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There can be more than one classification of Indian soils following different criteria. Here we take into account the geographical extent, physical appearance, chemical properties, significance for the purpose of agriculture and the nutritional deficiencies, if any.
Broadly, Indian soils can be studied under nine groups:
1. Alluvial Soils:
These soils cover 15 lakh square kilometres from the Satluj plains in the west to the lower Ganga- Brahmaputra valley in the east and along east and west coasts in the coastal plains. In the north Indian plains’, alluvial soils are derived from debris brought by rivers from, the Himalayas and the silt left behind by old sea. The coastal alluvium is of tidal origin. The desert alluvium or loess is brought-by wind erosion. These soils have a mature profile in flat lands and an immature profile in an undulating surface.
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These are the best agricultural soils, because:
(i) They contain a variety of salts derived from Himalayan rocks;
(ii) They are light and porous, therefore easily tillable; and
(iii) They are good for canal irrigation because of a high water table and an easily penetrable stratum.
But, because the water is allowed to go very deep, these soils are not suitable for crops requiring water retention around the roots. These soils are rich in potash and poor in nitrogen and organic matter.
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2. Black or Regur Soils:
These soils cover five lakh square kilometres in Maharashtra, parts of Karnataka, Tamil Nadu, Andhra Pradesh, Madhya Pradesh and Gujarat.
They are also known as regur soils and have a characteristically black appearance.
These soils are derived from two types of rocks—basaltic Deccan and Rajmahal traps and the ferrogenous gneisses and schists in Tamil Nadu and Andhra Pradesh. These soils are very clayey (upto 50 per cent clay content) and, therefore, are highly retentive of water. This makes them suitable for dryland farming.
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Because of a high clay content, these soils expand when wet and become difficult to plough. During the dry season, the black soils shrink and develop big cracks (sometimes upto one metre wide). These soils vary from calcerous neutral to mild alkaline in reaction, and are deep and rich, fertile in the lowlands but of poor fertility in the uplands.
Black soils are good for cotton and sugarcane cultivation. The black soil regions of Maharashtra and Gujarat have contributed tremendously to the growth of cotton textile industry in the Mumbai- Ahmedabad belt. Black soils are rich in iron, carbonates of magnesium, calcium, and in alumina; they are poor in nitrogen, phosphorus, potassium, and organic matter.
3. Red Soils:
These are extensively found covering Tamil Nadu, southern Maharashtra, Andhra Pradesh, Karnataka, eastern Madhya Pradesh, south Bihar, western Orissa and in the sedimentaries of northeastern mountains. Red soils are derived from weathering of old crystalline and metamorphic rocks under dry conditions, and are red due to the presence of iron oxides. These soils are loamy or sandy and have a low cation exchange capacity and a low base status. These soils are characterised by low water retention capacity.
Red soils are deep and fertile in the lowlands and poor and thin in the uplands. In the lowlands, red soils are found along with black soils and give good crops on irrigation. Red soils are poor in nitrogen, phosphorus, potassium and organic matter.
4. Laterite Soils:
These soils cover parts of Western Ghats in Kerala, coastal Orissa, coastal areas of West Bengal and Eastern Ghats and areas of high rainfall in North- East and Bihar.
These are typical soils of the tropics and are the end product of decomposition when high rainfall leaches away calcium and silica leaving behind iron with silica. The iron content gives these soils a red colour. These soils are agriculturally unimportant because of intensive leaching, a low Base Exchange capacity and their acidic nature. Lateritic soils are poor in nitrogen, phosphorus, potassium, calcium and organic matter.
5. Forest Soils:
These soils cover areas between 3,000 m and 3,100 m height in the coniferous region. These are dark soils, rich in decomposed organic matter, but low in pH.
6. Mountain Soils:
These soils are spread over altitudes between 2,000 m and 3,000 m. These soils are shallow, silty-loam to loam, well-drained, stony, poorly endowed in organic matter and moderately acidic. The carbon-nitrogen ratio is wide. These soils are subject to dislodgement due to landslides and snowfall. Mountain soils are used for forestry and growing potatoes and subtropical fruits.
7. Desert Soils:
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These soils cover 1.4 lakh square kilometres. The Thar Desert alone accounts for 1 lakh square kilometres f»nd the rest is spread over southern Punjab, Rajasthan and Haryana, where the annual rainfall is less than 50 centimetres.
These soils are covered with a thick brown mantle which inhibits soil growth. Desert soils are derived from adjoining rocks and the coastal region. They are sandy, coarse and alkaline, rich in soluble salts (phosphates and nitrates, though actual nitrate content is low). Therefore, these are actually fertile soils, water being the only limiting factor for agricultural purposes. Poor in organic matter, they have a varying percentage of lime. Desert soils can be reclaimed through good irrigation.
8. Saline and Alkaline Soils:
These soils cover arid and semi-arid regions of the northern plains and almost the whole of Maharashtra. The salts from the Himalayas or those derived from the weathering of Deccan rocks get mixed with underground water and during the dry period, come up to the surface through capillary action. The salts, when on the surface, form efflorescence. These are salts of calcium, magnesium and sodium.
These soils are associated with the subsoil impervious layer or pan, a high water table, canal irrigation (which has the effect of saturating the adjoining water table, especially if under seepage—thus helping the salts to come up to the surface) and an inadequate surface drainage. Local names of saline and alkaline soils include reh, usar, kallar, rakar and chopan. The presence of salt, a cemented layer and bad water retention make these soils unsuitable for agriculture.
9. Peaty Soil:
These soils cover the high rainfall areas of West Bengal, Orissa and Kerala. These soils are heavy, dark and acidic and are formed under conditions of submergence. Peaty soil areas remain submerged during the monsoon, and experience accumulation of organic matter with large quantities of soluble salts, such as ferrous and aluminium sulphates which can be, at times, toxic for plants.
Thus, dryness and deficiency of nitrogen and organic matter are the features which are common to almost all the Indian soils.