Black Soil in India: Regur Soil, Formation, Characteristics, Crops and the Deccan Trap

Overview

PHYSICAL GEOGRAPHY
Geography · GS‑I

Black Soil in India
Regur soil: Formation, characteristics, crops and the Deccan Trap
74 Mha under black soilsRegur other nameCotton classic cropBasalt parent rock
digitallylearn.comUPSC‑CSE General Studies

Black soil is the dark, clayey soil that covers much of the Deccan plateau. It is also called regur soil, and because it suits cotton so well it is known as black cotton soil. It forms from the weathering of the basalt lava of the Deccan Trap, develops deep cracks in hot dry weather, holds moisture for a long time, and is rich in lime, magnesium and potash but poor in phosphorus and humus. Soil scientists place the black soils of India among the Vertisols, a world soil group of expansive clays that crack as they dry.

Black Soil Names, Classification and Extent

Names of Black Soil: Regur, Black Cotton Soil and Vertisol

The same soil goes by several names, and examination questions use all of them. In the Indian tradition it is regur soil, a name that survives in textbooks. Because it is ideal for growing cotton, it is also called black cotton soil. In soil taxonomy it corresponds to the Vertisols.

Colour is not fixed. Depending on the parent material and the climate, Vertisols range from grey or red to the familiar deep black. The same kind of soil has local names elsewhere: Black earths in Australia, black cotton soils in East Africa, and black gumbo in East Texas.

In short, black cotton soil forms from weathered volcanic rock, holds much moisture and is very fertile.

Black Soil in India: States Where It Is Found

Map of west-central India with state lines. Maharashtra, Madhya Pradesh, Chhattisgarh and Gujarat, the states with black soil plateaus, are shaded. Purple labels name the four Deccan Trap subprovinces: Main Deccan, Malwa Plateau, Mandla Lobe and Saurashtra. Numbered places: 1 Latur, with deep black soil in the west of the district; 2 Aurangabad, near Ellora; 3 Ajanta and 4 Ellora, caves cut in basalt; 5 Mahabaleshwar, source of the Koyna; 6 the Koyna project of 1,960 MW; 7 Indore on the Malwa plateau; 8 Rajkot in Saurashtra; 9 Raipur on the Chhattisgarh plateau; 10 Solapur on the Deccan plateau.

Black soil is typical of the Deccan Trap region spread over the north-west Deccan plateau, where it rests on lava flows. It covers the plateaus of Maharashtra, Saurashtra, Malwa, Madhya Pradesh and Chhattisgarh and extends to the south-east along the Godavari and Krishna valleys.

A district example shows how patchy even a black soil state is. In Latur district of Maharashtra the east carries light and medium soil of inferior type, the west has deep black soil, and a part of one taluka has laterite. The heavy deep black cotton soils of the west allow both kharif and rabi crops to be grown.

The Deccan Trap itself, the lava plateau under the soil, is divided into four subprovinces, the main Deccan, the Malwa Plateau, the Mandla Lobe and the Saurashtran Plateau. Two of these names, Malwa and Saurashtra, are also black soil plateaus, which ties the rock to the soil.

Extent of Black Soil and Its Place Among India's Soil Groups

India's soils are grouped in more than one way. A simple scheme has six major types, namely alluvial, black, red, laterite, desert and mountain or alpine soil. Black soil is the group tied most closely to one rock type, which is why UPSC asks about it so often and why it shows clearly how parent rock shapes soil.

Black soils (Vertisols) cover an area of about 74 million hectares, mainly in the central, western and southern states of India. A fuller agricultural scheme puts India's soils in eight major groups, of which four, the alluvial, black, red and lateritic soils, are of agricultural importance. The other four are saline and sodic soils, desert soils, forest and hill soils, and peaty and marshy soils.

Formation of Black Soil from Deccan Trap Basalt

Deccan Trap: Flood Basalt Province Under the Black Soil

The Deccan Trap is a flood basalt province, one of the largest volcanic features on Earth. Flood basalt provinces are made by volcanoes that pour out highly fluid lava, which flows for long distances; some flows are more than 50 m thick, and individual flows may extend for hundreds of km. The Deccan lavas came out through cracks and fissures in the crust, and the wall-like structures called dykes, which are the commonest intrusive forms in western Maharashtra, are considered the feeders of these eruptions.

