Black soil, known in India as regur (from the Telugu reguda) and internationally classed as a Vertisol under the USDA and FAO soil taxonomies, is a dark-coloured, clayey soil derived chiefly from the weathering of the basaltic lava flows of the Deccan Trap during the late Cretaceous–early Eocene volcanic episode. In the Indian Council of Agricultural Research (ICAR) eight-fold soil classification, it ranks as the second most extensive soil group after alluvial soil, covering roughly 5.4 lakh sq km. Its characteristic black colour is attributed by most geographers to the presence of titaniferous magnetite, fine humus, and iron–aluminium compounds rather than to high organic content, which is in fact modest.
The soil's defining feature is its mineralogy: it is rich in montmorillonite and other 2:1 lattice expanding clays that swell when wet and shrink on drying. This produces deep cracks in the dry season — earning it the name "self-ploughing soil," since surface material falls into the fissures and is mixed downward. Black soil is chemically endowed with calcium carbonate, magnesium carbonate, potash, lime, and iron, but is deficient in phosphorus, nitrogen, and organic matter. Its exceptional moisture-retentive capacity makes it ideal for dry farming, allowing crops to mature on stored sub-soil water. Because it becomes sticky and unworkable when wet, it is best tilled immediately after the first monsoon showers. Depth varies from over a metre in lowland river valleys to thin, gravelly cover on uplands.
Geographically, black soil dominates the Deccan plateau and the trap-covered tracts of Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh, Karnataka, and parts of Tamil Nadu — broadly the Godavari, Krishna, Narmada, and Tapi basins. It is the classic cotton soil of the Indian peninsula, and also supports wheat, jowar, linseed, groundnut, tobacco, citrus, and sugarcane. As of 2026 it remains central to India's cotton belt (Vidarbha, Marathwada, Malwa) and to oilseed and pulse cultivation, though salinity build-up under irrigation and waterlogging in canal-fed tracts are recognised management concerns. Analogous Vertisol regions occur in the Black Belt of the southern United States, the Sudanese Gezira, and Australia's Darling Downs.
For UPSC, black soil is a high-frequency topic in the General Studies Paper I geography section and in the Geography optional. Prelims questions typically test the basalt-Deccan parent material, the regur synonym, the montmorillonite clay and self-ploughing property, the cotton association, and the nutrient profile (rich in lime/potash/iron, poor in nitrogen/phosphorus/humus). Mains and optional answers demand the link between geology, soil formation, and agro-economic land use, often paired with comparison to alluvial and red soils or with questions on soil degradation, salinity, and conservation policy. Candidates should be precise that black colour derives from iron/titanium compounds and humus, not from high organic richness — a common distractor in multiple-choice framing.
Example
The cotton economy of Maharashtra's Vidarbha region — central to farmer-distress debates examined by India's 2006 Radhakrishna Expert Group — rests on deep black regur soil weathered from Deccan basalt.
Frequently asked questions
Regur derives from the Telugu word reguda for this soil. It is called self-ploughing because its montmorillonite clay swells when wet and shrinks when dry, opening deep cracks into which surface material falls, mixing the soil naturally.