Origin of Soils

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.

Types Of Soil

Soils rich in decayed organic matter are said to be "peaty". Plants add to the vegetable waste matter by shedding leaves and by the mere presence of their roots. Roots accelerate decomposition of organic matter and rock particles by their activities of excretion and absorption. Thus soil that supports vegetation of any kind is constantly increasing not only its depth of surface soil but its own fertility. Soil is never static.

When the soil is made up of forty to sixty per cent of clay and the remainder of sand, it is classified as a loam, being a light or heavy loam according to its variation in sand content. It habitually contains quite a lot of humus.

Almost any soil is, or can be made by cultivation into good garden soil. Pure quartz sand or degenerated sandstone is almost entirely devoid of plant food, and is the most unsatisfactory type of soil for rose-growing. Blue granite produces very good soils. White granite usually makes poor soil, also limestone and often red ironstone; they need the addition of great quantities of humus. Shale soils, alluvial soils, and yellow ironstone soils are usually good. Clay should have at least six to eight inches of surface soil overlaying it. Its close-knit texture makes clay very retentive of water; it almost always needs to be drained.

When a sandy soil rests on a bed of impermeable clay, water is held up until the interstices between the grains of sand become filled with it and the soil becomes a morass. Such a soil is utterly useless until drained.

The main function of cultivation is to produce and maintain a fertile layer of surface soil of adequate depth. Roots penetrate only as deep as the soil is suitable to them. Suitability varies with soil texture, available food, available water, aeration, and drainage.

Loams that are deficient in organic matter set hard when dry and are very greasy when wet. They are difficult to work and, even though containing some plant food, they are unproductive because the physical condition is unsuitable for root development. The addition of lime, dolomite, or gypsum will coagulate the fine clay particles, rendering them less cohesive. The soil will not set so hard, nor become so greasy.

Very peaty soils are rare in Australia. They are too rich, even toxic to some plants, and are of high acidity. Putrefaction occurs in the excessive humus content. They need drainage and liming.

Air, required by roots in the performance of their normal functions, is present in the small interstices between soil particles. In healthy soil, moisture slowly soaks away, creating a steady suction of air into the spaces left. This happens after every rain or artificial watering. The moisture is reduced, too, by plant absorption. The combustion of organic plant-matter in the soil generates carbon dioxide. Most of this is taken into solution by water, but some forces its way to the surface, increasing soil ventilation. Earthworms give great assistance in soil aeration.

Plants will tolerate neither air pockets nor water pockets. Contact of roots with the former will stop their growth. Presence of the latter kills desirable soil bacteria, which will perish in any soil that is not well drained and well ventilated. Their death is usually followed by replacement with putrefactive bacteria, with consequent loss of fertility and production of toxicity to plants. All soil management must be aimed at avoidance of this calamity.

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