Soil Groups
Some of the major soil groups available in India are:
1. Alluvial Soils
2. Black Soils
3. Red Soils
4. Laterite and Lateritic Soils
5. Forest and Mountain Soils
6. Arid and Desert Soils
7. Saline and Alkaline Soils
8. Peaty and Marshy Soils!
India is a country of vast dimensions with varied conditions of geology, relief, climate and vegetation. Therefore, India has a large variety of soil groups, distinctly different from one another. Different criteria have been applied to classify Indian soils, the outstanding being geology, relief, fertility, chemical composition and physical structure, etc. Any classification based on any one of the aforesaid criteria has its own inherent drawback. Even the most competent pedologist would find it difficult to present an accurate, complete, comprehensive and generalised account of the Indian soils. During the ‘British rule in India, a vast body of fascinating accounts had emerged in district gazetteers and official reports. These accounts were generally directed towards the assessment of differential soil fertility and land revenue collection, but did not attempt classification of soil types in the country.
The earlier studies of Indian soils were made by foreign scholars like Volckar (1893), Leather (1898), Schokalskaya (1932), Champion (1936), etc. Indian scholars including Wadia (1935), Basu (1937), Vishwanath and Ukil (1944), Chatterjee, Krishnan, Roychaudhary (1954) made strenuous efforts to classify soils of India. In 1957, The National Atlas Organisation (Kolkata) published a soil map of India in which Indian soils were classified into 6 major groups and 11 broad types. The Irrigation Atlas of India (1972) and Spate’s India, Pakistan and Ceylon (1976) utilised the 7th approximation soil classification developed by the U.S. Department of Agriculture (USDA). The 7th approximation defines soil classes strictly in terms of their morphology and composition as produced by a set of natural and human forces. The classification is determined by quantifiable criteria. Geologically, Indian soils can broadly be divided into two main types: (a) Soils of peninsular India and (b) Soils of extra-peninsular India.
The soils of Peninsular India are those which have been formed by the deomposition of rocks in situ, i.e. directly from the underlying rocks. They are transported and redeposited to a limited extent and are known as sedentary soils. On the other hand, the soils of the Extra-Peninsula are formed due to the depositional work of rivers and wind. They are mainly found in the river valleys and deltas. They are very deep and constitute some of the most fertile tracts of the country. They are often referred to as transported or azonal soils. The Indian Council of Agricultural Research (ICAR) set up an All India Soil Survey Committee in 1953 which divided the Indian soils into eight major groups.
They are
1. Alluvial soils,
2. Black soils,
3. Red soils,
4. Laterite and Lateritic soils,
5. Forest and Mountain soils,
6. Arid and Desert soils,
7. Saline and Alkaline soils and
8. Peaty and Marshy soils (See Fig. 7.1).
This is a very logical classification of Indian soils and has gained wide acceptance. A brief account of these eight soils is given as under:
1. Alluvial Soils:
Alluvial soils are by far the largest and the most important soil group of India. Covering about 15 lakh sq km or about 45.6 per cent of the total land area of the country, these soils contribute the largest share of our agricultural wealth and support the bulk of India’s population. Most of the alluvial soils are derived from the sediments deposited by rivers as in the Indo-Gangetic plain although some alluvial soils in the coastal areas have been formed by the sea waves. Thus the parent material of these soils is all of transported origin.
The streams bring with them the products of weathering of rocks from the mountains and deposit them in the low-lying areas. The alluvial soils are yet immature and have weak profiles. They differ in consistency from drift sand to rich loams and from silts to stiff clays. A few occasional kankar beds are also present. However, pebbly, stony or gravelly soils are rare in this group. The chemical composition of the alluvial soils makes this group of soils as one of the most fertile in the world. The proportion of nitrogen is generally low, but potash, phosphoric acid and alkalies are adequate, while iron oxide and lime vary within a wide range. The porosity and texture provide good drainage and other conditions favourable for bumper crops. These soils are easily replenished by the recurrent river floods and support uninterruped crop growth.
The widest occurrence of the alluvial soils is in the Great Indo-Gangetic Plain starting from Punjab in the west to West Bengal and Assam in the east. They also occur in deltas of the Mahanadi, the Godavari, the Krishna and the Cauvery, where they are called deltaic alluvium. Along the coast they are known as coastal alluvium. Some alluvial soils are found in the Narmada and Tapi valleys. Northern parts of Gujarat also have some cover of alluvial soils. Geologically, the alluvium of the Great plain of India is divided into newer or younger khadar and older bhangar soils. The khadar soils are found in the low areas of valley bottom which are flooded almost every year.
