Soil types in India for UPSC: a simple revision table

Ask someone to name the soils of India and most people can produce the list: alluvial, black, red, laterite, desert, mountain, saline, peaty. The list is rarely the problem. The problem is the pairing — remembering which of the two alluviums is the older one, which soil is short of nitrogen, which one hardens into a brick when you leave it out in the sun.

Soil questions are almost always about these pairings. Which soil, in which region, under which crop, short of which nutrient. Once the eight are grouped by how they formed, most of those pairings stop needing to be memorised.

Why the list is short

Indian soils are classified by formation rather than by chemistry, which is why the names sound like descriptions of a process.

Alluvial soil is deposited by water. Black soil is weathered lava. Red soil is weathered crystalline rock. Laterite is what remains after intense leaching in the wet tropics. Desert soil is worked by wind. Mountain soil is thin and young. Saline soil accumulates salt where drainage fails. Peaty soil is the product of waterlogged decay.

Two of those, alluvial and laterite, are about what has been added or taken away. The rest are about the rock underneath and the climate above.

Alluvial: the soil of the plains

Alluvial soil covers roughly 43 per cent of India's land area, the largest share by a wide margin. It is deposited by the Indus, the Ganga and the Brahmaputra, and by the rivers of the coastal plains and deltas.

It is rich in potash and phosphoric acid, and poor in nitrogen and humus. That combination is worth holding on to, because it is the opposite of what most people assume about the most fertile soil in the country: it grows the food, and it still needs nitrogen.

The distinction that gets tested most often is inside the alluvium itself. Khadar is the newer alluvium, laid down by annual floods, found near the river channel, and renewed every year. Bhangar is the older alluvium, sitting on higher terraces beyond the flood limit, and it contains kankar — nodules of calcium carbonate. Khadar is newer and near the water; Bhangar is older and above it.

Black: cotton on old lava

Black soil, also called regur, is the weathering product of the Deccan basalt flows. It is found across Maharashtra, Gujarat, Madhya Pradesh, Karnataka, Andhra Pradesh and Tamil Nadu — the Deccan Trap region, in other words.

It is rich in lime, iron, magnesia and alumina, and poor in nitrogen, phosphorus and organic matter. It holds water well, which is why it supports cotton in a region with a long dry season, and it develops deep cracks as it dries. Those cracks turn the soil over between seasons, which is why it is called self-ploughing.

The colour comes from titanium and iron compounds in the basalt rather than from humus, which is why black soil can be fertile and still be low in organic matter.

Red, and the laterite that is not a crop soil

Red soil is weathered crystalline and metamorphic rock, coloured by iron oxide. It covers much of Tamil Nadu, Karnataka, Andhra Pradesh, Odisha, Chhattisgarh, Jharkhand and parts of Madhya Pradesh. It drains well and is short of nitrogen, phosphorus and humus; with irrigation and fertiliser it becomes productive, which is why the red-soil belt grows millets, pulses, groundnut and tobacco.

Laterite is the one that catches people out. It forms in high-rainfall, high-temperature conditions where leaching is intense: silica is carried away and iron and aluminium oxides are left behind. The result is a soil rich in iron and aluminium, poor in lime, magnesium and nitrogen, and distinctly acidic.

Two consequences follow. Laterite is poor for food crops without heavy amendment, which is why it is associated with plantation crops — tea, coffee, rubber, cashew, coconut — rather than paddy. And it hardens irreversibly when exposed to air, which is why it has been cut into bricks for centuries along the Malabar coast and the Western Ghats.

The four that occupy the margins

Desert soil is sandy, coarse and low in organic matter, with high soluble salt content and poor moisture retention. Bajra, guar and pulses grow where irrigation reaches.

Mountain soil is thin, immature and acidic, rich in humus where forest cover holds it, and vulnerable to erosion on steep slopes. Tea, coffee, fruit and potato are grown on terraces.

Saline soil, called reh, kallar or usar, accumulates sodium, potassium and magnesium salts where drainage is poor. Some of it is natural; some is man-made, produced by irrigation without drainage. The Indira Gandhi Canal command in Rajasthan is the standard example of waterlogging turning land saline.

Peaty and marshy soil forms where organic matter accumulates under water. It is acidic, dark, high in organic matter, and found in the Kerala backwaters, the Sundarbans and coastal Odisha and West Bengal.

The pairs that get mixed up

Erosion, and what it carries away

Three types of erosion are worth naming, each with a region attached.

Sheet erosion removes a thin, uniform layer of topsoil across a slope. It is the most widespread form in India precisely because it is the least dramatic; you notice it in yields, not in gullies.

Gully erosion cuts deep channels where water concentrates. The Chambal valley, with its ravines, is the standard example, along with parts of Gujarat and Madhya Pradesh.

Wind erosion lifts loose topsoil from dry, exposed surfaces, worst in Rajasthan and Gujarat in the pre-monsoon months when the ground is bare.

The remedies follow the mechanism: contour ploughing and terracing for slope wash, check dams and watershed treatment for gullies, shelter belts and stubble cover for wind. Neeranchal and the Integrated Watershed Management Programme are the two names usually attached to the public effort.

The nutrient problem

Roughly a third of India's land area is affected by degradation, by the Indian Council of Agricultural Research's estimate, and the more persistent damage is chemical rather than physical.

The recommended balance of nitrogen, phosphorus and potassium is about 4:2:1. In the most fertiliser-intensive districts of Punjab the ratio has drifted toward something like 40:8:1, because urea is cheap and subsidised while potash and phosphate are not. Excess nitrogen does not simply wash away; it leaves the crop more vulnerable, acidifies the soil over time, and crowds out the uptake of the nutrients that are now missing.

The Soil Health Card scheme, launched in 2015, was designed as the corrective: test the holding, tell the farmer which nutrient is actually short, and pay for balance rather than volume. Its weakness is the same as any advisory scheme — the subsidy on urea still pulls in the other direction.

Soils are where the climate topics in this section become visible. The monsoon decides whether khadar is renewed or washed away, the western disturbances decide whether the rabi crop on that soil gets its winter rain, and the river map shows where the alluvium was actually laid down.