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Soil formation and the four major soil groups

Soils and natural vegetation of India · Part 1 / 4 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

How soil forms and is layered, how India's soils are classified (ICAR's eight groups and the USDA taxonomy in outline), and the four most widespread groups: alluvial, black (regur), red and yellow, and laterite, with their origin, properties, areas and crops.

The earlier chapters on India described its relief, its rivers and its climate. This chapter turns to two things that grow out of all three: the soil that covers the land and the plants that cover the soil. Soil decides where wheat, rice, cotton or tea can be grown, how much water a field can hold and how easily the land is worn away, so it sits at the base of Indian farming. Part 1 explains how soil forms, how it is layered and how India's soils are grouped, and then looks closely at the four most widespread groups: alluvial soil, black soil (regur), red and yellow soil and laterite soil. Part 2 takes up the remaining soils and the problem of soil loss; Parts 3 and 4 move on to natural vegetation and forest cover.

Soil is the thin, loose cover on top of the land, a blend of rock particles, decayed plant and animal matter, water and air in which plants can take root. It is built slowly, as weathering breaks rock into fragments and living things add organic material. Five factors shape the result. The parent rock supplies the mineral grains and often the colour. Climate, above all heat and rainfall, sets the pace of weathering and decides whether minerals stay near the surface or are washed downward. Relief matters because steep slopes shed soil quickly, while gentle slopes and valley floors collect it. Plants, animals and micro-organisms churn the soil and add humus, the dark, decomposed organic matter that holds nutrients and water. The last factor is time: a mature soil with clear layers can take thousands of years to form.

Dig a pit and look at its wall, and you see the soil profile: a stack of layers called horizons. The top one, the A horizon, is the topsoil, darkened by humus and full of roots and small creatures; farming depends on it. Below lies the B horizon or subsoil, with less organic matter and a build-up of fine clay and minerals washed down from above. The C horizon is the parent rock in a half-broken state, and under it is solid bedrock. Soils also differ in texture, the proportion of sand, silt and clay in them. Sandy soil drains quickly and keeps little water; clay holds water but turns sticky when wet and hard when dry; loam, a balanced mix of the three, suits most crops. Because topsoil takes centuries to build and can be lost in a few seasons, guarding it is the heart of soil conservation, which Part 2 explains.

A useful first split is between transported soils, carried and laid down by rivers or wind, and residual soils, formed in place from the rock beneath them. Beyond that, Indian soils were once sorted mainly by colour. The Indian Council of Agricultural Research (ICAR) later grouped them into eight broad classes on the basis of how they formed, their colour, their make-up and where they occur: alluvial, black, red and yellow, laterite, arid, saline, peaty and marshy, and forest and mountain soils. Prelims questions usually follow this eight-fold scheme. For detailed survey maps, ICAR's National Bureau of Soil Survey and Land Use Planning at Nagpur uses the soil taxonomy of the United States Department of Agriculture, which names orders such as Entisols, Inceptisols, Alfisols and Vertisols; broadly, the clay-rich black soils of the Deccan belong to the Vertisols. The eight groups need to be known in detail and the modern taxonomy only in outline.

alluvial soil is the most widespread soil of India, covering roughly two-fifths of the country. It is a transported soil. The Indus, Ganga and Brahmaputra and their tributaries carry silt down from the Himalaya and spread it over the Northern Plains, while the Mahanadi, Godavari, Krishna and Kaveri lay down similar material in their deltas on the east coast. Narrower bands follow the river valleys of the Peninsula and parts of the coastal plains. Its texture ranges from sandy loam to clay, depending on how far and how fast the river carried the material before dropping it. The soil is generally deep and easy to work, and together with river water and canals this explains why the Ganga plain has fed large farming populations for many centuries.

Within the alluvium, two kinds already met in the physiography chapter matter for soils too. Khadar is the newer alluvium of the flood plains; fresh silt renews it in most flood seasons, so it is fine-grained and very fertile. Bhangar is the older alluvium on the higher ground beyond the reach of floods; it is less fertile and often holds lumps of lime called kankar. In chemical terms, alluvial soils are usually rich in potash but poor in phosphorus. They grow rice, wheat, sugarcane, pulses and oilseeds, and jute does especially well on the fresh alluvium of the lower Ganga-Brahmaputra delta. In the drier western plains, poor drainage and heavy canal irrigation can draw salts up to the surface and spoil the land, which links this soil to the saline soils described in Part 2.

black soil (regur), known in India as regur, covers most of the Deccan plateau. It developed largely from the basalt of the Deccan Traps, the huge sheets of lava that poured over western and central India about 66 million years ago, and the parent rock explains much of its character. Its heartland is Maharashtra, and it extends over large parts of Gujarat and western Madhya Pradesh and into Karnataka, Telangana, Andhra Pradesh and Tamil Nadu. It tends to be deepest on the lowlands and in the valleys of rivers such as the Godavari, Krishna, Narmada and Tapi, and thinner on the higher plateau surfaces. Because cotton thrives in it, it is widely called black cotton soil, and the cotton fields of Vidarbha, Marathwada and Gujarat lie largely on it. It is the classic example of a residual soil that reflects the rock beneath.

