Factors Involved In Soil formation

Soil, the regolith layer on the top of the earth's surface are formed by the combination of various physical Chemical and anthropogenic factors. It takes thousands of years of natural process to convert the parent rock material into topsoil.

Factors Involved In Soil Formation

The primary factors which influence the formation of soil include:

  • parent rock material
  • relief/topography
  • climate
  • vegetation

Parent rock material

  • The parent rock material plays an important role in determining the chemical composition, colour/appearance, and texture of the soil.
  • However, in some cases, the soil does not reflect the characteristics of the parent rock material, because of the influence of climatic factors. The processes of weathering (physical and chemical) and erosion modify the characteristics of the soil.
  • The parent rock material in India can be classified into the following categories - Ancient crystalline and metamorphic rocks, Cuddapah and Vindhayn rocks, Gondwana rocks, Deccan basalt, Tertiary and Mesozoic sedimentary rocks.

Ancient Crystalline and Metamorphic Rocks

  • These are the oldest rock systems in India, formed over 4 billion years ago.
  • They are formed after the solidification of molten magma, giving rise to rocks such as granites, gneisses, and schists.
  • The weathering of these rocks leads to the formation of red soils. These soils have good ferromagnetic properties. The red colour of the soils is due to the presence of iron oxides.

Cuddapah and Vindhyan Rocks

  • These are sedimentary rocks found in central and south India.
  • The rocks are very thick, sometimes extending 4000 m in thickness.
  • Upon weathering and erosion, they give rise to calcareous soils (which contain calcium carbonate and are chalky in appearance), and also argillaceous soils (which comprise of clay).
  • Because of their sedimentary origin, these soils do not contain any metalliferous minerals.

Gondwana Rocks

  • These are sedimentary rocks, just like the Cuddapah and Vindhyan system, but are much younger than the former.
  • These rocks give rise to soils which are less mature upon their weathering and erosion. The soil profile is often incomplete, with all the soil horizons not being present.
  • Though they have uniformity in character, they are not as fertile as other kinds of soils.

Deccan Basalts

  • Formed as a result of the outpouring of basaltic lava over the peninsular plateau, leading to the creation of Deccan traps.
  • The weathering and erosion of Deccan traps resulted in the formation of black soils. These soils are also known by their local name, regur.
  • They are black in colour because of the presence of oxides of titanium, magnesium, aluminium, and magnetite.
  • These are highly fertile soils, and have a good moisture retaining capacity. Hence they are extensively cultivated, cotton and sugarcane being the major crops. Hence, they also known as black cotton soils.

Tertiary and Mesozoic Sedimentary Rocks

  • These are the younger soils of India found in the extra peninsular regions (Himalayas and plains).
  • Soils formed due to the weathering and erosion of these rocks have a high porosity.
  • These soils are immature, without the complete soil profile.
  • Alluvial soils of the Gangetic plains are a classic example of this type of soil. They contain the fine silt and clay that was brought down by the Himalayan rivers. They are formed as a result of a long process of deposition of river sediments. They have little in common with the parent rock beneath their profile. Hence they are ex-situ soils. The fine silt and clay makes them very fertile
  • On the other hand, the soils of peninsular India contain coarse grains and reflect the characteristics of their parent rocks. They are less fertile in comparison to the alluvial soils.

Relief/Topography

  • The relief/topography is essential in determining the process of soil formation.
  • On steep slopes such as hills and plateau edges, soil formation is hindered. Rampant erosion happens on steep slopes. For example, Chambal ravines, upper reaches of the Himalayas with steep edges and barren surface experience rampant erosion.
  • On the other hand, regions with a low relief and a mildly sloping terrain experience deposition leading to the formation of deep soils. For example, the Indo-Gangetic plains have experienced deposition of alluvial silt because of their gentle slope and low relief.
  • The plateau regions also experience deposition in the river basins which have a low relief, leading to depositional soil layers which are sufficiently deep for the development of the entire soil profile.

Climate

  • Precipitation and temperature are the two important climatic factors influencing the process of soil formation.
  • The weathering of the parent rock material, the extent of water seepage into the rock strata, and the pace of decomposition by microorganisms are also dependant on the temperature and precipitation.
  • The same parent rock material can give rise to different kinds of soils under different climatic conditions. For example, granitic rocks upon their weathering in monsoon climate give rise to lateritic soils, due to extensive leaching. In the drier regions, the soils formed due to their weathering and erosion are non-lateritic.
  • On the other hand, two different parent materials can give rise to same soils under the influence of the same specific type of climate. For example, high temperatures and lack of rainfall led to the formation of black soil in some parts of Tamil Nadu, similar to that of the black cotton soil of the Deccan basalt.
  • The arid climate of Rajasthan resulted in the formation of sandy soils from both granite as well as sandstone based parent rocks.
  • In semi-arid and arid regions where there is little rainfall, and evaporation exceeds precipitation, there is little decomposition of the organic matter which could have added humus to the topsoil. Hence soils found in such regions are mostly light in appearance.
  • In the desert regions of Rajasthan, excess heat and evaporation has led to the accumulation and deposition of lime in the topsoil, due to capillary action of groundwater which brings along dissolved calcium salts that get deposited after the evaporation of water.
  • Under cold weather conditions, such as at the higher altitudes in the Himalayas, the process of decomposition of organic matter is slower than the pace of its accumulation. This led to the formation of acidic soils.
  • While excessive rainfall leaches the oxides of calcium from the soil and deposits them in the bottom layers of the soil, excessive evaporation leads to accumulation and deposition of calcium oxides in the topsoil due to capillary action.
  • In regions with alternate wet and dry climates, the leached material goes up and down with the seasons. The soils get baked in the sun giving them a brick-like appearance. These are the lateritic (brick-like) soils.

Vegetation

  • The effect of natural vegetation on soil formation is in combination with the influence of climate and topography.
  • The vegetative cover provides the organic matter which gets decomposed and adds to the humus content in soils, apart from making the soils appear dark.
  • However, if the relief is steeper, or if the temperature is low and there is not enough precipitation, the organic matter is slow to decompose or does not accumulate in the soil at all.
  • Hence, the densely forested regions of central and south India have soils with high humus content, with mature soils.

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