Overview
Previous Year UPSC-CSE Questions By the end you will be able to draft model answers for the following UPSC questions. Each question carries a collapsible framework showing how to approach it in the exam.
- UPSC Prelims 2021The black cotton soil of India has been formed due to the weathering of
How to approach this Prelims question
Approach: Identify the parent rock. Black Cotton Soil derives from the Deccan-Trap fissure-eruption basaltic lava sheets that flooded peninsular India in the late Cretaceous (around 66 million years ago). The weathering of this basalt under sub-humid tropical climate produces the montmorillonite clay that gives the soil its shrink-swell signature and its iron-magnesia-lime chemistry.
Trap to watch: Option (c) granite-and-schist is the parent-rock of red soil (crystalline peninsular gneiss), not Black Cotton Soil; option (d) shale-and-limestone yields calcareous soils, not the clay-rich Vertisol.
Key facts to recall:
- Deccan Trap is a flood-basalt province from fissure-eruption volcanism
- Montmorillonite is the dominant clay mineral of Black Soil
- Iron, magnesia, lime, and alumina inherit directly from basalt chemistry
Answer signal: Correct answer is (b): fissure volcanic rock (Deccan-Trap basalt).
Black Soil, also called Regur and Black Cotton Soil, is the clayey soil derived from weathering of basaltic Deccan-Trap lava flows that covers most of the Deccan Plateau across Maharashtra, Madhya Pradesh, Gujarat, parts of Karnataka, Telangana, and Andhra Pradesh. The soil is famous for its capacity to hold moisture through the dry season and for the deep summer cracks that produce a self-ploughing effect. Its chemistry is dominated by lime, iron, magnesia, and alumina; it contains potash but is deficient in nitrogen, phosphorus, and organic matter. The colour grades from deep black to grey. In the ICAR-USDA Soil Taxonomy it is the principal Indian member of the Vertisols order.
Background and Historical Context
Why it matters: Black soil underwrites the commercial cotton economy of India and the dry-farming systems of the Deccan Plateau. It covers roughly 5.46 lakh square kilometres, close to 16 per cent of India's geographical area, with Maharashtra holding the largest single-state share. UPSC Prelims has tested the basaltic weathering origin and the texture-chemistry link; Mains GS-I treats black-soil distribution as a foundational input to regional agricultural geography.
What is the significance of mastering black-soil geography? Three operational dimensions follow. The parent-rock-to-soil-chemistry link explains why basaltic Deccan Trap yields a clay-rich, lime-rich, iron-rich soil while granitic peninsular outcrops yield the sandy red soil covered in Soils Part 4. The shrink-swell behaviour of the montmorillonite clay mineral explains why black-soil tracts develop deep cracks during the hot pre-monsoon months and turn sticky when wet, shaping the timing of every agricultural operation. The moisture-retention capacity explains why the soil sustains rain-fed cotton, sorghum, and pulses across the rain-shadow Deccan interior despite limited monsoon rainfall.
Contemporary linkages: The Cotton Corporation of India and the Maharashtra cotton procurement system operate minimum-support-price purchase on black-soil tracts; the Pradhan Mantri Krishi Sinchayee Yojana targets micro-irrigation expansion in dry-farming belts; the National Mission on Edible Oils and pulse promotion pushes oilseed-pulse intensification to break cotton-cereal mono-cropping. Sustainability concerns include soil salinity from canal-irrigation seepage on irrigated tracts, compaction under heavy mechanisation, and the pink-bollworm resistance that is shifting the crop mix on traditional cotton land.
Introduction: Black Soil as the Deccan Trap Signature
Naming, definition, extent
Black Soil is the regional name for the clayey, dark-coloured soil that covers the Deccan Plateau across western and central India. The local Telugu name Regur (from Reguda) is the conventional usage in Andhra Pradesh and Telangana; the Marathi-Hindi name Kali Mitti is used across Maharashtra and Madhya Pradesh; the popular name Black Cotton Soil reflects the soil's near-exclusive role in the country's commercial cotton agriculture.
