Black Cotton Soil (Regur)

Black cotton soil, designated regur in Indian pedology and classified internationally as a Vertisol, is one of the eight major soil groups recognised by the Indian Council of Agricultural Research (ICAR) and the National Bureau of Soil Survey and Land Use Planning (NBSS&LUP). Its origin lies in the weathering of the basaltic lava flows of the Deccan Trap, extruded during the late Cretaceous to early Palaeogene volcanic episodes roughly 66 million years ago. The breakdown of these ferromagnesian basalts, under the alternating wet and dry conditions of a semi-arid to sub-humid climate, releases iron, magnesium, calcium, aluminium and lime while concentrating the clay mineral montmorillonite (smectite). The term regur derives from the Telugu word reguda; the colloquial English name "cotton soil" reflects its historic suitability for rain-fed cotton cultivation in peninsular India. The soil is not the product of in-situ basalt weathering alone—in parts of the Tapi, Narmada and Godavari valleys it is transported and redeposited as alluvium, which is why deep regur occurs in lowlands while shallow regur caps the residual uplands.

The defining mechanism of regur is its moisture behaviour, governed by the montmorillonite clay lattice. When wetted during the monsoon the expanding clay swells, becomes sticky and almost impermeable, and holds water tenaciously; on drying it contracts and develops deep, wide polygonal cracks that can extend a metre or more downward. This shrink–swell cycle produces self-ploughing or self-mulching: surface material falls into the cracks and is churned upward, inverting the profile and keeping it homogeneous and largely structureless. The same cracks allow deep-rooted crops to draw on stored sub-soil moisture through the dry season, which is why regur supports a rabi crop without irrigation. The soil's high cation-exchange capacity and retention of lime and potash make it chemically fertile, though it is generally deficient in nitrogen, phosphorus and humus and requires supplementation for high yields.

Regur is subdivided by depth and topographic position into deep black soils (over a metre, found in valleys and lowlands of Maharashtra, Gujarat and Madhya Pradesh), medium black soils, and shallow black soils (on uplands and slopes). Colour ranges from deep black to chestnut-brown, the darkness owing chiefly to the presence of titaniferous magnetite and humic colloidal complexes rather than to organic-matter abundance, which is in fact modest. The soil is typically rich in calcium carbonate, magnesium carbonate and potassium, with a neutral to mildly alkaline pH, and in poorly drained tracts develops salinity and alkalinity. Its clayey texture gives high water-retention but poor drainage and difficult tillage when wet, so cultivators traditionally ploughed it immediately after the first monsoon showers, while it remained moist enough to work yet not waterlogged.

In contemporary terms, black cotton soil covers approximately 5.4 lakh square kilometres—about 16.6 per cent of India's geographical area—spread across the Deccan plateau. Its principal extent lies in Maharashtra (the Vidarbha cotton belt around Nagpur and Amravati), Madhya Pradesh (Malwa plateau and the Narmada valley), Gujarat (Saurashtra and the Tapi basin), and parts of Telangana, Andhra Pradesh, Karnataka and Tamil Nadu. The crop economy built on regur—cotton, soybean, sorghum (jowar), pigeon pea (tur), wheat, gram, linseed, sugarcane, citrus and tobacco—anchors the agrarian distress debates of Vidarbha and the procurement policy discussions of the Cotton Corporation of India. Engineering agencies including the Indian Roads Congress and the Bureau of Indian Standards (IS 1498, IS 2911) treat regur as a problematic expansive subgrade requiring special foundation design.

Black cotton soil must be distinguished from the adjacent major soil groups in the ICAR scheme. Unlike laterite soil, which forms under heavy tropical rainfall by leaching that removes silica and bases while concentrating iron and aluminium oxides, regur retains its bases and is fertile rather than impoverished. It differs from alluvial soil, the nitrogen-poor but generally productive transported soil of the Indo-Gangetic plains, in being residual basalt-derived and dominated by swelling clay. It is also separate from red soil, which forms on crystalline Archaean rocks of the peninsula and owes its colour to ferric oxide under low-clay, well-drained conditions. The international equivalent is the FAO/USDA order Vertisol, from the Latin vertere, "to turn," referencing the self-churning action.

Controversy and edge cases attach chiefly to engineering and agronomy. The high swelling pressure of regur causes cracking of building foundations, road pavements and canal linings, prompting techniques such as under-reamed piles, cohesive non-swelling (CNS) layers and lime stabilisation. Agriculturally, the soil's poor internal drainage makes it prone to waterlogging and secondary salinisation under unscientific irrigation, a recurring issue in canal command areas of Maharashtra and Gujarat. The Vidarbha farmer-suicide crisis has repeatedly drawn the fertility of regur into policy debate, since rain-fed cotton on this soil is highly sensitive to monsoon failure despite the soil's moisture-retention capacity.

For the working practitioner—particularly civil-services aspirants preparing General Studies Paper I physical geography, agricultural-policy analysts, and land-use planners—regur exemplifies how lithology, climate and clay mineralogy jointly determine agrarian and infrastructural outcomes. A precise grasp of its basaltic origin, montmorillonite-driven shrink–swell behaviour, nutrient profile and regional distribution enables accurate distinction from laterite, alluvial and red soils in examination and field contexts alike, and explains the persistent economic and engineering challenges of India's central cotton belt.