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SOILS

SOILS

Introduction

  • Soil is the thin surface layer on the earth, comprising mineral particles formed by the breakdown of rocks, decayed organic materials, living organisms, water and air. Soil forming process is called pedogenesis. Soil is a very important natural resource. The rich, deep fertile soils of Ganga plain especially its delta and coastal plains of Kerala support high density of population through agricultural prosperity. On the other hand, the shallow and coarse grained soils of Telangana and Rajasthan support only small population because there soils do not provide a base for prosperous agriculture.
  • Nature takes thousands of years to build up soils and various environmental factors are, given below are responsible for it.

PARENT MATERIAL

  • The parent material of soil may be deposited by streams or derived from in situ weathering and mass wasting processes. The parent material determines the colour, mineral composition and texture of derived soils. The peninsular soils reflect parent rock very much e.g. Ancient Crystalline and metamorphic rocks which are basically granite, gneiss and schist form red soils on weathering because they contain iron oxide. Soils derived from lava rocks are black coloured. Similarly, sandy soils are derived from sandstone. But the soils of Northern Plains are transported and deposited by the rivers from Himalayan and peninsular rocks. So they have very little relation with rock material in situ.

RELIEF

  • Slope is the most important factor in relief. Steep slope increases flow of water and mass wasting. Hence soil development takes place in area with gentle slope. As increased slope increases erosion, the soil thickness and fertility also depend on it.

CLIMATE

  • Temperature and particularly rainfall control the weathering of rocks, percolation of water in soil and type of micro-organisms present in the soil e.g. in alternate wet and dry climate laterite soil develops while in Rajasthan both granite and sandstone give birth to sandy soil irrespective of parent rock because of high temperature and wind erosion.

VEGETATION

  • The decayed leaf material adds humus to the soil which increases its fertility. Roots of the plants hold the soil together as also the roots-soil ecosystem contains many soil affecting organisms.

VERTICAL PROFILE OF SOIL

CLASSIFICATION OF INDIAN SOILS

  • The regional diversity in rock materials, climate, vegetation, relief as well as economic practices of farmers and people has influenced soil types in India. Indian Council of Agricultural Research (ICAR) has classified Indian Soils into 8 groups based on genesis, colour, composition and location.
  • Alluvial Soil: Alluvial soils are found in Satluj-Ganga-Brahmaputra plains and in river deltas of the east coast west coast and some river valleys of peninsula. Alluvial soils are depositional soils transported by the streams and winds. They are grey to ash grey and the texture is sandy to silty loam. These soils are both poorly drained.
  • These soils may be divided into: (i) Khadar Soil: The Khadar soils are low lying, frequently inundated by floods during rainy season. Thus they are enriched by fresh silt deposits every year. It is sandier than bhangar soil. It drier areas, it also exhibits stretches of saline and alkaline efflorescence locally known as reh, kallar or thur. (ii) The Bhangar Soil is above flood level. It is generally well drained. They are drier than khadar. Both Bhangar and Khadar contain calcareous concretions (Kankars).
  • Alluvial soils are the most fertile soils supporting variety of crops specially cereals and pulses. Besides these, commercial crops like cotton, sugarcane and jute are also grown. Alluvial soils are deficient in potash while rich in humus, phosphoric acid, lime and organic matter.

BLACK SOIL/REGUR SOIL

  • These soils are spread over Maharashtra, parts of Karnataka, Tamil Nadu, Andhra Pradesh, Madhya Pradesh and Gujarat. They are also known as black cotton soils and have characteristically black to dark grey appearance. They get their color from an admixture of humus and also black constituents from rocky materials. These soils are derived from two types of rocks — basaltic Deccan and Rajmahal traps and the ferruginous (iron containing) gneisses and schists in Tamil Nadu and Andhra Pradesh. These soils are very clayey and therefore highly retentive of water. This makes them suitable for dry land farming. Because of high clay content these soils expand when wet and became difficult to plough. During the dry season the black soils shrink and develop crack into which loosened particles fall down. This process is called self ploughing. These soils are deep and rich, fertile in lowlands but of poor fertility in uplands.
  • Black soils are good for cotton and sugarcane cultivation. They have contributed immensely in the growth of cotton textile industry in Mumbai-Ahmedabad belt. Black soil are deficient in organic content, nitrogen, phosphorus, potassium while rich in iron, lime, potash, aluminium and magnesium.

RED SOIL

  • These are extensively found covering Tamil Nadu, Andhra Pradesh, Karnataka, Konkan, eastern Madhya Pradesh, south Bihar, western Odisha, and North-eastern mountains. Red soils are derived from weathering of old crystalline and metamorphic rocks under dry condition. They are red due to presence of iron oxides and look yellow when occur in hydrated form. Thus very often surface soil is red and subsurface is yellow. These soils are loamy or sandy. They have low water retention capacity.
  • Red soils are deep and fertile in the lowlands and poor and thin in the uplands. In the lowlands red soils are found along with black soil and give good crops on irrigation. Red soils is poor in nitrogen, phosphorus, potassium and organic matter.

