Black soils, locally called regard or black cotton soils, and internationally known as 'tropical black earths' or 'tropical chernozems' have been developed by the weathering of the Deccan lava in majorparts of Maharashtra, western MadhyaPrades (Hoshangabad, Narsinghpur, Damoh, Jabalpur, Raisen and Shahdol districts), Gujarat (Surat, Bharuch, Vadodara, Kheda, Sabarkantha and Dang districts), Andhra Pradesh (Adilabad, Warangal, Khammam, Mahbubnagar, Kurnool, Guntur and Karimnagar districts), Karnataka (Bijapur, Dharwar, Gulbarga, Bidar, Belgaum, Raichur, Bellari and Chitradurga districts), Rajasthan (Kota, Bundi, Sawai Madhopur, Bharatpur and Banswara districts), Tamil Nadu (Ramnathpuram, Tirunelvelli, Coimbatore, Madurai and South Arcot districts) and Uttar Pradesh (Jalaun, Hamirpur, Banda and Jhansi districts).
According to Krebs the regur soil is essentially a mature soil which has been produced by relief and climate rather than by a particular type of rock. It occurs where the annual rainfall is between 50 cm to 75 cm and the number of rainy days is from 30 to 50.
The colour of these soils varies from deep black to light black and chestnut and is dependent on the colour of the mechanical fractions.
The black color is attributed to the presence of titaniferous magnetite, compounds of iron and aluminum, accumulated humus and colloidal hydrated double iron and aluminum silicate. In general these soils have clay texture, average clay content being 50% and the range being 40-50%. Except in cases where there is stratification, the clay content down the profile is uniform.
The structure of these soils is usually cloddish but occasionally friable. Regard soils are calcareous neutral to mild alkaline in reaction, high in carbon exchange capacity and low in organic matter. In general these soils are rich in iron, lime, calcium, potash, aluminum and magnesium carbonates but poor in nitrogen, phosphorus and organic matter. The chemical test near Nagpur yields soluble matter 68.71%, ferric oxide 11.24%, alumina 9.3%, water and organic matter 5.83%, lime 1.81% and magnesium 1.79%.
Black soils are highly retentive of moisture, extremely compact and tenacious when wet, considerably contracted developing deep wide cracks on drying and self-ploughing. Black soils are credited with high fertility. These are well suited to leguminous crops like cotton, turn and citrus fruits. Other crops include wheat, jowar, millets, linseed, castor, tobacco, sugarcane, safflower, vegetables etc. On the uplands these soils are comparatively less fertile than on the low lands.
On the basis of the proportion of clay and silt regard soils are divided into two broad groups:
(a) Trappean Black Clayey Soil-it occupies major parts of the Peninsular India. This soil is very heavy owing to finer constituents (65% to 80%).
(b) Trappean Black Loamy Soil-in this soil the silt-content varies between 30 and 40 per cent. It occurs in patches in the Wainganga valley and northern Konkan coast.
Based on the thickness of layers black soils may be divided into three sub-groups:
(a) Shallow Black Soil-its thickness is less than 30 cm. It mainly occupies Satpura hills (Madhya Pradesh), Bhandara, Nagpur and Satara (Maharashtra), Bijapur and Gulbarga districts (Karnataka). The soil is utilised in the cultivation of jowar, rice, wheat, gram and cotton.
(b) Medium Black Soil-its thickness ranges between 30 cm and 100 cm. It covers a larger area in Maharashtra, Gujarat, Madhya Pradesh, Tamil Nadu and Andhra Pradesh.
(c) Deep Black Soil-its thickness is more than 1 meter. It covers large areas in lowland zones of the Peninsular India. The clay content ranges between 40 to 60 per cent. Its reaction is alkaline. The soil is fertile and is utilised in raising the crops of cotton, sugarcane, rice, citrus fruits, vegetables etc.
Similarly on the basis of the colour we may have (a) deep coloured, and (b) light coloured black soils.