kinds, diorite and dolorite.

4. Limestone is of two great classes. We have (1) Common, (2) Magnesian. The following are the analyses of these two classes by Dr Anderson:—

Clays are formed by the disintegration of any of the crystalline rocks; the purest clays being formed from felspar. A pure clay consists simply of silica and alumina, all the other constituents having been washed out. Disintegration, however, seldom reaches such an extent; otherwise clay soils would be completely barren, which they are notably not. The impurities present in clay, which consist of alkalies, especially potash and other mineral ingredients of the plant, are what confer on clay soils their fertility. Clays differ, however, very considerably in their composition. The following is an analysis of a clay soil by Dr Anderson:—

NOTE VII. (p. 91).

Forms in which Plant-foods are present in Soil.

The forms in which the bases necessary for plant-food are present in the soil, are chiefly as hydrated silicates, and in combination with organic acids, forming humates, &c., as well as in the form of sulphates and chlorides.

Phosphoric acid is present in combination with iron, alumina, or lime, or possibly also as magnesium-ammonium-phosphate. Sulphuric acid is generally present in a more or less insoluble condition, in combination with iron and lime; whereas chlorine is combined with the alkali bases in an easily soluble form. An important point is as to the form in which the plant absorbs these food constituents. In this connection reference may be made to a theory put forward by a very distinguished French agricultural chemist, Professor Grandeau. His theory is that the necessary ingredients of plant-food are absorbed into the plant as humates, or, at any rate, that the medium of this transference is humic acid, and organic acids of a similar nature. This theory, however, while ingenious, has not yet been supported by sufficient evidence to make its acceptance advisable. It is probable that it is only in the form of soluble salts that the plant can absorb its food. It is quite probable, however, at the same time, that the exact form in which the different food substances enter the plant may be largely determined by circumstances. According to Nobbe, chloride of potassium is the most suitable form of potassium salts, although the plant may absorb its potassium as sulphate, phosphate, or even silicate.

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