Soils and Classification of Soils
• Soil is most important layer of earth’s crust. This is a valuable resource. bulk of our food and much of our clothing is derived from land-based crops that grow in soil.
• soil on which we depend so much for our day-today needs has evolved over thousands of years.
• various agents of weathering and gradation have acted upon parent rock material to produce a thin layer of soil.
• Soil is mixture of rock debris and organic materials which develop on earth’s surface.
• major factors affecting formation of soil are relief, parent material, climate, vegetation, other life forms and time. Besides these, human activities influence soil to a large extent.
• Components of soil are mineral particles, humus, water & air. actual amount of each of these depends upon type of soil. Some soils are deficient in one or more of these, while some others have varied combinations.
• Soil consists of three layers which are known as horizons.
• ‘Horizon A’ is topmost zone, where organic materials have got incorporated with mineral matter, nutrients & water, which are necessary for growth of plants.
• ‘Horizon B’ is a transition zone between ‘horizon A’ and ‘horizon C’, and contains matter derived from below as well as from above. It has some organic matter in it, although mineral matter is noticeably weathered.
• ‘Horizon C’ is composed of loose parent material. This layer is first stage in soil formation process and eventually forms above two layers.
• This arrangement of layers is called soil profile. Underneath these three horizons is rock which is called parent rock or bedrock.
• India has varied relief features, landforms, climatic realms and vegetation types. These have contributed to development of various type of soil in India.
• Soils were classified based on their inherent characteristics and external features such as texture, colour, slope of land and moisture content in it.
• Based on texture, main soil types were identified as sandy, clayey, silty & loam.
• On basis of colour, they were red, yellow, black.
• Soil Survey of India, established in 1956, National Bureau of Soil Survey and Land Use Planning, an Institute under control of Indian Council of Agricultural Research [ICAR] did a lot of studies on Indian soils.
• ICAR has classified Indian soils on basis of their nature and character as per United States Department of Agriculture [USDA] Soil Taxonomy.
• On basis of genesis, colour, composition and location, soils of India have been classified into eight types.
Alluvial Soils
• Alluvial soils are widespread in northern plains and river valleys. These soils cover about 40% of total area of country. They are depositional soils, transported and deposited by rivers and streams.
• alluvial soils vary in nature from sandy loam to clay.
• Khadar and Bhangar are two types of Alluvial soil.
• Khadar is new alluvium and is deposited by floods annually, Bhangar, represents older alluvium and it is deposited away from flood plains.
• colour of alluvial soils varies from light grey to ash grey.
• Alluvial soils are intensively cultivated.
Laterite Soil
• laterite soils develop in areas with high temperatures and high rainfall. These are result of intense leaching due to tropical rains.
• With rain, lime & silica are leached away, and soils rich in iron oxide and aluminium compounds are left behind.
• Humus content of soil is removed fast by bacteria that thrive well in high temperatures.
• These soils are poor in organic matter, nitrogen, phosphate & calcium, while iron oxide and potash are in excess.
• Hence, laterite soils are not suitable for cultivation; however, application of manures and fertilisers are required for making soil fertile for cultivation.
• Red laterite soil in Tamil Nadu, Andhra Pradesh and Kerala is more suitable for tree crops like a cashew nuts.
• laterite soils are commonly found in Karnataka, Kerala, Tamil Nadu, Madhya Pradesh and hilly areas of Odisha and Assam.
Black Soil
• Black soil covers most of Deccan Plateau which includes parts of Maharashtra, Madhya Pradesh, Gujarat, Andhra Pradesh and some parts of Tamil Nadu.
• Black soils are called Regur Soil or Black Cotton Soil.
• black soils are usually clayey, deep & impermeable. They swell and become sticky when wet and shrink when dried. So, during dry season, this soil develop wide cracks.
• Chemically, black soils are rich in lime, iron, magnesia & alumina.
Red and Yellow Soil
• Red soil develops on crystalline igneous rocks in areas of low rainfall in eastern and southern parts of Deccan Plateau.
• Yellow and red soils are found in parts of Odisha and Chhattisgarh and southern parts of middle Ganga plain.
• red soil develops a reddish colour due to a wide diffusion of iron in crystalline and metamorphic rocks. It looks yellow when it occurs in a hydrated form.
• These soils are usually poor in nitrogen, phosphorous and humus.
Arid Soils
• Arid soils range from red to brown. They are usually sandy in structure and saline in nature.
