Soils

Soil is the layer of unconsolidated matirial at the surface of the earth. It is derived from the weathering and erasion of socks to which decayed elements of wont and animal life, known as humus, are added. Soil formation is mainly related to the parent rock material, surface relief, climate and natural vegetation. Animals, insects and man also play important role in soil formation. Some of the important factors of soil formation are mentioned below:

Factors of Soil Formation

1. Parent Material

The material for soil formation is mainly derived from the rocks and is termed as the parent material by soil scientists. The surface rocks are exposed to the process of weathering and suffer decay and decomposition. In this process, the rocks are converted into fine grains and provide a base for the soil formation. The soils of the Northern Plain of India have been largely derived from the depositional work of the Himalayan rivers. This depositional work has been continuing for thousands of years. These are alluvial fertile soils consisting of the silts and clay. These soils have little relation with the original rocks. On the other hand, the soils of peninsular plateau are generally coarse-grained and are closely related to the parent rocks. The peninsular soils are generally less fertile.

2. Relief

Relief influences the process of soil formation in man ways, the most important being the slope of land. Steep slope encourages the swift flow of water and hinders the process of soil formation. There may even be soil erosion in areas of steep slope. The areas of low relief or gentle slope generally experience deposition and have thick layer of soil. Because of this reason, there are thick layers of fertile alluvial soils in the northern plain of India whereas the soils are generally shallow in the plateau areas. The exceptions in the plateau are river basins where the soil layers are sufficiently deep. The degree of slope also largely determines the fertility of soil.

3. Climate

Climate is the most important single factor in soil formation. Most important climatic factors affecting soil formation are the amount and seasonal distribution of rainfall and temperature. Climate controls the type and effectiveness of weathering of the parent material, the quantity of water seeping through the solid and the type of micro-organisms present therein. When the climatic control acts for a sufficiently long period, it reduces the differences in the parent materials.

4. Natural Vegetation

Natural vegetation reflects the combined effects of relief and climate. The formation and development of soil is very much influenced by the growth of vegetation. The decayed leaf material adds much needed humus to soil thereby increasing its fertility. e densely forested areas contain some of the best soils in India. There is a close relationship between the vegetation types and soil types in India.

Classification of Soils

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 formed by the depositional work of rivers in the river valleys, flood plains and deltas. The Sutlej-Ganga plain has the largest deposition of alluvial soils. These are very fertile soils and this plain is considered to be one of the world’s most fertile areas. The deltas of the Mahanadi, the Godavari, the Krishna and the Cuavery also abound in alluvial soils. Alluvial soils are also found in Assam Valley, Gujarat plains and the Western Coastal Plain. About 40% area is covered by alluvial soils.

The physical properties of alluvial soils are determined by climate and vegetation. These soils vary in nature from sandy loam to clay. They are generally rich in potash but poor in phosphorus. In the Upper and Middle Ganga plain, two different types of alluvial soils have developed, viz. Khadar and Bhangar. Khadar is the new alluvium and is deposited by floods annually, which enriches the soil by depositing the silts. Bhangar represents a system of older alluvium, deposited away from the flood plains.

Both khadar and bhangar contain calcareous concretions (kankars) which are used for white washing the houses in the rural areas. These days, it forms an important raw material for cement industry. These soils have lime, potash in abundance but are det in phosphoric acid and nitrogenous and organic contents. These soils are more loamy and clayey in the lower and middle Ganga plain and the Brahmaputra valley. The sand content decreases from west is east. The colour of 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 very useful for growing a variety of crops especially cereals and pulses. Besides some commercial crops like cotton, sugarcane and jute are also grown. As such these soils are most intensively cultivated.

2. Black Soils

These soils are also known as Regur soils or cotton soils as cotton is abundantly grown in these soils. The black oils have been formed by the solidification of lava spread over large area s during volcanic activities in the Deccan Plateau over thousands of years ago. At present, these soils are found over an area of five lakh sq. km including parts of Maharashtra. Western Madhya Pradesh, Karnataka, Gujarat, Andhra Pradesh and Tamil Nadu. In the lower reaches of the Godavari and the Krishna, and the northern part of the Deccan Plateau, the black soil is quite deep. Black soils cover about 30 per cent of the total areas of the country.

