Black (regur) soils

Detail

Names and place in the classification

• Black soil is a dark, clay-rich soil formed mainly from lava rock. It has three names:
• Regur, the Indian name.
• Black cotton soil, because cotton grows very well in it. Cotton has been the main traditional crop wherever it is found [6].

• Vertisol, its name in world soil classification (see below).

• ICAR classification (8 groups): India has eight major soil groups. Black soil is one of the four groups that matter most for farming [3].

• Area in India: about 74 million ha, mostly in the central, western and southern states [3].

What a Vertisol is

• Vertisol is a soil order (a top-level class in world soil classification). It covers heavy clay soils that "churn": they keep mixing themselves as they swell and shrink [4].
• Rules for being a Vertisol:
• The soil must have 30% or more clay in every layer, down to at least 100 cm [4].

• Most of that clay must be 2:1 lattice clay, a clay mineral built in thin sheets that soak up water and swell [4].

• Montmorillonite is the main swelling clay. Vertisols are defined by their shrink-swell potential, meaning how much they swell when wet and shrink when dry. This potential depends on:

• the soil's total clay content

• how much of that clay is montmorillonite [5]

• Cracking is part of the definition. Whether cracks open in the dry season, and how long they stay open, both count [5].

How it forms: parent rock + climate

• Parent rock: basalt, a dark volcanic rock. It forms when lava (melted rock poured out from inside the Earth) cools.
• The Deccan Trap is the huge region of layered basalt flows that covers the north-western Deccan Plateau.

• "Trap" comes from the step-like look of the stacked lava layers.

• Climate: black soil forms where rainfall is moderate and basalt lies below [6]. It does not form from rock alone. The rock and the climate act together.

• Process (cause → process → result):
• Cause: basalt is rich in calcium, magnesium and iron.
• Process: moderate rain and long hot, dry spells break the basalt down chemically into fine clay.

• Result: a deep, dark clay soil that is rich in bases (calcium and magnesium) but low in humus.

• Black soils are products of trap lava [6].

• Rocks other than basalt: Vertisols can also form from sediments rich in smectite (swelling) clay. They can also form from other base-rich rocks, such as calcium- and magnesium-rich basement rocks [4]. This explains the black soils of parts of Karnataka, Telangana and Tamil Nadu, which formed partly from rocks other than the Deccan basalt.

Where it is found

Zone	Areas
Core (Deccan Trap)	Plateaus of Maharashtra, Saurashtra (Gujarat), Malwa (western Madhya Pradesh), Madhya Pradesh and Chhattisgarh
South-east extension	Along the Godavari and Krishna valleys, towards the east coast
Interior Gujarat and Karnataka	On the Deccan lava plateau and Malwa Plateau [6]
Other areas (other parent rocks)	Parts of Karnataka, Telangana and Tamil Nadu

• Pattern: black soil is thickest in the north-west Deccan, where the lava is. It then thins out towards the south-east along the river valleys.
• World: Vertisols cover about 335 million ha [4].
• About 150 million ha of this could be farmed [4].
• About 200 million ha lie in the tropics [4].
• Major areas besides India's Deccan Trap plateaus: Ethiopia, Sudan, Australia, south-western USA (Texas), and Uruguay, Paraguay and Argentina. In Africa, the largest areas are in the Sahel and along the East African Rift Valley [4].

Local name	Country
Regur	India [4]
Black cotton soils	USA [4]
Vlei soils	South Africa [4]
Margalites	Indonesia [4]
Gilgai	Australia [4]

Properties

• Texture: very fine clay. Black soil is very dark and has a very high clay content [3].
• Moisture: it holds water very well [3]. It responds well to irrigation [6].
• Rich in: calcium carbonate, magnesium, potash and lime.
• Poor in:
• phosphorus

• nitrogen and organic matter. It is poor in humus [6].

• Fertility: it is naturally very fertile [3].

• Erosion: its granular structure is rich in iron, so it resists wind and water erosion [6].
• Colour: the dark colour comes from the basalt parent rock (titanium- and iron-bearing minerals) and from the clay–humus mix.
• Special surface features:
• Gilgai: a small-scale pattern of humps and hollows on the surface. The humps are 2–8 m apart and stand 15–50 cm above the hollows [4].
• Slickensides: shiny, grooved faces inside the soil. They form when blocks of soil slide past each other during swelling and shrinking [4].

Self-ploughing of black soil

• Self-ploughing means the soil turns and opens itself, partly doing the work of a plough. It happens because the clay swells when wet and shrinks when dry.

Wet season (monsoon)

• Rain soaks into the clay.
• The sheets of montmorillonite take in water and swell [5].
• The soil turns sticky and very heavy [3].