The Deccan Trap is many layers of solidified flood basalt that together are more than about 2 km thick, cover an area of about 0.5 million square km, and may originally have covered about 1.5 million square km. Erosion and plate movement have reduced it to its present size, though the lava still covers most of the Maharashtra plateau. At least 95 per cent of the lava is tholeiitic basalt, a basic rock. The step-like hills of the region gave the rock formations their name, since trap comes from the Swedish word for stairs.

Questions on black soil are also questions on rock. The basalt is the parent material, and the plateau of flat-topped hills it forms carries the soil, the rivers and the farming of western India.

Age of the Deccan Trap and the Indian Plate

The outpouring is tied to plate movement. While the Indian plate moved north towards the Eurasian plate, a major event was the outpouring of lava and the formation of the Deccan Trap; the subcontinent was then still close to the equator. NCERT dates the start to about 60 million years ago. Other accounts put the start at about 66 million years ago, at the end of the Cretaceous, with the main eruption between 66 and 65 million years ago. Where the two differ, NCERT's figure of about 60 million years is the textbook value.

The eruptions are thought to have been produced in major part by the Réunion hotspot, which also created the Mascarene Islands. Earlier writers blamed the eruptions for the end-Cretaceous mass extinction. The present consensus is that the Chicxulub impact was the primary cause and that the Deccan eruptions played a minor role at most.

Weathering of Basalt into Black Soil

Four steps in a row. Basalt lava of the Deccan Trap, a fissure flood lava and a basic rock. Weathering in wet and dry seasons breaks the rock into very fine material. That material is expansive clay, montmorillonite, which swells, shrinks and holds water. The result is black soil, or regur, clayey, rich in lime and potash and poor in phosphorus. Below: Rivers move the basalt, so black soil also forms on outer tracts. Parent rock and climate together form black soil.

Two factors make black soil, the parent rock and the climate. The parent rock is basalt. Vertisols typically form from highly basic rocks such as basalt, in climates that are seasonally humid or subject to erratic droughts and floods, or where drainage is impeded. As the basalt weathers in the wet and dry seasons of the Deccan, it breaks down into very fine material, and much of that material is expansive clay, a good deal of it belonging to the group of minerals called montmorillonites.

The soil stays close to the rock that made it. That is why black soil is the soil of the Deccan Trap region, and why the same soil is also found on many outer tracts where rivers have carried the basalt from its original place. The movement only increases the share of the loose fragments in the soil.

Characteristics of Black Soil

Physical Properties: Clay, Swelling, Cracking and Self-Ploughing

Two cross-sections of black soil over basalt. Wet season: The clay swells and turns sticky; it is hard to plough when wet and is tilled after the first shower. Dry season: The clay shrinks and deep cracks open, air reaches the deep layers, and surface soil falls into the cracks, which is called self-ploughing. Alternate swelling and shrinking keeps mixing the soil.

Black soils are made up of extremely fine, clayey material. Clay in a Vertisol is expansive; it swells when water enters the soil and shrinks as the soil dries. The soil swells and becomes sticky when wet, and develops deep cracks during hot weather. Even so, black soils resist wind and water erosion because of their iron-rich granular structure.

The cracks matter for the farmer. They help in the proper aeration of the soil by letting air reach the deeper layers. In some Vertisols the loose surface material falls into the cracks and the alternate shrinking and swelling keeps mixing the soil, an action called self-ploughing; as a result some of these soils have a very deep upper horizon and no real subsoil layer. Where the underlying material is heaved to the surface, it can raise a hummocky microrelief called gilgai.

The same shrink and swell has a cost outside the field. It can damage buildings and roads and lead to subsidence, and in dry periods livestock have been injured by falling into cracks.

The natural vegetation of Vertisols is grassland, savanna or grassy woodland. The heavy texture and unstable behaviour of the soil make it difficult for many tree species to grow, and forest is uncommon.