Black soils are generally clayey, deep and impermeable. These soils are capable of sustaining moisture for long periods. In dry and hot season, moisture is evaporated and the soils develop cracks where loosened soil particles get accumulated. Thus, there occurs a kind of ‘self ploughing’. During rains, they become sticky and difficult to plough. These are very fertile soils but they have lower fertility on the uplands as compared to lowlands. The black soils are very rich in mineral contents because they have been formed due to volcanic activities. Their chemical composition consists of lime, iron, magnesia and alumina. They also contain potash. But they lack in phosphorus, nitrogen and organic matter.

3. Red and Yellow Soils

These soils are reddish in colour due to wide diffusion of iron in crystalline and metamorphic rocks. These soils are mainly found in tropical and comparatively dry areas of the Peninsular India. Red soils cover almost the whole of Tamil Nadu, Karnataka, Telangana, Andhra Pradesh, south-eastern Maharashtra, eastern parts of Madhya Pradesh, Chhattisgarh, parts of Odisha, Jharkhand and Bundelkhand. Such soils lack in nitrogenous, organic and phosphoric acid contents and are less fertile is soil looks yellow when it occurs in a hydrated form. The d-grained red and yellow soils are normally fertile, whereas coarse-grained soils found in dry upland areas are poor in fertility.

4. Laterite Soil

Laterite has been derived from the Lain word ‘Later’ which means brick. Laterite soils are formed as a result of leaching away of siliceous matter of the rocks due to dry and wet conditions which take place in the typical monsoon climate. Generally speaking, the soils of higher altitudes are more acidic than those of lower altitudes. With rain, lime and silica are leached away, and soils rich in iron oxide and aluminium compounds are le 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. These soils are mainly found in the highland areas of the plateau and are common in Karnataka, Kerala, Tamil Nadu, Maharashtra, Madhya Pradesh and the hilly areas of Odisha and Assam. Laterite soils are not much suitable for agriculture but grass, bushes and shrubs grow abundantly in them. It becomes hard like a brick when it dries up. Thus, it is not useful for agriculture but provides very good building material. However these soils can be used for growing crops with proper dose of manures and fertilisers. Red laterite soils in Tamil Nadu, Andhra Pradesh and Kerala are more suitable for tree crops like cashewnut.

5. Arid Soils

As the name indicates, arid soils are found in arid lands of Rajasthan and adjoining areas of south-west Punjab and south-west Haryana. The colour of these soils range from red to brown. Most of the arid soils are sandy and saline. In some areas, the salt content is so high that common salt is obtained by evaporating the saline water. Accelerated evaporation in hot and dry climate leads to lack of moisture and humus. Although phosphate content is normal, there is deficency of nitrogen. Lower horizons of the soil are occupied by ‘kankar’ layers because of the increasing calcium content downwards. The ‘Kankar’ layer formation in the bottom horizons restricts the infiltration of water, and as such when irrigation is made available, the soil moisture is readily available for a sustainable plant growth.

6. Saline Soils

Also known as Usara soils, the saline soils contain higher proportion of sodium, potassium and magnesium. As such these soils are infertile and have scarce vegetation. They are saline i.e., they contain more salt due to dry climate and poor drainage. Obviously such soils are found in arid and semi-arid regions, and in the swampy and waterlogged areas. They are mostly sandy and loamy and lack in nitrogen and calcium. Saline soils are mainly found in Western Gujarat, deltas of Eastern Coast and in the Sunderban areas. In the Rann of Kuchchh, the Southwest Monsoon brings salt particles and deposits there as a crust. Seawater intrusions in the deltas promote the occurrence of saline soils. In the areas of intensive cultivation with excessive use of irrigation, especially in areas of green revolution, the fertile alluvial soils are becoming saline. 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, adequate dose of gypsum can solve the problem of salinity in the soil.

7. Peaty Soils

Peaty soils are found in areas of heavy rainfall and high humidity and occur largely in Bihar, Uttarakhand, and coastal areas West Bengal, Odisha and Tamil Nadu. They have good growth of vegetation and are rich in humus and organic content. Organic matter may account for as much as 40 to 50 per cent in these soils. Peaty soils are normally heavy and brown in colour but at several places they are alkaline also.

8. Forest Soils

These soils are found in forest areas where rainfall occurs and there is thick forest growth. Most of the forest soils occur in the Himalayan region where the structure and texture of these soils varies depending on the mountain environment. They are loamy and silty on valley sides and coarse-grained in the upper slopes. 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.

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