The soil covers roughly 5.46 lakh square kilometres, close to 16 per cent of India's geographical area; in the ICAR-USDA Soil Taxonomy it is the principal Indian member of the Vertisols order, and India holds about a quarter of the world's Vertisols. The contiguous belt covers Maharashtra (largest single-state share), western Madhya Pradesh, Gujarat (the Saurashtra pocket), parts of Karnataka, Telangana, and the western districts of Andhra Pradesh. Smaller outliers occur in Tamil Nadu and southern Rajasthan.
Formation: Basaltic Lava Weathering and Montmorillonite Chemistry
How Deccan-Trap basalt produces Vertisol clay
Why the parent rock sets the soil chemistry: The chemistry of the parent rock decides the chemistry of the soil. Black soil owes its colour, clay content, shrink-swell behaviour, and high lime-iron-magnesia content to a single geological event: the Deccan Trap volcanic episode in the late Cretaceous, around 66 million years ago, that flooded peninsular India with successive basaltic lava sheets. The basalt is rich in ferromagnesian minerals and aluminium silicates.
- (i) Source rock: Deccan-Trap basalt; ferromagnesian minerals (pyroxene, olivine, plagioclase feldspar) supply iron, magnesium, calcium, and aluminium.
- (ii) Weathering regime: Sub-humid tropical climate with hot dry summers and a strong monsoon; alternating wet and dry cycles drive intense hydration-dehydration weathering of the basaltic parent rock.
- (iii) Clay product: Montmorillonite is the dominant clay mineral; it has an expanding 2:1 lattice that holds water between layers, swells dramatically when wet, and shrinks sharply when dry. This single mineral property determines the soil’s signature shrink-swell behaviour.
Weathering concentrates the iron-magnesia-lime fingerprint of the parent basalt into the soil profile while leaching out silica selectively. The dark colour comes from the iron-rich mineral residue plus organic matter trapped in the fine clay pore network.
Characteristics: Self-Ploughing Cracks and Moisture Retention
Shrink-swell behaviour, summer cracks, monsoon stickiness
How the shrink-swell cycle drives every operation: The montmorillonite clay mineral controls almost every working feature of black soil. During the hot pre-monsoon months the soil dehydrates, contracts, and develops deep cracks that can reach a metre in depth.
These cracks ventilate the lower horizons, let surface litter fall inward, and produce a self-ploughing effect that reduces the need for primary tillage in some traditional systems. When the monsoon arrives, water enters the cracks; the clay swells, the cracks seal, and the surface turns sticky and hard to work.
- Distinguishing feature: deep summer cracks. Pre-monsoon dehydration drives the clay to contract; cracks develop along the soil surface and extend downward; air circulation through the cracks supports microbial activity and oxidises iron compounds.
- Distinguishing feature: monsoon stickiness. Wet montmorillonite swells, fills the cracks, and produces a plastic, sticky working surface; mechanical cultivation is delayed until the surface dries to a tilth window, typically between two and four days after a major rainfall event.
- Distinguishing feature: moisture-retention capacity. The fine clay pore network holds water tightly; Black Soil supplies sub-soil moisture to deep-rooted crops well into the post-monsoon dry months, sustaining rain-fed cotton, sorghum, and pulses.
The optimal tillage practice is to enter the field immediately after the first monsoon showers while the surface clay is wet enough to break the crust but not yet plastic. This narrow operational window shapes the labour and machinery scheduling across the entire Vertisol belt.
Chemistry: Lime-Iron-Magnesia-Alumina Plus Nitrogen Deficiency
What the basaltic origin leaves behind in the nutrient profile
Observable outcomes of the basaltic weathering regime. Three nutrient characteristics shape every fertiliser recommendation for Black Soil cropping systems and underwrite the Soil Health Card prescriptions issued across the Vertisol belt.
- (a) Rich in lime, iron, magnesia, and alumina. The basaltic parent rock supplies calcium, iron, magnesium, and aluminium; weathering concentrates these into the soil profile. Lime accumulation as nodules in lower horizons is common.
