THE SOIL

Soil Formation. Soil is produced from rock by the pros of weathering and by the activities of plants, animals, and man. Primitive or igneous rocks,¹ formed by the solidification of the magma in the process of the cooling of the earth consist of aggregates of mineral crystals which are large or small according to the rate at which cooling took place. Each mineral is a chemical compound with specific chemical and physical properties. As soon as igneous rocks are exposed to changing temperatures, moisture, etc. slow processes of disintegration and decomposition begin.

The weathering of a rock is generally due to a combination of physical and chemical actions. The weathered products of rock alone do not constitute a soil. Plants establish themselves very soon after weathering begins, and the mineral material thus becomes mixed with plant remains. These remains, in the process of decay, form an addition to the products of rock weathering. Soil is therefore a mixture of organic and inorganic material containing a large and complex population of living things.

The general character of a soil depends to a considerable extent on the nature of the parent material. Thus a coarse-grained sandstone will generally produce a sandy soil, and a stratum of shale a "heavy" soil.

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