Moisture Retention and Tillage

The black soils are well known for their capacity to hold moisture. The clay absorbs water slowly and gives it up slowly, so the soil keeps moisture for a very long time. That is why rain-fed crops sown on black soil can carry on into the dry weather. Black soils are poor in humus yet highly moisture-retentive, and so they respond well to irrigation.

The other side of the same property is the difficulty of working the soil. It is very hard when dry and very sticky when wet. It is therefore difficult to work unless it is tilled immediately after the first shower or during the pre-monsoon period. Rain-fed farming on Vertisols is difficult for this reason, because the soil can be worked only under a narrow range of moisture conditions.

Chemical Properties and Nutrient Status

Black soil takes its chemistry from basalt. It is rich in nutrients such as calcium carbonate, magnesium, potash and lime. It is generally poor in phosphoric content. It is also poor in humus. Even so, these soils are inherently very fertile.

Table 1. Nutrient status of black soil
Nutrient or constituent Status in black soil
Lime and calcium carbonate Rich
Magnesium Rich
Potash Rich
Phosphorus Poor
Humus Poor

Compare the chemical profile with alluvial soil, whose adequate share of potash, phosphoric acid and lime makes it ideal for sugarcane, paddy and wheat. Black soil's special strength is water more than nutrients.

Crops and Farming on Black Soil

Crops Grown in Black Soil: Cotton, Soybean, Sorghum and Pulses

Black soil is ideal for growing cotton, which is why it is called black cotton soil. Under rain-fed conditions the black soils are used for growing cotton, millets, soybean, sorghum and pigeon pea. Under irrigated conditions they can take a wider range, such as sugarcane, wheat, tobacco and citrus crops.

Cotton grows well in the drier parts of the black cotton soil of the Deccan plateau. It requires high temperature, light rainfall or irrigation, 210 frost-free days and bright sunshine. It is a kharif crop and takes 6 to 8 months to mature, which lines up with the soil's ability to hold the monsoon's moisture into the dry months.

The agro-climatic zones give the regional pattern. The Western Plateau and Hills zone, covering Maharashtra and western Madhya Pradesh, grows cotton, soybean, sorghum and pulses; the Southern Plateau and Hills zone, including Telangana, grows millets, pulses, oilseeds and cotton; and the Gujarat Plains and Hills zone grows cotton, groundnut, tobacco and cumin.

Black Soil Regions in the Agro-Ecological Map

Bar chart of five agro-ecological regions by area in million hectares. Hot semi-arid, shallow and medium black soils of the Deccan plateau: 31.0, millets, cotton and pulses. Hot subhumid, red and black soils of the Malwa and Bundelkhand highlands: 22.3, rice, wheat, sorghum and soybean. Hot semi-arid, medium and deep black soils of the Malwa highlands, Gujarat plains and Kathiawar: 17.6, millets, wheat and pulses. Hot semi-arid, red and black soils of the Telangana plateau: 16.5, millets, oilseeds, rice and cotton. Hot arid, red and black soils of the Deccan plateau: 4.9, sorghum, safflower and cotton.

The agro-ecological regions of India combine soil, climate and growing period, and five of them are named for black soils or mixed red and black soils. Together they spread over the Deccan plateau, the Malwa highlands, the Gujarat plains and Kathiawar, the Telangana plateau and Bundelkhand. In the region of medium and deep black soils on the Malwa highlands, Gujarat plains and Kathiawar peninsula, millets, wheat and pulses dominate; in the Deccan plateau region of shallow and medium black soils, millets, cotton, pulses and, under irrigation, sugarcane are the major crops.

The region names also give the depth of the soil. The Deccan plateau has shallow and medium black soils, while the Malwa highlands, the Gujarat plains and Kathiawar have medium and deep black soils. The same soil group appears under different depths, and the leading crops differ with the region.

Problems in Using and Managing Black Soil

Black soil is fertile but not easy to farm. The main difficulties are physical, then chemical.

