The investigations of Voelcker in 1893, and those of Leather in 1898, led to a classification if India soils into four major types and three minor types: (i) the Indo-Gangetic alluvium; (ii) the black cotton soils; (iii) the red soils lying on metamorphic rocks; and (iv) the lateritic soils.
(i) Indo-Gengetic alluvium - The Indo-Gangetic alluvium is by far the largest and most important of the soil groups of India. The soils of this group cover about 777,000 square kilometers. They are distributed mainly the Punjab, Haryana, Uttar Pradesh, Bihar, Bengal and parts of Assam and Orissa. They produce bumber crops of wheat and rice. Geologically the alluvium is divided into: (i) Khadar, or new alluvium of sandy composition, generally light in colour, about 10,000 years old, and (ii) Bhangar, or the older alluvium of Pleistocene elate, of more clayey composition, generally of dark colour, and fun of pebbles or kankar. The soils differ in consistency from drift sand to loams, and from fine silts to stilts clays. A few pebble beds are also occasionally met with. The presence of impervious clays obstructs the drainage, and also promotes the accumu- lation of injurious salts of sodium arid magnesium, which make the soils sterile. The formation of hard pans at certain levels in the soil profile as a result of the binding of soil grains by the infiltrating silica or calcareous matter is often observed in these alluvial soils. A majority of the soils are loams or sandy loams, with a soil crust of varying depth. Soluble salts are present in considerable quantities.
The alluvial soils of Tamil Nadu are transported soils, found mainly in the deltaic areas and on the coastal line, A section of the profile shows alternate layers of sand and silt. The composition of the strata varies with the nature of the silt brought by the rivers which, in turn, varies with the catchment areas and the tracts through which the streams flow.
(ii) Black cotton soils - The typical soil of the Deccan Trap is the regur or black cotton soil. It is common in Maharashtra, in the western parts of Madhya Pradesh, Karnataka, and ‘some parts of Tamil Nadu, including the districts of Ramnad and Tirunelvely in the extreme south. It is comparable with the chernozems of Russia and with the prairie soil of the cotton-growing tracts of the United States of America, especially the black adobe of California. It is derived from two types of rocks: the Deccan and Rajmahal Trap, and the ferruginous gneisses and schists occur- ring in Tamil Nadu under semi-arid conditions. The former attains sometimes considerable depths, whereas the latter are generally shallow. The black soil areas have, generally, a high degree of fertility, though some mainly in the uplands are of low productivity. The soils on the slopes and the uplands are somewhat sandy, but those in the broken country between the hills and the plains are darker, deeper and richer, and are constantly enriched by deposits washed down from the hills.
(iii) Red soils - Red soils extend practically over the whole Archaean basement of Peninsular India, from Bundelkhand to the extreme south, covering 2,072,000 square km, embracing south Bengal, Orissa, parts of Madhya Pradesh eastern Andhra Pradesh Karnataka, and a major part of Tamil Nadu. These soils also occur in Santhal Parganas in Bihar, and in the Mirzapur, Jhansi and Hamirpur districts of Uttar Pradesh. They were produced as a result to meteoric weathering of
ancient crystalline and metamorphic rocks. These soils started developing around the Mesozoic and Tertiary ages. The colour if these soils is generally red, grading sometimes into brown chocolate, yellow; grey and even black. The redness is due more to a general diffusion than to a high proportion of iron content. The soils grade from the poor thin gravelly and light coloured varieties of the uplands to the much more fertile deep dark varieties of the plains and the valleys. They are generally; poor in nitrogen phosphorus and humus. Compared with regur, they are poor in lime, potash and iron oxide, and are also uniformly low in phosphorus. The clay fraction of the soils is rich in kaolinite. More than two-thirds of the cultivated area in Tamil Nadu is covered by red soils they are in-situ formations produced from the rock below under the influence of climatic conditions. The rocks are acidic, consisting of mica or red granites. The soils are shallow and open in texture. They have a low exchange capacity and are deficient in organic matter and plant nutrients.
(iv) Laterites - Laterite is a soil type peculiar to India and some other tropical countries, characterized by the intermittent occurrence of moist climate. In formation it varies from compact to vesicular rock composed essentially of a mixture of hydrated oxides of aluminium and iron with small quantities of manganese oxides, titanium, etc. It is produced by the atmospheric weathering of several types of rocks. Laterites occur in Madhya Pradesh, the coastal region of Orissa, south Maharashtra, Malabar and part of Assam. All lateritic soils are generally very poor in lime and magnesia and deficient in nitrogen. Occasionally, the P2O5 content may be high, but there is deficient of K2O. In Tamil Nadu, there are both high-level and low-level laterites which are formed from a variety of rock materials under certain climatic and weather conditions. The laterites at lower elevations grow rice whereas those at higher elevations grow tea, cinchona, rubber and coffee. The soils are rich in nutrients and contain 10 to 20 per cent organic matter. (v) Forest and hill soils - The soil formation is governed mainly by the character of the deposition of organic matter derived from the forest growth. Broadly, two conditions of soil formation may be distinguished: (i) soils formed under acid condition, with acid humus and low base status, and (ii) soils formed under slightly acid or neutral condition with high base status, which is favour- able to the formation of brown earths. Forest and Hill soils occur in Assam and in Uttar Pradesh, the Sub Himalayan tract comprises three distinct parts viz., bhabar area immediately below the hills, tarai and the plains. The tarai areas are characterized by extreme unhealthiness owing to excessive soil moisture and prolific growth of vegetation. The soils in Coorg has deep surface soil of great fertility, as it receives annually the decomposed products of the virgin forest. The areas towards the west are for the greater part reserved under forests and mountain areas. The land surface is full of pebbles, is easily drained, and has a laterite bed.
(vi) Desert soils - A large part of the arid region of Rajasthan and the Punjab and Haryana, lying between the Sutlej and the Aravallis, is affected by desert conditions, which geologically are of recent origin. This part is covered under a mantle of blown sand, and is dominated by conditions, which inhibit soil growth. Some of the soils contain a high percentage of soluble salts and varying percentages of calcium carbonate, and possess high pH. They are, however, poor in organic matter. Reclamation is possible only if proper irrigation facilities are made available. (vii) Saline and alkaline soils - These soils are extensively distributed throughout India in all the
climatic zones. These soils occur in Bihar, Uttar Pradesh, Punjab, Haryana and Rajasthan. The injurious salts are confined to the top layers, being deposited there by the capillary transference
of saline solutions from the lower strata. It has been estimated that nearly 850,000 hectares in
Over 10,000 hectares are being affected every year in the Punjab and Haryana. Alkali soils are met with all over Maharashtra.