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.