  • Narrow tillage window: The soil is very hard when dry and very sticky when wet, so it can be ploughed only within a narrow range of moisture and soon after the first shower or in the pre-monsoon period.
  • Rain-fed risk: In rain-fed areas, where the farmer cannot choose the moment, a delayed shower can mean a late sowing.
  • Low phosphorus and humus: The soil is poor in phosphorus and humus, so fertiliser and organic additions are needed even where the clay is rich.
  • Engineering damage: Shrinking and swelling can crack buildings and roads and cause subsidence.
  • Cracks in the field: The cracks that give aeration are also a hazard for livestock in dry periods.

Black Soil Compared with Other Indian Soils

Difference Between Black, Red, Laterite and Alluvial Soil

Each main soil takes its character from its parent material and the climate. Black soil stands apart from the others in three ways, a basalt parent, a clay texture and a store of lime and potash.

Table 2. Black, red, laterite and alluvial soil compared
Soil Parent material and formation Nutrient and water character Typical crops
Black soil Weathering of Deccan Trap basalt in a seasonal climate Clayey, moisture-retentive, cracks when dry; rich in lime and potash, poor in phosphorus Cotton, soybean, sorghum, pulses; sugarcane with irrigation
Red and yellow soil Crystalline igneous rocks in low rainfall areas of the east and south of the Deccan plateau Reddish from the diffusion of iron in crystalline and metamorphic rocks; looks yellow in hydrated form Millets, rice, groundnut and maize under good management
Laterite soil Intense leaching under heavy rain in a tropical climate with alternate wet and dry seasons Deep, acidic, generally deficient in plant nutrients; prone to erosion Tea and coffee on conserved hill slopes; cashew nut on red laterite
Alluvial soil Deposited by the Indus, the Ganga and the Brahmaputra and by deltaic rivers Adequate potash, phosphoric acid and lime; very fertile Sugarcane, paddy, wheat and other cereals and pulses

Soils fall into three kinds by origin. Black soil is a residual soil, formed on the rock it came from. Alluvial soil is a transported soil, laid down by rivers. Laterite is a leached soil, formed where heavy rain washes the soluble minerals out and leaves iron and aluminium behind.

Soils That Share the Deccan with Black Soil

Black soil and red soil both occur on the Deccan, which is why they are so often confused. The difference lies in the parent rock and the rainfall. Black soil lies on the basalt of the north-west. Red soil develops on crystalline igneous rocks in areas of low rainfall in the eastern and southern parts of the plateau. Laterite occurs mostly in the southern states, the Western Ghats region of Maharashtra and Odisha.

The Deccan Trap as a Natural Resource

Soil, Water and Power from the Deccan Trap

Cross-section of stacked lava flows under black soil: Weathered, jointed basalt, then vesicular basalt 15 to 25 m thick, which forms the potential zone, then massive basalt, repeated below. A dug well taps phreatic water to 15 to 20 m and yields 25 to 250 cubic metres a day. A bore well taps semi-confined to confined water and yields 0.50 to 52.00 cubic metres a day. Promising deeper aquifers occur even below 100 m. Figures from the Latur district.

The Deccan Trap gives western India much more than its soil. Three resources follow from the rock itself:

  • Soil and farming: Black soil on the weathered basalt supports the cotton, soybean, sorghum and pulse farming of the plateau.
  • Groundwater: In the Deccan Trap basalt of Latur district, water occurs in weathered, fractured and jointed basalt, and in vesicular or massive basalt, under phreatic and semi-confined to confined conditions. It occurs under phreatic conditions at shallow depths up to 15 to 20 m, and under semi-confined to confined conditions at deeper levels. The vesicular portion of the lava flows varies in thickness from 15 to 25 m and forms the potential zones. The yield of dug wells in the upper phreatic aquifer ranges between 25 and 250 cubic metres a day, while the yield of bore wells varies from 0.50 to 52.00 cubic metres a day, depending on local conditions. Dug wells are the main structures of the district. Exploration data show promising deeper confined aquifers even below 100 m.
  • Hydropower: The Koyna river rises at Mahabaleshwar in the Sahyadri ranges, and the Koyna Hydroelectric Project, with a total installed capacity of 1,960 MW, is one of the largest hydroelectric power plants in India.