They are pale brown, sandy clays and loams, more dry and leached, less calcareous and carbonaceous i.e. they are less kankary. Bhangar, on the other hand, is found on the higher reaches about 30 metres above the flood level. It is of a more clayey composition and is generally dark coloured. A few metres below the surface of the bhangar are beds of lime nodules known as kankar. Along the Shiwalik foothills, there are alluvial fans having coarse, often pebbly soils. This zone is called bhabar. To the south of the bhabar is a long narrow strip of swampy lowland with silty soils. It covers an area of 56,600 sq km and is called tarai. The tarai soils are rich in nitrogen and organic matter but are deficient in phosphate. These soils are generally covered by tall grasses and forests but are suitable for a number of crops such as wheat, rice, sugarcane, jute and soyabean under reclaimed conditions.
Due to their softness of the strata and fertility the alluvial soils are best suited to irrigation and respond well to canal and well/tube-well irrigation. When properly irrigated, the alluvial soils yield splendid crops of rice, wheat, sugarcane, tobacco, cotton, jute, maize, oilseeds, vegetables and fruits.
2. Black Soils:
The black soils are also called regur (from the Telugu word Reguda) and black cotton soils because cotton is the most important crop grown on these soils. Several theories have been put forward regarding the origin of this group of soils but most pedologists believe that these soils have been formed due to the solidifaction of lava spread over large areas during volcanic activity in the Deccan Plateau, thousands of years ago.
Most of the black soils are derived from two types of rocks, the Deccan and the Rajmahal trap, and ferruginous gneisses and schists occurring in Tamil Nadu. The former are sufficiently deep while the later are generally shallow. Krebs holds that the regur is essentially a mature soil which has been produced by relief and climate, rather than by a particular type of rock. According to him, this soil occurs where the annual rainfall is between 50 to 80 cm and the number of rainy days range from 30 to 50. The occurrence of this soil in the west deccan where the rainfall is about 100 cm and the number of rainy days more than 50, is considered by him to be an exception. In some parts of Gujarat and Tamil Nadu, the origin of black cotton soils is ascribed to old lagoons in which the rivers deposited the materials brought down from the interior of Peninsula covered with lava.
Geographically, black soils are spread over 5.46 lakh sq km (i.e. 16.6 per cent of the total geographical area of the country) encompassed between 15°N to 25°N latitudes and 72°E to 82°E longitudes. This is the region of high temperature and low rainfall. It is, therefore, a soil group of the dry and hot regions of the Peninsula. These soils are mainly found in Maharashtra, Madhya Pradesh, parts of Karnataka, Andhra Pradesh, Gujarat and Tamil Nadu. The black colour of these soils has been attributed by some scientists to the presence of a small proportion of titaniferous magnetite or even to iron and black constituents of the parent rock. The black colour of this soil may even be derived from crystalline schists and basic gneisses such as in Tamil Nadu and parts of Andhra Pradesh. Various tints of the black colour such as deep black, medium black, shallow black or even a mixture of red and black may be found in this group of soils. The black soil is very retentive of moisture. It swells greatly and becomes sticky when wet in rainy season. Under such conditions, it is almost impossible to work on such soil because the plough gets stuck in the mud.
However, in the hot dry season, the moisture evaporates, the soil shrinks and is seamed with broad and deep cracks, often 10 to 15 cm wide and upto a metre deep. This permits oxygenation of the soil to sufficient depths and the soil has extraordinary fertility. Remarkably “selfploughed” by loosened particles fallen from the ground into the cracks, the soil “swallows” itself and retains soil moisture. This soil has been used for growing a variety of crops for centuries without adding fertilizers and manures, or even fallowing with little or no evidence of exhaustion.
A typical black soil is highly argillaceous with a large clay factor, 62 per cent or more, without gravel or coarse sand. It also contains 10 per cent of alumina, 9-10 per cent of iron oxide and 6-8 percent of lime and magnesium carbonates. Potash is variable (less than 0.5 per cent) and phosphates, nitrogen and humus are low. The structure is cloddish but occasionally friable. In all regur soils in general, and in those derived from ferromagnesian schists in particular, there is a layer rich in kankar nodules formed by segregation of calcium carbonate at lower depths. As a general rule, black soils of uplands are of low fertility but they are darker, deeper and richer in the valleys. Because of their high fertility and retentivity of moisture, the black soils are widely used for producing several important crops. Some of the major crops grown on the black soils are cotton, wheat, jowar, linseed, Virginia tobacco, castor, sunflower and millets. Rice and sugarcane are equally important where irrigation facilities are available. Large varieties of vegetables and fruits are also successfully grown on the black soils.
