Why does Deccan black soil crack in summer?
Ancient basalt helped form many Deccan black soils. Their swelling clay shapes summer cracks, stored moisture and farming.
Why does black soil in parts of the Deccan crack in summer and become sticky after rain? Its clay swells when wet and shrinks when dry. The story of how that clay formed leads back to ancient lava flows.
Series: Why India Looks This Way
From volcanic rock to soil
The Deccan Traps are layers of basalt, a rock formed when lava cools. They cover a large part of west-central India. As basalt weathered—broke down and changed—it supplied material for many of the region's black soils.
ICAR's National Bureau of Soil Survey and Land Use Planning identifies smectite, a family of clay minerals that swell and shrink, as an early product of basalt weathering. Some material also moved downhill and collected in valleys. The soil in a field therefore need not have formed entirely from the rock beneath it.
Basalt alone does not determine the soil. Rainfall, drainage and moving sediment also shape it. The bureau's research describes clay that formed during an earlier humid climate and survived under later conditions in which less water drained through the soil, carrying dissolved material with it. Lava did not simply crumble into a ready-made field.
Why wet clay swells and dry soil cracks
As the clay takes up water, it expands; as it dries, it contracts. That repeated change in volume pulls the soil apart into summer cracks, which close when rain wets it again.
The Kerala Soil Survey department's description of Vertisols describes clay-rich soils that are sticky when wet and very firm when dry. Vertisols are a group of soils with pronounced swelling and shrinking. India's broader black-soil region includes related soils too; its fields do not all behave identically.
In a long-term fertilizer experiment at Akola, researchers describe black soil containing smectite with these swelling and shrinking properties. It is one documented example of the clay behind the seasonal cracks.
Stored moisture helps, but drainage matters
The clay can hold water that supports crops after rain stops. Where rain arrives in a distinct season, the moisture stored below the surface can matter long after a shower has passed.
Holding water also means managing the excess. An ICRISAT account published by the FAO describes both the value of stored moisture for rainfed farming and the drainage problems of these soils. Once the wet soil is full, more water can enter slowly, increasing runoff and the risk of waterlogging. A field may need drainage during rain, then rely on its stored moisture later.
Farmers also have a narrower window to work the ground: it is sticky when wet and hard when dry. The clay that helps store water makes cultivation harder at either extreme.
Why people call it black cotton soil
Black soil is also called regur or black cotton soil. NCERT's geography text associates it with cotton cultivation and the Deccan basalt region, including the plateaus of Maharashtra, Malwa, Madhya Pradesh and Chhattisgarh and the Godavari and Krishna valleys.
The name does not limit what can grow there. The Akola experiment, for example, studies a sorghum–wheat sequence on black soil. Farmers choose crops according to rainfall, soil depth, drainage and management as well as soil type.
Dark colour does not guarantee plentiful nutrients. Akola's experimental soil was low in available nitrogen despite having high available potash. A soil test can reveal what its appearance cannot.
Is every black soil made from Deccan lava?
Many Deccan black soils have a basalt connection, but others come from different origins. The ICRISAT account describes soils derived from other rocks, calcareous clays and transported material. A field's dark soil does not, by itself, identify the rock beneath it.
To understand a particular field, the rock is a clue, while its clay and drainage tell farmers how the soil will behave through wet and dry seasons.