- (b) Contains potash; deficient in nitrogen, phosphorus, and humus. The nutrient profile drives the heavy nitrogenous-fertiliser and phosphatic-fertiliser consumption that characterises Maharashtra and Madhya Pradesh cotton cropping; pulse rotations are promoted to break the nutrient drain.
- (c) Colour grades deep black to grey. The darkest profiles occur on level land with stable monsoon recharge; greyer profiles dominate the upland slopes where leaching removes organic accumulations faster.
| Nutrient or property | Status in black soil | Agronomic consequence |
|---|---|---|
| Lime (calcium carbonate) | Rich; often as kankar nodules | Neutral to alkaline pH; favours cotton |
| Iron, magnesia, alumina | Rich; inherited from basalt | Dark colour; high clay activity |
| Potash | Adequate to rich | Lower potassic-fertiliser need |
| Nitrogen | Deficient | Heavy urea use; pulse rotation advised |
| Phosphorus | Deficient | Phosphatic-fertiliser dependence |
| Humus (organic matter) | Low | Soil Health Card stresses organic inputs |
The combination of lime-rich plus nitrogen-poor chemistry explains why black soil suits cotton, a deep-rooted crop tolerant of high pH, yet handles shallow-rooted, nitrogen-hungry winter wheat poorly. The pH usually runs neutral to slightly alkaline; salt build-up in canal-irrigated tracts pushes the soil toward salinity that needs gypsum reclamation.
Distribution and Agricultural Significance
The Maharashtra-MP-Gujarat-Karnataka-Telangana belt
Where the black-soil belt drives the rural economy: The black-soil belt carries India's commercial cotton economy and a large share of its dry-farmed pulses and oilseeds, and it lies behind the Cotton Corporation of India minimum-support-price purchase system. The Indian Council of Agricultural Research's Central Institute for Cotton Research at Nagpur leads belt research, while the National Bureau of Soil Survey and Land Use Planning publishes the district soil maps.
- Maharashtra: Largest single-state share; the Vidarbha and Marathwada divisions concentrate cotton cultivation; Saurashtra-style deep Vertisols also occur in western Maharashtra.
- Madhya Pradesh: Malwa Plateau and Narmada valley Vertisols support cotton, soybean, and pulses; soybean has overtaken cotton in some districts.
- Gujarat: Saurashtra deep Vertisols and the Surat plain carry groundnut, cotton, and pulses; salinity-prone tracts near the Kachchh-Saurashtra coast.
- Karnataka and Telangana: Northern Karnataka (Belgaum-Dharwad-Gulbarga) and the Krishna-Godavari upper-reach Vertisols carry cotton, jowar, and pulses.
- Andhra Pradesh and Tamil Nadu outliers: Rayalaseema and western Tamil Nadu Vertisol patches support cotton and pulses on a smaller scale.
Prelims MCQ practice
Each question below tests one specific concept on the topic. Click to reveal the answer and a full option-wise explanation.
Q1. Consider the following statements about black soils (regur) in India:
- Black soils are formed primarily from the weathering of Deccan trap basaltic lava.
- Black soils are concentrated in Maharashtra, Madhya Pradesh, Gujarat, Karnataka, Andhra Pradesh, and parts of Tamil Nadu.
- Black soils are azonal soils formed by river deposition rather than in-situ weathering.
Which of the statements given above are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1 and 2 only
Explanation.
Correct: a (1 and 2 only). Statement 1 is correct: black soils form from in-situ weathering of Deccan trap basaltic lava. Statement 2 is correct: they cover the Deccan plateau across Maharashtra, MP, Gujarat, Karnataka, AP, and parts of TN. Statement 3 is wrong: black soils are RESIDUAL soils formed by in-situ weathering, NOT azonal river-deposited soils.
Q2. Consider the following statements about the agronomic properties of black soils:
- Black soils are clayey-textured with high moisture-retention capacity.
- Black soils develop deep cracks during the dry season and become sticky when wet, complicating tillage.
- Black soils are agronomically suited primarily to cotton, jowar, sugarcane, and pulses.
