SOIL, that part of the disintegrated surface of the earth's crust in which the roots of plants ramify, and from which growing plants derive the mineral sub stances necessary for their proper devel opment. Soils are formed by the disinte gration of the rocks through the continued action of water and air at various tem peratures, and by the accumulation of the decaying remains of vegetable organisms. In the process of disintegration water acts variously, as in the bursting action of frost, the gradual wear of running water, and the sawing, grinding, and dashing of the glacier and mountain tor rent. The action of the atmosphere is dependent upon the carbonic acid which it contains. This acid is capable of de composing the silicates of potash and soda, which form part of rocks, seizing the pot ash and soda and converting the same into carbonates, which are subsequently washed out by water. Besides carbonic acid, chloride of sodium, or common salt, possesses a very powerful solvent action, having in solution the power of dissolv ing the phosphates of the alkaline earths. The tendency of water and atmosphere combined is to level the earth's surface by destroying eminences and filling up hollows. We often find, therefore, in the
plains a soil differing widely from its un derlying stratum, on account of its being a mixture of the deposits of streams and the debris of neighboring or more distant mountains. On hills the soil is usually of the same nature as the subsoil.
The proportion of organic matter varies exceedingly in different soils. The mix ture of various earths and humus, termed loams, constitute the best of soils. These are classified according to the earths which prevail in them, as a sandy loam, etc.; according to their degree of friabil ity, as a free loam, a stiff loam, etc.; or according to both, as a free calcareous loam, etc. They are generally laid on the sides of valleys, along the bases of hills or mountains, or on the banks of upland rivers. In general, much more depends on the texture of a soil, and on its capacity for retaining or parting with water and heat, than on its chemical composition.