Without reference to some particular system of soil classification, the distribution of soils throughout the world can be discussed only in the most general terms. Of this purpose, soils can be divided into five broad types: (i) soils of humid lands, (ii) soils of subhumid grasslands and deserts, (iii) soils of tundra regions, (iv) mountain soils, and (v) alluvial soils. The first three show strongly the influence of climate and tend to occur in broad zones.
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Zonal soils are those which occur over large areas or climatic zones having geographical characteristics of their own, sited on well-drained undulating land, having well-developed profiles and other properties, developed on parent material which has remained in its original place for a sufficiently long time to have been affected by climatic and organism processes.
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Zonal soils display a characteristic latitudinal belt distribution. The zonal order is comprised of soils which can be differentiated on the basis of humid and arid, semiarid, and subhumid climatic zones, in the humid climate class we have podzol, grey, brown podzolic, red-yellow podzolic, latosols, and tundra soils.
The Podzols are the most widely distributed of the zonal soils of the humid climates. They occur in the sub-arctic climatic and northern parts of humid continental climate areas. They develop under conditions of a cold winter and an adequate precipitation spread throughout the year. Grey-brown podzolic soil occurs in humid climates. Eastern U.S.A., the North European plain, and the Northern Chinese plains are typical areas of its occurrence.
Red-yellow podzolic soil occurs south of the grey-brown podzolic belt. Pine vegetation is associated with this soil also. South-eastern U.S.A. is a typical area of its occurrence. Latosols or latritic soils are characteristic of the humid tropics. The soil is associated with luxuriant rain forests of hardwoods, and lianas.
India, South-east Asia, the Amazon Basin, and the Congo Basin are typical areas of occurrence of laterite soils. The Tundra soil is widespread and coextensive with the Tundra climatic and vegetation type. Arid, semi-arid, sub-humid zonal order soils include chernozem, chestnut, chestnut-brown, grey desert, and red desert varieties. They occur in areas in increasing order of temperature and aridity.
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Chernozem is associated with a continental, warm, moist early summer, later summer and winter drought, and dry snow-free winter climate, and long-grass steppe. The chernozem is generally acidic.
A typical area of its occurrence is Ukraine but it is also widespread in the Central U.S.A., Central Africa, South America, and Australia. Chernozem is highly productive for small grain crops, such as wheat, oats, barley, and rye. The Ukraine ‘the bread-basket of the U.S.S.R’ is in fact based on chernozem soil. Very similar to chernozem is the prairie soil.
Chestnut soil occurs towards the arid side of the chernozem belt in the semi-arid middle latitude steppe lands of North America and Asia. Chestnut soil is replaced by chestnut brown type in more arid areas.
The soil is typical of the middle-latitude steppes and is suitable for livestock grazing. Grey-desert soil, also termed sierozem, occurs in vast east-west belts between 30° and 50° N latitudes in the Northern Hemisphere. It is co-extensive with the spread of the middle-latitude desert climate. Desert soils are suitable for cultivation along the flood plains and on the terraces where their texture is fine.
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The Indian Council of Agricultural Research (ICAR) divides the soil found in the country into 8 major groups which are (i) Alluvial soils including the coastal and deltaic alluvium, (ii) Black soils, of varying types (iii) Red soils, including red loams, yellow earths etc. (iv) Laterite and lateritic soils (v) Forest soils (vi) Arid and Desert soil (vii) Saline and Alkali soils and (viii) Peaty and Organic soils. Climate and nature of the parent rock are the two most important factors that determine the types of soil found in India.
Soils may also be categorized according to their formation:
(a) Residual Soil:
These are found where they are formed hence called “In Situ”. The red, laterite, black, podzolic soils of forests, saline and Alkaline and peaty and other organic soils are residual soils. Of these, the red and laterite soils are zonal soils developed tinder hot and humid conditions through the laterisation process on a variety of rocks including the Archaean granite. The black soil is an intrazonal soil developed on Deccan lavas.
(b) Transported Soil:
These are the soils which are carried down by agents of gradations such as rivers, wind.
ICAR Classification:
(i) Alluvial Soils:
It is by far the largest and the most important soil group of India contributing the largest share of the country’s agricultural production. Alluvial soils cover about 22.1 per cent of the country’s total land surface. Composed of sediments deposited by rivers in the interior and sea waves along the coasts, these soils constitute the surface of the Great Plains from Punjab to Assam.
They also occur in the interior and sea waves along the coasts, these soils constitute the surface of the Great Plains from Punjab to Assam. They also occur in the valleys of the Narmada and Tapti in Madhya Pradesh and Gujarat, Mahanadi in Madhya Pradesh and Orissa, Godavari in Andhra Pradesh, and Cauvery in Tamil Nadu.
Along the coast of Kerala, they are referred to as coastal alluvium and in the deltas of Mahanadi, Godavari, Krishna and Cauvery as deltaic alluvium. Alluvial soils are generally deficient in Nitrogen and humus; this necessitates heavy fertilisation particularly with nitrogenous fertilisers. Phosphorus is also deficient in some areas.
These soils are suitable for the cultivation of almost all kinds of cereals, pulses, oil seeds, cotton, sugar-cane and vegetables. Jute can be grown in the eastern areas. In the upper and middle Ganga plain two different types of alluvial soils have developed viz. Khadar and Bhangar.