The limits are as important as the resources. In Latur the dug wells far outyield the bore wells, so a district that relies on them is exposed when the shallow aquifer runs low. Watershed management, rainwater storage and crops suited to the soil ease these limits.

Laterite, Stone and Heritage of the Deccan Trap Country

Laterite is derived from the leaching of parent rocks that include basalts. Its name comes from the Latin word for brick, since the compacted, cemented soil can easily be cut into brick-shaped blocks for building.

Basalt cliffs also gave India some of its finest rock-cut architecture. The caves at Ajanta are excavated out of a vertical cliff above the left bank of the river Waghora; they are thirty in number, including the unfinished ones. The 34 monasteries and temples of Ellora, extending over more than 2 km, were dug side by side in the wall of a high basalt cliff, not far from Aurangabad in Maharashtra. The Deccan Trap is exposed at Ajanta.

Previous Year UPSC‑CSE Questions

Each Prelims question is explained option by option, and each Mains question comes with an answer outline.

  1. UPSC Prelims 2021The black cotton soil of India has been formed due to the weathering of
    1. a brown forest soil
    2. b fissure volcanic rock
    3. c granite and schist
    4. d shale and limestone
    How to approach this Prelims question

    Question type: Single correct answer

    Approach: Name the parent rock of black soil and test each option against it. Black soil is typical of the Deccan Trap (basalt) region, made of lava flows, and that basalt came out through fissures as flood lava. Weathering of that basalt in a seasonal climate gives the clay-rich black soil.

    Trap to watch: Granite and schist sound like hard parent rocks, but crystalline rocks give the red soils, not the black soil.

    Why each option is right or wrong:

    • (a) brown forest soil: Wrong. Forest soils belong to hilly and mountainous areas, not to the lava plateau of the Deccan; black soil forms from rock, not from an older soil.
    • (b) fissure volcanic rock: Correct. The Deccan Trap basalt came out of fissures as flood lava, and its weathering made the black soil.
    • (c) granite and schist: Wrong. Red and yellow soils form on crystalline and metamorphic rocks, with their colour from iron, so these rocks give red soil and not black soil.
    • (d) shale and limestone: Wrong. These are sedimentary rocks, while the parent rock of black soil is the Deccan Trap basalt, an igneous rock.

    Answer: (b) fissure volcanic rock. Black soil is the weathering product of the Deccan Trap, a basalt that erupted through fissures.

  2. UPSC Mains 2022 GS‑IDiscuss the natural resource potentials of 'Deccan Trap'.
    Answer outline

    The outline sets out what the question asks, the main arguments, relevant examples and the direction of the conclusion; it is not a full model answer.

    Directive verb: Discuss

    Approach: Introduce the Deccan Trap as a flood basalt province, then take each resource in turn with its limit, and close with how the resources can be used sustainably.

    Introduction: The Deccan Trap is a flood basalt province of west-central India, more than about 2 km thick and covering about 0.5 million square km.

    Body (sub-themes to develop):

    • Soil: Weathering gives black soil, moisture-retentive, rich in lime, magnesium and potash, poor in phosphorus, ideal for cotton, with soybean, sorghum and pulses.
    • Water: Groundwater in weathered, fractured and vesicular basalt; dug wells yield 25 to 250 cubic metres a day against 0.50 to 52.00 for bore wells.
    • Power: The Koyna project of 1,960 MW in the Sahyadri country.
    • Building material and heritage: Laterite cut into bricks, and the rock-cut caves of Ajanta and Ellora in basalt cliffs.
    • Limits: Low-yield aquifers and a soil that is hard to work.

    Conclusion: Watershed management, rainwater storage and crops suited to the soil turn these resources into lasting gains.

Sources

Editorial Disclaimer

This article draws on the NCERT geography textbooks, the FAO, ICAR, the Central Ground Water Board, UNESCO and the other sources in the Sources list. Map markers show named places, not the exact extent of the soil.

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