LATERITE SOIL

  • Laterites are mainly developed in high altitude areas of peninsular plateau. They cover parts of Western Ghats, coastal Bengal, Eastern Ghats, areas of high rainfall in North-East and Bihar. Laterite soil develops in regions of alternate dry and wet weather. The laterites are typical soils of tropical regions with heavy seasonal rainfall, which promotes leaching of the soil. ( leaching – downward movement of water comprising of dissolved organic and inorganic matter). With rain, lime and silica are leached away and soils rich in iron oxide and aluminium compound which are relatively insoluble, are left behind. Humus content of the soil is removed fast by the bacteria that thrive well in high temperature. The organic matter, nitrogen, phosphate and calcium are low while iron and potash are in excess. Hence laterites are not fertile enough. But they are useful for plantation crops like tea, coffee, etc. They are also used for making bricks.

ARID OR DESERT SOIL

  • They are developed in Western Rajasthan, Haryana and Southern Punjab where annual rainfall is less than 50 cm. These soils are covered with thick brown mantle which inhibits soil growth and red to brown in colour. Desert soils are derived from adjoining rocks and the coastal region. They are sandy, coarse and rich in soluble salts like phosphates and nitrates. It is poor in organic matter with varying percentage of lime. These are actually fertile soils, water being the only limiting factor. They can be reclaimed through good irrigation. Coarse grains like bajra, jowar, ragi and oil seeds grow in these soils.

SALINE AND ALKALINE SOILS

  • The saline soils are widespread in Rann of Kachchh, deltas of eastern coast and Sundarban areas of West Bengal. They are also found in excessively irrigated areas of Northern Plains and Western Maharashtra. Saline soil contains larger proportion of sodium, potassium and magnesium and thus they are infertile and not support any vegetative growth. They occur in arid and semi-arid areas and water logged and swampy areas. Excessive irrigation (e.g. canal irrigation) with dry climatic conditions promotes capillary action which results into deposition of salt on the top layer of soil.

PEATY AND ORGANIC SOILS

  • These soils cover high rainfall areas of West Bengal, Odisha and Kerala. These soils are heavy, dark and acidic and formed under conditions of submergence. Peaty soil areas remain submerged during monsoon and experience accumulation of organic matter with large quantities of soluble salts which can be toxic for plants.

FOREST AND MOUNTAIN SOIL

  • These soils vary in structure and texture depending on mountain environment. They are loamy and silty on valley sides and coarse grained, well drained on the slopes. The slopes are used for horticulture and plantation crops like apple, peach, tea, coffee, etc. while rice and wheat are grown in valleys. These soils are subject to denudation due to landslides and snowfall.

PROBLEMS OF INDIAN SOILS

  • The important problems faced by Indian soils are —
    • Soil Erosion
    • Declining fertility (soil exhaustion)
    • Water logging
    • Salinity and alkalinity
    • Desertification
    • Wasteland
    • Encroachment of the agricultural land
  • As per Planning Commission, 173 million ha of land in India is degraded. This land is in urgent need of reclamation considering food security and growing urbanization.

Erosion of soil

  • Soil Erosion: Soil erosion means removal of top layer of the soil. Soil erosion is significantly affecting agricultural productivity and is a growing menace in the whole country. Among natural agents water and wind significantly erode the soil.
  • Water Erosion: Heavy rains in areas with less vegetation cover remove a lot of soil. Sheet erosion takes place when the top layer of soil is removed. When heavy rains continue then finger shaped grooves are formed particularly on steeper slopes. -It is called sill erosion. These sills further enlarge to form gullies. It is gully erosion. When a gully bed cuts into soil with an immediate drop of 3 to 4 meters and gradually flattens out, a ravine is formed. A badland topography consists of all the above features while ravines are found in Chambal basin and southern Tamil Nadu, water erosion and wind erosion are found all over India. We also see other varieties such as coastal erosion and glacial erosion.
  • Wind Erosion: It is common in arid and semiarid areas of Gujarat, Rajasthan and Haryana. Light soils are more susceptible to wind erosion than heavier soils.
  • Besides natural factors, humans are also agents of soil erosion through (i) Deforestation; (ii) Overgrazing; (iii) Faulty methods of agriculture such wrong ploughing, shifting cultivation, etc.

Soil Exhaustion:

  • Removal of the organic and mineral content of the soils is known as soil exhaustion. Soil degradation results from its misuse. Farmers have to use more inputs every year to get the return to the level of previous year. This testifies declining fertility of land. Cultivation of leguminous crops, organic farming are methods to improve soil fertility.

Water Logging:

  • In low lying areas and level surfaces, water table in the soil rises and saturates the soil, Seepage from unlined channels, canals and sea
    coasts cause water logging. Water logging is believed to be one of the chief causes of salinity. Development of drainage and lining of canals are some solutions in reclamation of water logging areas.

Salinity and Alkalinity:

  • Salinity means predominance of chlorides and sulphates of sodium, calcium and magnesium in such quantity that it interferes with plant growth. Alkalinity implies dominance of sodium salts, especially sodium carbonate. Salinity and alkalinity are found in the relatively less rainfall areas where rate of evaporation is generally higher than the rate of precipitation.
  • They also develop in khaddar lands and canal irrigated areas. Under such conditions, ground water level rises and saline and alkaline efflorescence appear on the surface as a layer of white salt through capillary action. About 2.4% of country’s area is affected by salinity and alkalinity. Satluj-Ganga-Yamuna belt, Indira Gandhi Canal area in Rajasthan, Gulf of Khambhat, Western Maharashtra, Krishna-Tungabhadra belt and deltas of all major rivers are affected by salinity.

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