Khadar is a newer alluvium developed behind the levees of the numerous streams flowing is this section of the Ganga plain. Bhangar represents a system of older alluvium developed on the upper reaches of the streams where floods generally do not reach.
(ii) Black Cotton Soils:
These soils are black in colour and they are eminently suitable for the cultivation of cotton. In some areas they are also called regur. These soils have developed over Deccan lavas, gneisses and granites under semi-arid conditions and they occupy many areas of Maharashtra, Gujarat, Madhya Pradesh, Karnataka, Andhra Pradesh, Tamil Nadu, Uttar Pradesh, and Rajasthan.
The black colour is variously attributed to the presence of titaniferous magnetite, compounds of iron and aluminium, accumulated humus and colloidal hydrated double iron and aluminium silicate. They are usually deficient in nitrogen, phosphoric acid and organic matter, but rich in potash, lime, aluminium, calcium and magnesium carbonates.
They are sticky when wet and develop deep wide cracks on drying which helps in the process of self-aeration and absorption of nitrogen from the atmosphere. An extreme degree of moisture retentiveness is another characteristic of these soils. Black soils are well-known for their fertility.
Cotton, cereals and oilseeds, like linseed and safflower, many kinds of vegetables and citrus are some of the crops well suited to black soils. Very good results have also been obtained in crops like sugar-cane and tobacco. On account of the moisture-retentive qualities, the black soils are ideally suited to dry farming.
(iii) Red Soils:
Red soil is formed in situ by the decomposition of metamorphic rocks. These soils comprising red loams and yellow earths and derived from crystalline and metamorphic rocks rich in ferromagnesium minerals occupy much of the Peninsula reaching up to Rajmahal Hills in the East, Jhansi in the North and Kachchh in the west.
These soils are generally characterised by light texture with porous and friable structure, absence of lime and free carbonates and presence of soluble salts in a small quantity. They are neutral to acid reaction and deficient in nitrogen, humus, phosphoric acid and lime.
These soils occupy over two-thirds of the total area of Tamil Nadu. Almost all kinds of crops are grown on red soils, though they seem to be more suitable for the cultivation of rice, ragi, tobacco and vegetables. Groundnut and potato can be grown on coarse soils at higher level and sugarcane on heavy clays at lower level. Red soils are airy and need irrigation support for cultivation. It is suited for dry farming as it does not require much moistures.
(iv) Laterite and Lateritic Soils:
Lateritic soils are formed under conditions of high rainfall and temperature with alternate wet and dry periods. It develops insitu as a result of leaching in areas of heavy rain. Leaching is a process by which the nutrients in the soil are washed away by heavy rains.
The soil consists of a honeycombed mass of iron oxides which turn black after exposure to rain. Usually laterite soils are acidic in nature and are poor in nitrogen, phosphoric acid, potash, lime and Magnesia.
When they are of low fertility generally, they readily respond to maturing and valley soils are found to be suitable for a variety of crops particularly rice, ragi and sugar-cane. Lateritic soils occur especially on the summits of the Sahyadric, Eastern Ghats, Rajmahal Hills and many other hills in the eastern parts of the peninsula, East Godavari districts in Andhra Pradesh, some districts in Orissa and West Bengal also have laterite soils.
(v) Forest Soils:
The forest soils are characterized by the deposition of organic matter derived from forest growth. Humus predominates in all forest soils and it is more raw at higher levels leading to acidic conditions.
The Himalayas and the other ranges in the north and the high hill summits in the Sahyadris, Eastern Ghats and the Peninsula have forest soils. Forest soils are deficient in potash, phosphorus and lime and need fertilisation for good yields. Plantations of tea, coffee, spices and tropical fruits are laid out on these soils in Karnataka, Tamil Nadu, Kerala and Manipur, Temperate fruits, Maize, wheat and barley are raised on them in Jammu and Kashmir and Himachal Pradesh.
(vi) Arid and Desert Soils:
These soils are formed under arid and semiarid conditions in the northwestern parts of the country. The entire area west of the Aravalli Range in Rajasthan has desert soils. The soils extend to the southern districts of Haryana and Punjab in the north and the Rann of Kachchh in the south.
These soils often have a high soluble salt content and a low to very low humus content. They are quite rich in phosphate but poor in nitrogen. Generally, desert soils improve in fertility towards east. It is formed due to ill-drainage which causes water logging.
(vii) Saline and Alkali Soils:
It is formed due to ill-drainage which causes water logging. Soils of many parts of the arid and semi-arid areas of Rajasthan, Punjab, Haryana, Uttar Pradesh and Bihar have saline and alkaline effervescences mainly of sodium, calcium and magnesium.
These soils called reh, kallar and usar variously, are infertile. Saline soils contain free sodium and other salts whereas alkali soils contain large quantities of sodium chloride. These salt-impregnated soils can be reclaimed by providing good drainage.
(viii) Peaty and Other Organic Soils:
Peaty soils have developed under humid conditions as a result of an accumulation of large amounts of organic matter. In addition, they contain considerable amount of soluble salts.
These soils are highly saline, rich in organic matter but deficient in phosphate and potash. Marshy soils with a high quantity of vegetable matter frequently occur in the coastal areas of Orissa, West Bengal and Tamil Nadu, in central and northern Bihar and Almora district of Uttar Pradesh.