• In some areas, salt content is so high that common salt is obtained by evaporating saline water.
• Due to dry climate, high temperature and accelerated evaporation, they lack moisture and humus. Nitrogen is insufficient and phosphate content is normal.
• Lower horizons of soil is occupied by ‘kankar’ layer because of increasing calcium content downwards.
• Arid soils are characteristically developed in western Rajasthan, which exhibits characteristic of arid topography.
• These soils are poor and contain little humus and organic matter.
Saline Soils
• Saline soils are called Usara soils.
• Saline soils contain a larger proportion of sodium, potassium & 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. They occur in arid and semi-arid regions and waterlogged and swampy areas. Their structure ranges from sandy to loamy. They lack nitrogen and calcium.
• Saline soils are more widespread in western Gujarat, deltas on eastern coast and in Sunderban areas of West Bengal. In Rann of Kuchchh, Southwest monsoon brings salt particles and get deposited there as crust.
• Seawater intrusions in deltas promote occurrence of saline soils.
Forest Soils
• As name suggests, forest soils are formed in forest areas where sufficient rainfall is available.
• soils vary in structure and texture depending on mountain environment where they are formed. They are loamy and silty on valley sides and coarsegrained in upper slopes. In snow-bound areas of Himalayas, they experience denudation and are acidic with low humus content.
• soils found in lower valleys are fertile.
Peaty Soils
• They are found in areas of heavy rainfall and high humidity, where there is a good growth of vegetation.
• Thus, a large quantity of dead organic matter accumulates in these areas, and this gives rich humus and organic content to soil.
• These soils are normally heavy and black and in many places, they are alkaline also. It occurs widely in northern part of Bihar, southern part of Uttarakhand and coastal areas of West Bengal, Odisha & Tamil Nadu.
Soil Degradation and Soil Erosion
• Soil degradation can be defined as decline in soil fertility when nutritional status declines and depth of soil goes down due to erosion and misuse.
• Soil degradation is main factor leading to depleting soil resource-base in India.
• Topography, wind velocity and amount of rain affects soil degradation.
• destruction of soil cover is described as soil erosion.
• rate of removal of fine particles from surface is same as rate of addition of particles to soil layer.
• Sometimes, such a balance is disturbed by natural or human factors, leading to a greater rate of removal of soil.
• Human activities too are responsible for soil erosion to a great extent.
• Wind and water are powerful agents of soil erosion because of their ability to remove soil and transport it.
• Wind erosion is significant in arid and semi-arid regions.
• In regions with heavy rainfall and steep slopes, erosion by running water is more significant.
• Water erosion which is more serious and occurs extensively in different parts of India takes place mainly in form of sheet and gully erosion.
• A region with a large number of deep gullies or ravines is known as a badland topography.
• Ravines are widespread, in Chambal basin. Besides this, they are found in Tamil Nadu and West Bengal.
• Soil erosion is a serious problem for Indian agriculture and its negative effects are seen in other spheres also.
• Deforestation is one of major causes of soil erosion.
• Plants keep soil bound in locks of roots, and thus, prevent its erosion. They add humus to soil by shedding leaves and twigs.
• Chemical fertilisers in absence of organic manures are harmful to soil.
• According to estimates, about half of total land of India is under some degree of degradation.
Soil Conservation
• Soil conservation is a methodology to maintain soil fertility, prevent soil erosion and exhaustion, and improve degraded condition of soil.
• A land with a slope gradient of 15 – 25% should not be used for cultivation.
• Terrace farming can check soil erosion.
• Overgrazing and shifting cultivation can be controlled by educating villagers and spreading awareness among them.
• Contour bunding, contour terracing, regulated forestry, controlled grazing, cover cropping, mixed farming and crop rotation are some of remedial measures which are often adopted to reduce soil erosion.
• Efforts should be made to prevent gully erosion and control their formation.
• In arid and semi-arid areas, efforts should be made to protect cultivable lands from encroachment by dunes by developing shelter belts of trees and agro-forestry.
• Lands not suitable for cultivation should be converted into pastures for grazing.
• Experiments have been made to stabilise dunes in western Rajasthan by Central Arid Zone Research Institute [CAZRI].
• Central Soil Conservation Board, set up by Government of India, has prepared several plans for soil conservation in different parts of country.
• final responsibility for achieving conservation of land will rest on people who operate on it and receive benefits.