Geography Quiz: Soils of India

A delta is built out of silt that a river carries down and drops where it meets the sea, so deltaic land is alluvial by definition, and the eastern coastal deltas of the Mahanadi, Godavari, Krishna and Kaveri are among the most fertile rice tracts of the country.

What it is rich in is calcium carbonate, magnesium, potash and lime, and remembering that short list is the safest way to answer any statement question on this soil.

In sheet erosion water spreads over a wide slope and carries away a thin, even film of topsoil, so nothing obvious is left behind to see, and the loss is noticed only years later as falling yields.

Weathering is the natural breaking down of rock by temperature, water, wind and living things, and it is the process that makes soil in the first place; it is not a technique that a farmer applies to save soil, so it cannot be a conservation method.

Fertility needs mineral nutrients such as nitrogen, phosphorus and potassium from the weathered rock, and organic matter, that is humus from decayed plants and animals, which holds water, feeds soil organisms and releases nutrients slowly.

Black soil, called regur, formed from the lava of the Deccan trap and is so closely tied to this one crop that it is popularly called black cotton soil.

Bhangar is the older alluvium of the northern plains, lying above the flood level on slightly raised terraces, coarser in texture and marked by nodules of calcium carbonate known as kankar, which lower its fertility and hinder the movement of water.

Laterite soil forms on hill slopes of Karnataka, Kerala, Tamil Nadu, Odisha and Assam under heavy rain and high temperature, and after manuring it carries these plantation crops well; cashew in particular does well on the laterite of the Kerala and Karnataka coast.

Gypsum, that is calcium sulphate, supplies calcium that replaces the sodium held on the soil particles, and once the sodium is set free it can be washed out of the field by irrigation and drainage, restoring structure to land that had become hard and impermeable.

Running water cuts gullies in the soft alluvium of the Chambal basin, and where those gullies deepen and spread over a wide tract the land becomes a maze of steep channels and ridges unfit for cultivation, which geographers call badland and local people call the ravines or beehad.

The National Bureau of Soil Survey and Land Use Planning, an institute of the Indian Council of Agricultural Research, works from Nagpur and prepares the soil maps and land capability surveys on which district planning rests.