Soil

Soil is the thin top layer of the Earth's crust in which plants grow. It forms very slowly, from the breaking down of rocks and the rotting of dead plants and animals. For a farmer it is the most important resource: it gives crops their nutrients, water and support.

How does soil form? It starts with bedrock, the original solid rock underneath. Over time the bedrock cracks and crumbles (weathering), helped by plants and other living things. Layers of rock bits and dead plant matter build up, until finally there is proper topsoil with humus, able to support thick plant growth. This can take anywhere from a century to several thousand years.

Humus is the dark, rich material formed when dead plants and animals rot. It is full of nutrients and holds water in the soil. ('Humus' is Latin for earth or soil.) Weathering is the breaking of rocks into soil particles by physical, chemical or living (biological) means.

Ancient texts such as the Amarakoṣha describe twelve types of farmland, based on how fertile the soil is. Today we divide India's soils into six main types:

Alluvial soil — silt (fine mud and sand) carried down by rivers from the mountains and plateaus and spread over the plains. It is rich in nutrients, which is why the great river plains feed so many people. Silt is finer than sand but coarser than clay, so rivers carry it easily.

Black soil, also called cotton soil — formed from weathered volcanic rock; it holds a lot of water and is very fertile. (That is one reason Maharashtra grows so much cotton.) Red soil — formed from slowly cooled lava or old rocks; it looks red because it contains iron, which turns reddish with air and water, like rust; it is not very fertile.

Laterite soil, also called brick earth — formed where heavy rain weathers the rock; it goes hard in hot weather. The rain washes most minerals away, leaving mainly iron and aluminium, so it is not fertile.

Sandy soil — mostly small grains of sand, with a little water, air and organic matter. Alpine soil — formed in the mountains as ice freezes and melts and breaks the rock; it is thin, rough and rocky. Six soils, and each one decides what can grow on it.