3. Red Soil
This comprehensive term designates the largest soil group of India, comprising several minor types. Most of the red soils have come into existence due to weathering of ancient crystalline and metamorphic rocks. The main parent rocks are acid granites and gneisses, quartzitic and felspathic. The colour of these soils is generally red, often grading into brown, chocolate, yellow, grey or even black. The red colour is due more to the wide diffusion rather than to high percentage of iron content. The red soils occupy a vast area of about 3.5 lakh sq km which is about 10.6 per cent of the total geographical area of the country. These soils are spread on almost the whole of Tamil Nadu, parts of Karnataka, south-east of Maharashtra, eastern parts of Andhra Pradesh and Madhya Pradesh, Chhattisgarh, Orissa and Chota Nagpur in Jharkhand.
In the north the red soil area extends in large parts of south Bihar; the Birbhum and Bankura districts of West Bengal; Mirzapur, Jhansi, Banda and Hamirpur districts of Uttar Pradesh; Aravallis and the eastern half of Rajasthan, parts of Assam, Nagaland, Manipur, Mizoram, Tripura and Meghalaya. By and large, the red soils are poor in lime, magnesia, phosphates, nitrogen and humus, but are fairly rich in potash. In their chemical composition they are mainly siliceous and aluminous; with free quartz as sand the alkali content is fair, some parts being quite rich in potassium. The texture of these soils varies from sand to clay, the majority being loams. On the uplands, the red soils are thin, poor and gravelly, sandy or stoney and porous, but in the lower areas they are rich, deep dark and fertile. The red soils respond well to the proper use of fertilizers and irrigation and give excellent yields of cotton, wheat, rice, pulses, millets, tobacco, oil seeds, potatoes and fruits.
4. Laterite and Lateritic Soils:
The word ‘laterite’ (from Latin letter meaning brick) was first applied by Buchanan in 1810 to a clayey rock, hardening on exposure, observed in Malabar. But many authors agree with Fermor’s restriction of this term to soils formed as to 90-100 per cent of iron, aluminium, titanium and manganese oxides.
According to majority opinion, the laterite soil is formed under conditions of high temperature and heavy rainfall with alternate wet and dry periods. According to Polynov, laterite soils may be “the end products of weathering given sufficiently long time”. In the opinion of George Kuriyan, “It is probably the end product of decomposition found in regions of heavy rainfall, more than 200 cm” Such climatic conditions promote leaching of soil whereby lime and silica are leached away and a soil rich in oxides of iron and aluminium compounds is left behind. We have numerous varieties of laterite which have bauxite at one end and an indefinite mixture of ferric oxides at the other. Almost all laterite soils are very poor in lime and magnesia and deficient in nitrogen. Sometimes, the phosphate content may be high, probably present in the form of iron phosphate but potash is deficient. At some places, there may be higher content of humus.
Laterite and lateritic soils are widely spread in India and cover an area of 2.48 lakh sq km. They are mainly found on the summits of Western Ghats at 1000 to 1500 m above mean sea level, Eastern Ghats, the Rajamahal Hills, Vindhyas, Satpuras and Malwa Plateau. They also occur at lower levels and in valleys in several other parts of the country. They are well developed in south Maharashtra, parts of Karnataka, Andhra Pradesh, Orissa, West Bengal, Kerala, Jharkhand, Assam and Meghalaya. Due to intensive leaching and low base exchange capacity, typical laterite soils generally lack fertility and are of little use for crop production. But when manured and irrigated, some laterites and lateritics are suitable for growing plantation crops like tea, coffee, rubber, cinchona, coconut, arecanut, etc. In low lying areas paddy is also grown.
Some of the laterite soils in Kerala, Karnataka, Chota Nagpur region of Jharkhand, Orissa and Assam respond well to the application of fertilizers like nitrogen, phosphorus and potassium. In some areas, these soils support grazing grounds and scrub forests.Laterite and lateritic soils have a unique distinction of providing valuable building material. These soils can be easily cut with a spade but hardens like iron when exposed to air. Because it is the end- product of weathering, it cannot be weathered much further and is indefinitely durable.
5. Forest and Mountain Soils:
Such soils are mainly found on the hill slopes covered by forests. These soils occupy about 2.85 lakh sq km which is about 8.67 per cent of the total land area of India. The formation of these soils is mainly governed by the characteristic deposition of organic matter derived from forest growth.
