Black soil

Also called: Regur soil, Black cotton soil, Regur · Topic: Soils of India: Types, Degradation and Conservation · NCERT: Class 7, Ch 1 "The Story of Indian Farming"; Class 10, Ch 1 "Resources and Development"

Meaning

Black soil (also called regur or black cotton soil, and Vertisol in world soil classification) is a dark, very clayey soil. It forms mainly when Deccan Trap basalt (dark volcanic rock) breaks down under moderate rainfall and long hot, dry spells. It holds water well. It is rich in calcium carbonate, magnesium, potash and lime, but poor in phosphorus, nitrogen and organic matter.

It matters because it is one of the four farming-important groups among ICAR's eight major soil groups [3]. It is also the base of India's cotton belt. Its swelling, cracking clay makes it both fertile and hard to manage.

Explanation

How it forms: parent rock + climate

• Parent rock: basalt. This dark volcanic rock forms when lava (melted rock from inside the Earth) cools.
• The Deccan Trap is a huge area of layered basalt flows over the north-western Deccan Plateau.
• "Trap" comes from the step-like look of the stacked lava layers.

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

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

• 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. It is rich in bases (calcium and magnesium) but low in humus.

• Other parent rocks: 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].

What makes it a Vertisol

• A 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:
• 30% or more clay in every layer, down to at least 100 cm [4].

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

• Montmorillonite is the main swelling clay. Shrink-swell potential means how much the soil swells when wet and shrinks when dry. It depends on two things [5]:

• the soil's total clay content

• how much of that clay is montmorillonite

• Cracking is part of the definition. Both things count: whether cracks open in the dry season, and how long they stay open [5].

Properties

• Texture: very fine clay. It is very dark and has a very high clay content [3].
• Moisture: it holds water very well [3] and 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 two sources:
• minerals in the basalt that carry titanium and iron

• the mix of clay and humus

• Surface features:

• Gilgai: a small 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 as the soil swells and shrinks [4].

Self-ploughing

• Self-ploughing means the soil turns and opens itself, so it partly does a plough's work.
• Wet season (monsoon):
• The montmorillonite sheets take in water and swell [5].

• The soil turns sticky and heavy [3].

• Hot, dry season:

• The clay shrinks, and deep, wide cracks open from the surface downward [4].

• The clay packs into hard lumps. 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.

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 window when ploughing is possible

• Ploughing time: 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 before the monsoon.

Crops

• Cotton is the main crop. Its roots grow straight down (a vertical root system), so cracking in the soil does not badly damage them [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].

Where it occurs

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

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 parent rocks	Parts of Karnataka, Telangana and Tamil Nadu

• Pattern: the soil is thickest in the north-west Deccan, where the lava is. It thins out towards the south-east along the river valleys.
• Cotton link: India produced 341.91 lakh bales of cotton in 2022-23. Maharashtra led with 80.25 lakh bales, and Madhya Pradesh produced 15.19 lakh bales [2].

World: Vertisols cover about 335 million ha [4]

• About 150 million ha of this could be farmed, and about 200 million ha lie in the tropics [4].
• Major areas outside India: 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]

Don't confuse with

• Red soil: it forms from old crystalline rocks, and iron makes it red. Black soil forms mainly from Deccan Trap basalt and holds far more moisture.
• Laterite soil: it forms when heavy rain washes out (leaches) bases, leaving iron and aluminium behind. Black soil is rich in bases and forms under moderate rainfall. Black soil also resists erosion [6], unlike laterite and red soils.
• Alluvial soil: rivers carry and drop (deposit) this soil, so it is transported. Black soil forms in place from the rock below it.
• Gilgai (feature) vs Vertisol (soil): gilgai is the surface pattern of humps and hollows on Vertisols [4]. It is also the Australian local name for these soils, but it is not a separate soil type.

Prelims Hooks

• Regur = black cotton soil = Vertisol. The parent rock is Deccan Trap basalt. It forms from parent rock + climate, not climate alone.
• Trap: black soil is rich in calcium carbonate, magnesium, potash and lime. It is poor in phosphorus, nitrogen and organic matter (humus) [6].
• Self-ploughing comes from the swelling and shrinking of montmorillonite (2:1) clay [5]. A Vertisol needs ≥30% clay to a depth of ≥100 cm [4].
• India has about 74 million ha of black soil. It is one of the four farming-important groups among ICAR's 8 major soil groups [3].
• Match: Regur – India; Vlei – South Africa; Margalites – Indonesia; Gilgai – Australia [4].
• Core plateaus: Maharashtra, Saurashtra, Malwa, MP and Chhattisgarh, extending along the Godavari and Krishna valleys. Maharashtra was the top cotton producer in 2022-23 (80.25 lakh bales out of 341.91 lakh bales) [2].

Mains Points

• Rock and climate decide soil (GS-I): Deccan basalt, moderate rainfall and a long dry season together made black soil.
• This explains why the soil is concentrated in the north-western Deccan.

• It also explains why it spreads south-east along the Godavari and Krishna valleys.

• Fertile but hard to manage (GS-I/III): black soil is naturally fertile and holds a lot of water [3].

• This supports cotton, soybean and tur grown on rain alone across Maharashtra and MP.

• But it is hard to plough and becomes waterlogged once its cracks close. Waterlogging in the rainy season is the first management problem of Vertisols [4].

• Canal areas, nutrients and buildings (GS-III):

• Canal command areas (land watered by a canal network) get extra irrigation water that drains away slowly. This causes waterlogging and, later, salty soil. Fixes include field drains, broad bed and furrow systems, careful (deficit) irrigation and drip irrigation.
• Low nitrogen, phosphorus and organic matter mean fertiliser should be based on soil tests, as under the Soil Health Card scheme (2015).
• Shrink-swell clay cracks foundations, roads and pipes. So building on the Deccan plateau needs deeper foundations or soil treatment, which raises the cost.

Related concepts

• Self-ploughing of black soil

Read more

• Black (regur) soils