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A History of Agriculture in India v1.c8.s1: Formation of Soils

A History of Agriculture in India v1.c8.s1

Language
English

Contents

Formation of Soils

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A mature soil is the handiwork of geoclimatic and geobiological forces operating jointly. How do these forces operate? Soils are formed by the weathering and disintegration of rocks. Soils are either sedentary, i.e. those formed out of the underlying rock, or are transported, i.e. those formed out of the disintegrated parts of rocks, brought down by the action of rivers from a distance. The process of breaking down of rock masses and their gradual development into soil is called weathering. The agencies of weathering are physical, i.e. those that break the rock into smaller pieces, and chemical, i.e. those that change the composition of the minerals forming the rock and, in so doing, exert a marked influence upon its physical character. The work of the physical agencies is called decomposition. Both these processes are complementary to each other.

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The agencies operating in the disintegration of soils are: aqueous, atmospheric, physical and organic.

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The aqueous agency operating in the disintegration of rocks and soils is the most potent of all. What enormous quantities of solid matter are dislocated by rain and brought down by streams and rivers may be judged from Everest's calculation of silt carried down by the Ganges alone. Everest has calculated that 355,361,461 tonnes of solid matter are carried down annually to the sea by the Ganges. The erosive action of running water is due to sand, pebbles and rocks carried by it. The hardest and heaviest rocks become converted into rounded boulders and pebbles by the action of the moving water containing sand in motion. Water acts not only mechanically in denuding rocks, but it is also a solvent. Potash, soda, silica and lime get dissolved in water, and the rocks are denuded by the solvent action Fig. 3. The occurrence of Archaean and Purana Rocks and the Deccan Trap in India of water. The solvent action of water and minerals is increased by salts in solution and gases in suspension. Sea waves beating against cliffs also help in the formation of soils. The action of glaciers in tearing down rocks and in the formation of moraines and erratic boulders may be also included under this head. The hydration of rocks in the presence of water may also be mentioned.

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The atmospheric agency acts on rocks as follows: The carbon dioxide gas in the atmosphere renders the calcium carbonate soluble. Limestones, chalk and kankar thus get dissolved and become available as plant food, and the rain-water from calcareous rocks charged with calcium carbonate flows into the sea, where shell-fish, corals and foraminifera utilize the lime in building up their own bodies, which in time settle in the form of dead shells and form new rocks. The dew and water-vapour of the atmosphere which get into the interstices of rocks in cold regions are frozen and the resulting expansion disintegrates the particles of rocks. When water freezes, it expands, and the volume of ice formed is increased by nine per cent. This increase in volume exerts a force of 1,640 tonnes per square metre. The oxygen of the atmosphere is a very potent agent in oxidizing and disintegrating the surfaces of hard rocks. Strong currents of wind carry sand and finer particles of matter (such as common salt) from the sea-shore and the dry beds of rivers into the interior.

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Physical agencies operate in disintegrating soils chiefly in the form of heat. Rocks are poor conductors and hence the effects of heat and cold extend only to a slight depth from the surface. The greater expansion or contraction of the surface produces a strain which causes a layer of rock to break off. Earthquakes, hot springs and volcanoes alter even the superficial layers of the earth's surface.

Action of Plants

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Disintegration of rocks takes place by hydration, oxidation and physical action. Nitrification then proceeds with the help of bacteria. Lichens and blue-green algae appear. Gradually, the quantity of soil on the hard surface of the rock increases, and the growth of vegetation becomes more vigorous, mosses, liverworts, ferns and grasses gradually taking the place of lichens. When visible soil accumulates, and fissures and cracks appear on the rock, herbs and shrubs multiply and by their root-action further help to disintegrate the rock to some depth. The formation of soils now goes on apace. Lichens and bacteria are able to draw nourishment from the most insoluble rocks, not only from basalts, granites and schists, but also from quartz. Even quartz gets covered with lichens when exposed long enough to the air. The action of higher vegetation on rocks is partly mechanical and partly chemical. Roots get into the clefts of rocks and tear them asunder. Chemical action is concerned in the solution of some of the ingredients of the rock. The solvent action of roots is partly due to the formation of acids in them which act on the particles of soil.

Action of Animals

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The larger rodents, like rats, squirrels and rabbits, burrow into the ground and thus large quantities of partially decomposed rocks are exposed to the agencies of weathering. Earthworms and ants also help to decompose and disintegrate the rocks. They carry down vegetable matter, which on decomposition helps to break down the minerals. Earthworms pass large quantities of soil through their bodies. This passage of soil through their bodies amounts to more than 22 tonnes of soil per hectare. This process greatly helps to mix the soil with the subsoil. Worm-casts are particularly useful to the farmer, as they help to loosen and perforate the soil to facilitate the penetration of roots, water and air. These worms also drag down leaves, pieces of straw, etc., into their holes, thus incorporating organic matter into the soil, and making heavy soils lighter and light soils heavier.

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As the soil particles become smaller, their water-holding capacity increases, as each soil particle is surrounded by a film of capillary water. This water is very effective in the decomposition of the soil particles. Hydration, oxidation and carbonation work simultaneously. These decomposition processes are always accompanied with increases in bulk and these tend to disrupt the rock and further assist to disintegrate it.