These soils are heterogeneous in nature and their character changes with parent rocks, ground-configuration and climate. Consequently, they differ greatly even if they occur in close proximity to one another. In the Himalayan region, such soils are mainly found in valley basins, depressions, and less steeply inclined slopes. Generally, it is the north facing slopes which support soil cover; the southern slopes being too precipitous and exposed to denudation to be covered with soil. Apart from the Himalayan region, the forest soils occur on Western and Eastern Ghats as well as in some parts of the Peninsular plateau. The forest soils are very rich in humus but are deficient in potash, phosphorus and lime. Therefore, they require good deal of fertilizers for high yields. They are especially suitable for plantations of tea, coffee, spices and tropical fruits in Karnataka, Tamil Nadu and Kerala and wheat, maize, barley and temperate fruits in Jammu and Kashmir, Himachal Pradesh and Uttaranchal.
6. Arid and Desert Soils:
A large part of the arid and semi-arid region in Rajasthan and adjoining areas of Punjab and Haryana lying between the Indus and the Aravalis, covering an area of 1.42 lakh sq km (or 4.32% of total area) and receiving less than 50 cm of annual rainfall, is affected by desert conditions.
The Rann of Kuchchh in Gujarat is an extension of this desert. This area is covered by a mantle of sand which inhibits soil growth. This sand has originated from the mechanical disintegration of the ground rocks or is blown from the Indus basin and the coast by the prevailing south-west monsoon winds. Barren sandy soils without clay factor are also common in coastal regions of Orissa, Tamil Nadu and Kerala. The desert soils consist of aeolian sand (90 to 95 per cent) and clay (5 to 10 per cent). Some of these soils contain high percentages of soluble salts, are alkaline with varying degree of calcium carbonate and are poor in organic matter. Over large parts, the calcium content increases downwards and in certain areas the subsoil has ten times calcium as compared to that of the top soil. The phosphate content of these soils is as high as in normal alluvial soils. Nitrogen is originally low but its deficiency is made up to some extent by the availability of nitrogen in the form of nitrates. Thus, the presence of phosphates and nitrates make them fertile soils wherever moisture is available.
There is, therefore, great possibility of reclaiming these soils if proper irrigation facilities are available. The changes in the cropping pattern in the Indira Gandhi Canal Command Area are a living example of the utility of the desert soils. However, in large areas of desert soils, only the drought resistant and salt tolerant crops such as barley, rape, cotton, wheat, millets, maize and pulses are grown. Consequently, these soils support a low density of population.
7. Saline and Alkaline Soils:
These soils are found in Andhra Pradesh and Karnataka. In the drier parts of Bihar, Uttar Pradesh, Haryana, Punjab, Rajasthan and Maharashtra, there are salt-impregnated or alkaline soils occupying 68,000 sq km of area. These soils are liable to saline and alkaline efflorescences and are known by different names such as reh, kallar, usar, thur, rakar, karl and chopan.
There are many undecomposed rock and mineral fragments which on weathering liberate sodium, magnesium and calcium salts and sulphurous acid. Some of the salts are transported in solution by the rivers, which percolate in the sub-soils of the plains. In canal irrigated areas and in areas of high sub-soil water table, the injurious salts are transferred from below to the top soil by the capillary action as a result of evaporation in dry season. The accumulation of these salts makes the soil infertile and renders it unfit for agriculture. It has been estimated that about 1.25 million hectares of land in Uttar Pradesh and 1.21 million hectares in Punjab has been affected by usar. In Gujarat, the area round the Gulf of Khambhat is affected by the sea tides carrying salt-laden deposits. Vast areas comprising the estuaries of the Narmada, the Tapi, the Mahi and the Sabarmati have thus become infertile.
8. Peaty and Marshy Soils:
Peaty soils originate in humid regions as a result of accumulation of large amounts of organic matter in the soils. These soils contain considerable amount of soluble salts and 10-40 per cent of organic matter. Soils belonging to this group are found in Kottayam and Alappuzha districts of Kerala where it is called kari.
Marshy soils with a high proportion of vegetable matter also occur in the coastal areas of Orissa and Tamil Nadu, Sunderbans of West Bengal, in Bihar and Almora district of Uttaranchal. The peaty soils are black, heavy and highly acidic. They are deficient in potash and phosphate. Most of the peaty soils are under water during the rainy season but as soon the rains cease, they are put under paddy cultivation.