ORIGIN OF SOILS

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Geologists tell us that in the very beginning of the Earth's existence there was no such thing as soil. There was an outer crust made up of rock, which varied from place to place in texture and chemical composition. Colour varied with mineral content. As the millions of years have passed many influences have acted on that crust, the most powerful being temperature variation. This has been governed by latitude, altitude, and proximity to large expanses of either land or water. The greater the influence, the greater has been the change, and so soils have been formed varying in texture from coarse to fine. Coarse particles make up what we call sand-light and friable; fine particles form what we call clay-heavy and cohesive.

The particles making up any soil are extremely slowly soluble. They constitute the mass and the anchorage for plants, but, in slowly dissolving, provide minute quantities of mineral plant foods. Hence the difference between soils, such as those with an ironstone or limestone basis.

Rock disintegration has occurred to its greatest extent in the surface soil-the upper few inches. Under this layer is the subsoil; it varies greatly in depth and gradually alters in character with depth. Quite near the surface the subsoil is little other than slightly decomposed rock between actual rock masses of varying sizes. It is complementary to the surface soil containing many mineral foods but little organic matter and a few soil bacteria. It retains water that has soaked down from higher levels, bringing with it a little plant food in dilute solution. Topsoil gradually deepens with cultivation, plant activities, and movement of soil by volcano, water, or wind.

The subsoil nearest the surface is slowly encroached upon. Trenching hastens the process. At still deeper levels fairly solid rock is found with only seams of soft material following the irregular courses of the cracks in the rock.

Soil becomes fertile by the addition of humus to the rock particles. Humus is decaying organic matter of plant and animal origin of which plants make up well over ninety-five per cent. Into this humus come myriads of soil bacteria; soil fertility is produced by their action on the organic matter. They cause decomposition and render organic plant foods soluble. They help in slowly dissolving rock particles, and in general rock disintegration.