Soil is the mixture of rock debris and organic materials which develop on the earth’s surface. The major factors affecting the formation of soil are relief, parent material, climate, vegetation and other life-forms and time. Components of the soil are mineral particles, humus, water and air.
Classification of Soils
The first scientific classification of soil was done by Voelekar and Leather. According to them, Indian soils were classified into four categories:
The All India soil and land use survey organization attempted a classification on the basis of texture, color, structure, pH value, porosity, etc in 1957. After that, the Indian Council of Agricultural Research, on the basis of texture, structure, color, pH value, porosity, etc.
India has varied relief features, land forms, climatic realms and vegetation types. These have contributed in the development of various types of soils in India. On the basis of genesis, colour, composition and location, the soils of India have been classified into:
- Alluvial soils
- Black soils
- Red and Yellow soils
- Laterite soils
- Arid soils
- Saline soils
- Peaty soils
- Forest soils
1. Alluvial Soils
Alluvial soils are widespread in the northern plains (Satluj Ganga Brahmaputra Plains) and the river valleys. These soils cover about 40 percent of the total area of the country. They are depositional soils, transported and deposited by rivers and streams. The alluvial soils vary in nature from sandy loam to clay. They are generally rich in potash but poor in phosphorous. The colour of the alluvial soils varies from the light grey to ash grey. Its shades depend on the depth of the deposition, the texture of the materials, and the time taken for attaining maturity. Alluvial soils are intensively cultivated.
In the Upper and Middle Ganga plain, two different types of alluvial soils have developed:
Khadar is the new alluvium and is deposited by floods annually, which enriches the soil by depositing fine silts. They are low lying, frequently inundated by floods during the rainy season. It occupies the flood plains of rivers. The khaddar tracts called as kankar are rich in concentration.
Bhangar represents a system of older alluvium, deposited away from the flood plains. This soil lies above the flood level. The texture of soil varies from the loamy soil to clayey soil.
2. Black Soil
Black soil covers most of the Deccan Plateau which includes parts of Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh and some parts of Tamil Nadu. In the upper reaches of the Godavari and the Krishna, and the north western part of the Deccan Plateau, the black soil is very deep. These soils are also known as the ‘Regur Soil’ or the ‘Black Cotton Soil’.
The black soils are generally clayey, deep and impermeable. They swell and become sticky when wet and shrink when dried. So, during the dry season, these soil develop wide cracks. Thus, there occurs a kind of ‘self ploughing’. Because of this character of slow absorption and loss of moisture, the black soil retains the moisture for a very long time, which helps the crops, especially, the rain fed ones, to sustain even during the dry season.
The soil is rich in iron, lime, calcium, potash, magnesium and aluminum. It has high water retaining capacity and good for the cotton cultivation, Tobacco, citrus fruits, castor, and linseed.
3. Red and Yellow Soil
Red soil develops on crystalline igneous rocks in areas of low rainfall in the eastern and southern part of the Deccan Plateau. Yellow and red soils are also found in parts of Odisha and Chattisgarh and in the southern parts of the middle Ganga plain. The soil develops a reddish colour due to a wide diffusion of iron (presence of ferric oxides ) in crystalline and metamorphic rocks. It looks yellow when it occurs in a hydrated form.
The fine-grained red and yellow soils are normally fertile, whereas coarse-grained soils found in dry upland areas are poor in fertility. They are generally poor in nitrogen, phosphorous and humus. This soil is good for the cultivation of wheat, cotton, pulses, tobacco, millets, orchards, potato, and oilseeds.
4. Laterite Soil
Laterite has been derived from the Latin word ‘Later’ which means brick. The laterite soils develop in areas with high temperature and high rainfall. These are the result of intense leaching due to tropical rains. With rain, lime and silica are leached away, and soils rich in iron oxide and aluminium compound are left behind.
Humus content of the soil is removed fast by bacteria that thrives well in high temperature. These soils are poor in organic matter, nitrogen, phosphate and calcium, while iron oxide and potash are in excess. Hence, laterites are not suitable for cultivation; however, application of manures and fertilizers are required for making the soils fertile for cultivation.
Red laterite soils in Tamil Nadu, Andhra Pradesh and Kerala are more suitable for tree crops like cashewnut. These iron and aluminum rich soils are suitable for the cultivation of rice, ragi, sugarcane and cashew nuts.
5. Arid Soils
Arid soils range from red to brown in colour. They are generally sandy in structure and saline in nature. Due to the dry climate, high temperature and accelerated evaporation, they lack moisture and humus. Nitrogen is insufficient and the phosphate content is normal.
Arid soils are characteristically developed in western Rajasthan, which exhibit characteristic arid topography. These soils are poor and contain little humus and organic matter.
6. Saline Soils
They are also known as Usara soils. Saline soils contain a larger proportion of sodium, potassium and magnesium, and thus, they are infertile, and do not support any vegetative growth. They have more salts, largely because of dry climate and poor drainage.
Their structure ranges from sandy to loamy. They lack in nitrogen and calcium. Saline soils are more widespread in western Gujarat, deltas of the eastern coast and in Sunderban areas of West Bengal.
Excessive irrigation with dry climatic conditions promotes capillary action, which results in the deposition of salt on the top layer of the soil. In such areas, especially in Punjab and Haryana, farmers are advised to add gypsum to solve the problem of salinity in the soil.
7. Peaty Soils
They are found in the areas of heavy rainfall and high humidity, where there is a good growth of vegetation. Thus, large quantity of dead organic matter accumulates in these areas, and this gives a rich humus and organic content to the soil.
It occurs widely in the northern part of Bihar, southern part of Uttaranchal and the coastal areas of West Bengal, Orissa and Tamil Nadu.
8. Forest Soils
Forest soils are formed in the forest areas where sufficient rainfall is available. The soils vary in structure and texture depending on the mountain environment where they are formed.
In the snow-bound areas of the Himalayas, they experience denudation, and are acidic with low humus content. The soils found in the lower valleys are fertile.