Which of the statements given above are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1, 2 and 3
Explanation.
Correct: d (1, 2 and 3). All three statements are correct. Black soils are clayey with high moisture retention. They crack during the dry season (the self-mulching property) and become sticky when wet, making tillage difficult except in specific moisture windows. They are agronomically suited primarily to cotton (which is why they are called 'black cotton soil'), jowar, sugarcane, and pulses.
Q3. Consider the following statements about black soil chemistry:
- Black soils are rich in calcium carbonate, magnesium, potash, and lime.
- Black soils are generally deficient in nitrogen, phosphorus, and humus.
- Black soils are extremely acidic with pH typically below 4.0.
Which of the statements given above are correct?
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Show answer and explanation
Answer: 1 and 2 only
Explanation.
Correct: a (1 and 2 only). Statement 1 is correct: black soils are rich in calcium carbonate, magnesium, potash, and lime. Statement 2 is correct: they are deficient in nitrogen, phosphorus, and humus. Statement 3 is wrong: black soils are NEUTRAL to ALKALINE (pH around 7.0 to 8.5) due to the calcium-carbonate content, NOT extremely acidic; acidic soils are characteristic of laterite and forest soils, not regur.
Q4. Consider the following statements about black-soil classification:
- Black soils are classified as Vertisols in the USDA soil taxonomy due to their shrink-swell behaviour.
- Regur is the colloquial Indian name for black soil; the term derives from Marathi-Telugu usage in the Deccan.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer and explanation
Answer: Both 1 and 2
Explanation.
Correct: c (Both 1 and 2). Statement 1 is correct: black soils are classified as Vertisols under USDA taxonomy due to their characteristic shrink-swell (vertic) behaviour as clay minerals expand on wetting and shrink on drying. Statement 2 is correct: regur is the Marathi-Telugu colloquial name for the Deccan black-cotton soil.
Q5. Consider the following statements about black soils and cotton cultivation:
- Black soils' high moisture-retention capacity makes them well-suited to dryland cotton cultivation in low-rainfall tracts.
- Maharashtra, Gujarat, and Telangana are major cotton-producing states on the black-soil belt.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer and explanation
Answer: Both 1 and 2
Explanation.
Correct: c (Both 1 and 2). Statement 1 is correct: black soils' moisture-retention capacity (clay minerals hold water for extended periods) makes them suited to dryland cotton in low-rainfall Deccan tracts. Statement 2 is correct: Maharashtra, Gujarat, and Telangana are major cotton-producing states on the black-soil belt.
Q6. Consider the following statements about the extent of black soil in India:
- Black soils cover approximately 16-17 per cent of India's total land area.
- Black soils correspond closely to the Deccan trap basalt outcrop boundary in central and Peninsular India.
Which of the statements given above is/are correct?
- 1 only
- 2 only
- Both 1 and 2
- Neither 1 nor 2
Show answer and explanation
Answer: Both 1 and 2
Explanation.
Correct: c (Both 1 and 2). Statement 1 is correct: black soils cover approximately 16-17 per cent of India's total land area. Statement 2 is correct: the black-soil distribution corresponds closely to the Deccan trap basalt outcrop in central and Peninsular India (Maharashtra, MP, Gujarat, Karnataka, AP); the geological parent material is the defining control.
Sources
- NCERT Class 11 India Physical Environment, Chapter 6 (Soils), p 73
- Indian Council of Agricultural Research (ICAR)
- National Bureau of Soil Survey and Land Use Planning (NBSS&LUP), Nagpur
- ICAR-Indian Institute of Soil Science, Bhopal
- Geological Survey of India: Deccan Trap basaltic province
- USDA Natural Resources Conservation Service: Vertisols order
- Ministry of Agriculture and Farmers Welfare, Soil Health Card
- Wikipedia: Black soil (India)
- Wikipedia: Deccan Traps
Disclaimer
This article on black soil (regur) is intended for UPSC preparation. Figures and named institutions are drawn from the authoritative sources listed below. Readers should consult the latest official data for current statistics.
