Soils of India: Types, Distribution, Erosion and PYQs

Soil is the thin top layer of loose material on the earth's surface in which plants grow. It is formed over long periods by the weathering of parent rock and the addition of decayed plant and animal matter, and its character depends on the relief, the climate, the vegetation and the time available for it to form. The Indian Council of Agricultural Research groups Indian soils into eight major types, and questions in every general studies paper come from the table of type, region and crop.

How soil is formed and how it is read

A vertical cut through the ground shows the soil profile, with the topsoil rich in humus called the A horizon, the subsoil of accumulated clay and minerals called the B horizon, and the weathered parent rock called the C horizon. Fertility depends on the right mixture of minerals and organic matter, on the ability of the soil to hold water and air, and on its reaction. A soil with a high concentration of hydrogen ions is acidic, one with an excess of sodium and other bases is alkaline, and lime is added to correct the first while gypsum is added to correct the second. Soil survey and mapping in India is the work of the National Bureau of Soil Survey and Land Use Planning at Nagpur, an institute of the Indian Council of Agricultural Research.

The major soil types

Alluvial soil

Alluvial soil is the most widespread soil of India and the most productive, laid down by the Indus, the Ganga and the Brahmaputra and by the peninsular rivers in their deltas. It is rich in potash, phosphoric acid and lime but poor in nitrogen and humus. It is divided by age into bhangar, the older alluvium that lies on higher terraces and contains lime nodules called kankar, and khadar, the newer alluvium of the floodplains that is finer, renewed at each flood and more fertile.

Black soil

Black soil, also called regur or black cotton soil, formed from the weathering of the Deccan lava. It is clayey and deep, holds moisture for a long time, and is rich in calcium carbonate, magnesium, potash and lime but poor in phosphoric acid, nitrogen and organic matter. In the hot dry season it develops wide cracks, which let air reach the lower layers, so the soil is sometimes described as self-ploughing. It is the natural cotton soil of the country.

Red, laterite and arid soils

Red soil develops on crystalline igneous and metamorphic rocks in areas of low rainfall. Its colour comes from the diffusion of iron in the rock, and it looks yellow when the iron is present in hydrated form. It is deficient in nitrogen, phosphorus and humus. Laterite soil is the product of heavy rain and high temperature, where intense leaching washes away the lime and silica and bacteria destroy the humus, leaving a soil rich in iron oxide and potash but poor in organic matter, nitrogen and calcium. It hardens on exposure to air, which is why it is cut into bricks for building. Arid soil is sandy and saline, red to brown in colour, low in humus, and often carries a kankar layer below that blocks the movement of water.

Soil erosion and conservation

The removal of the topsoil by water or wind is soil erosion. Sheet erosion removes a thin uniform layer over a wide area and is the least visible and therefore the most dangerous form. Gully erosion cuts deep channels, and where these spread over a wide tract the land becomes unfit for cultivation and is called badland, of which the ravines of the Chambal valley are the standard Indian example. Wind erosion dominates in the dry west. The methods of conservation are contour ploughing, terrace farming on slopes, strip cropping, the planting of shelter belts of trees across the wind, mulching, and the building of contour bunds and check dams. Weathering, by contrast, is the natural breaking of rock that creates soil in the first place and is never a conservation method.