Black soil is rich in clay, and that clay behaves unusually. Soaked by rain, the soil swells and turns sticky, so it is hard to plough just after a shower; as it dries it shrinks and splits into deep, wide cracks. Air enters the cracks and loose surface material falls into them, so the soil partly turns itself over, which is why it is called self-ploughing. Its great power to hold moisture lets crops such as cotton come through dry spells with little irrigation, an advantage in the rain-shadow interior of the Deccan. Chemically, black soil is generally rich in lime, iron, magnesia, alumina and potash, but poor in phosphorus, nitrogen and organic matter. Besides cotton, it grows jowar, wheat, pulses and oilseeds, and sugarcane where irrigation is available. The same stickiness makes it a difficult base for roads and buildings.

red and yellow soil forms on old crystalline rocks such as granite and gneiss, mostly where rainfall is low to moderate. Its main belt lies across the eastern and southern Deccan, in Tamil Nadu, Karnataka, Andhra Pradesh, Telangana, Odisha and Chhattisgarh, and it also appears along the foot of the Western Ghats and in patches to the south of the middle Ganga plain. The red colour comes from iron spread widely through the parent rock, and where that iron oxide has taken up water, the shade turns yellow, so both colours are often seen close together. In general the soil is light and porous and is short of nitrogen, phosphorus and humus. Fine-textured red soils on lower ground can be quite productive, whereas coarse, gravelly ones on dry uplands are poor and need manure and water. Millets such as ragi, groundnut, pulses and tobacco are typical crops.

The name laterite soil comes from the Latin word later, meaning brick. This soil forms where high temperatures and heavy rain alternate with a dry season. The rain dissolves lime and silica and carries them down out of the upper layers, a process called leaching, leaving behind oxides of iron and aluminium that give the soil its reddish tint. Heat also helps bacteria break down fallen leaves quickly, so little humus collects even under thick forest. The result is an acidic soil short of organic matter, nitrogen, phosphate and calcium. It occurs mainly in Kerala, Karnataka, Tamil Nadu and Madhya Pradesh and in the hilly parts of Odisha and Assam, often on high, rain-washed ground. Blocks cut from it harden in the air, so it has long served as building material. With manure and fertiliser it supports tea and coffee on hill slopes and cashew nearer the coast.

Seen together, the four groups show how parent rock and climate share the work. alluvial soil owes more to rivers than to local rock, so it follows the plains and deltas. black soil (regur) mirrors the Deccan basalt beneath it. red and yellow soil mirrors older granite and gneiss under moderate rain. laterite soil mirrors climate more than rock: given enough heat and rain, even basalt can weather into laterite, which is why laterite caps some high, wet hilltops of the Western Ghats while black soil lies on the drier plateau below. For each group, four facts carry most of the weight: how it forms, its colour, what it is rich and poor in, and the crops it suits. The remaining four groups, and the ways soil is lost and protected, follow in Part 2.

Exam traps

• Black soil is POOR in phosphorus, nitrogen and organic matter; it is RICH in lime, iron, magnesia, alumina and potash. Questions often swap the two lists.
• Alluvial soil is generally rich in POTASH and poor in PHOSPHORUS, not the other way round.
• Laterite forms under HIGH temperature and HEAVY rainfall with a dry season (intense leaching), not in dry areas; it is poor in humus even under dense forest.
• Khadar is the NEWER, more fertile flood-plain alluvium; Bhangar is the OLDER alluvium above flood level, with kankar nodules.
• The colour of red soil comes from IRON spread through crystalline rocks, not from humus; the yellow shade appears where the iron oxide is hydrated.
• Black soil is a RESIDUAL soil formed from the rock beneath it (Deccan basalt); alluvial soil is a TRANSPORTED soil.
• ICAR's eight-group scheme is the one Prelims usually follows; the USDA taxonomy (Entisols, Vertisols and so on) is what NBSS&LUP uses for detailed maps. Black soils broadly fall among the Vertisols.

Recap

• Soil forms from parent rock through weathering, shaped by climate, relief, living things (humus) and time.
• Soil profile: A horizon (topsoil, humus), B horizon (subsoil, washed-in clay and minerals), C horizon (broken parent rock), then bedrock.
• ICAR's eight groups: alluvial, black, red and yellow, laterite, arid, saline, peaty and marshy, forest and mountain.
• Alluvial: about two-fifths of India; transported by rivers; Khadar (new, fertile) and Bhangar (old, kankar); rich in potash, poor in phosphorus.
• Black (regur): from Deccan Trap basalt; clayey, holds moisture, cracks when dry, 'self-ploughing'; best known for cotton.
• Red and yellow: on granite and gneiss in the eastern and southern Deccan; red from iron, yellow when hydrated; poor in nitrogen, phosphorus and humus.
• Laterite: from Latin 'later' (brick); leaching under heat and heavy rain leaves iron and aluminium oxides; acidic, low in humus; used for building blocks; tea, coffee, cashew with manure.

Practice questions

Sources

• ICAR National Bureau of Soil Survey and Land Use Planning, Nagpur: soil resource maps of India and descriptions of the major soil groups and soil orders — https://nbsslup.icar.gov.in
• Indian Council of Agricultural Research: classification of Indian soils into major groups and the soils' agricultural use — https://icar.org.in
• Geological Survey of India: the Deccan Traps basalt province and the crystalline rocks of the Peninsula — https://www.gsi.gov.in