Hot, dry season

• The water dries out and the clay shrinks.
• Deep, wide cracks open from the surface downward [4]. This happens in most years.
• The clay sheets pack tightly together into hard lumps with few air spaces [5].
• Bulk density (mass of soil per unit volume) rises from about 1.33 g/cm³ when moist to more than 1.8 g/cm³ when fully dry [5].

Result

• Air reaches deep into the soil through the cracks. This is aeration.
• Topsoil and crop leftovers fall into the cracks. When the soil swells again, they get mixed into deeper layers. This is the "churning".

State	What happens	Effect for farmers
Wet	Clay swells and turns sticky	Too sticky to plough. Waterlogging.
Dry	Clay shrinks and cracks deeply	Very hard. Cracks let air in.
Just right	Moist, but not sticky	Short time window when tillage is possible

• Tillage timing: the soil is sticky when wet and very hard when dry [3]. Only a narrow range of moisture allows ploughing [4]. So farmers plough:
• right after the first shower, or
• in the pre-monsoon period.

Crops

• Cotton is the main crop. One reason it does well: cotton has a vertical (downward) root system that cracking in the soil does not badly damage [3].

Condition	Crops
Rainfed (rain only)	Cotton, millets (jowar), soybean, sorghum, pigeon pea (tur), groundnut [3]
Irrigated	Sugarcane, wheat, tobacco, citrus [3]
After the rains (rabi)	Wheat, chickpea, barley [4]

• Not suited to: tree crops. The swelling and shrinking tear their roots [4].
• Cotton output: India produced 341.91 lakh bales of cotton in 2022-23 [2]. Maharashtra, the heart of the Deccan Trap black soil, led with 80.25 lakh bales. Madhya Pradesh produced 15.19 lakh bales [2].

Problems

• Waterlogging and poor drainage:
• Once the soil is wet, the cracks close and water sinks in very slowly. This is low infiltration [4].
• In canal command areas (land watered by a canal network), extra irrigation water stands on the surface. Over time this can make the soil salty.

• Waterlogging in the rainy season is the first management problem of Vertisols [4].

• Hard to work: extremely hard when dry and sticky when wet [4].

• Building damage: the swelling and shrinking clay pushes on and pulls away from foundations, roads and pipes. Foundations crack, so builders must dig deeper foundations or treat the soil first.
• Nutrient gaps: low nitrogen, phosphorus and organic matter. Farmers need balanced fertiliser, a gap that soil testing under the Soil Health Card scheme (2015) is meant to show.

Prelims Hooks

• Regur = black cotton soil = Vertisol. Parent rock: Deccan Trap basalt. Formed by parent rock + climate, not climate alone.
• Black soil is rich in calcium carbonate, magnesium, potash and lime, and poor in phosphorus, nitrogen and organic matter. Trap: it is not rich in phosphorus or humus.
• Self-ploughing comes from the swelling and shrinking of montmorillonite (2:1) clay. Deep cracks in the dry season let air into the soil.
• A Vertisol needs ≥30% clay to a depth of ≥100 cm [4].
• India has about 74 million ha of black soil, one of the four farming-important groups among ICAR's 8 major soil groups [3].
• World Vertisols cover about 335 million ha. Main regions: India, Sudan, Ethiopia, Australia, Texas (USA) [4].
• Match: Gilgai = surface humps and hollows on Vertisols, also the Australian name; Vlei = South Africa; Margalites = Indonesia [4].
• Core plateaus: Maharashtra, Saurashtra, Malwa, MP, Chhattisgarh. It extends south-east along the Godavari and Krishna valleys.
• Black soil resists erosion (granular, iron-rich structure) [6], unlike laterite and red soils.
• Maharashtra was the top cotton producer in 2022-23 (80.25 lakh bales out of 341.91 lakh bales) [2].

Mains Points

• Rock plus climate decide soil: black soil shows that parent rock (Deccan basalt) together with moderate rainfall and a long dry season shapes Indian soils. This explains why it is concentrated in the north-western Deccan and spreads south-east along the Godavari and Krishna valleys (GS-I).
• Rich soil, hard to manage: black soil is naturally fertile and holds a lot of water [3]. But it is hard to till and becomes waterlogged once its cracks close [4]. This supports cotton, soybean and tur farming on rain alone across Maharashtra and MP, but makes farming risky when rains fail (GS-I/III).
• Problems in canal areas: over-irrigation on slow-draining Vertisols causes waterlogging and later salinity. Fixes include field drains, broad bed and furrow systems, careful (deficit) irrigation and drip irrigation (GS-III, resources).
• Nutrients and infrastructure: low N, P and organic matter call for soil-test-based fertiliser use under the Soil Health Card scheme (2015). The shrink-swell clay also raises building costs for roads, canals and houses on the Deccan plateau, because foundations must be deeper or the soil must be stabilised (GS-III).