Soil and its Formation


Soil is the uppermost layer of the land surface that plants use and depend on for nutrients, water and physical support.

FACTORS OF SOIL FORMATION

The five factors, which control the formation of soil are parent rock, relief, time, climate and plant and animal organisms. The former three are called the passive factors while the later two are the active factors. The parent material and climate are the most important because these two affect the other factors.

(a) Parent rock

A soil is derived from the underlying rock or the parent rock material containing different minerals. The parent rock gets broken into tiny pieces and is decomposed slowly by physical and, chemical weathering. It furnishes inorganic mineral particles of the soil. The parent rock also influences the rate of soil formation, the chemical composition, colour, texture, structure, mineral content and fertility.

(b) Relief

Topography of an area affects the degree of erosion of the parent rock material and the rate of surface run off of water. Thus, the relief affects directly and indirectly the processes involved in soil formation.

Steep slopes are subjected to more rapid run-off of surface water than the gentle slopes. Therefore, there is less infiltration of water on steeper slopes, which retards soil forming processes. In addition, rapid run-off on steep slopes often erodes their surface faster than soil can develop. It is because of this that the mountainous topography develops coarse, thin and infertile soil and the plain areas have rich well developed fertile soils.

(c) Time

The soil forming process is very slow. A well developed soil results as an end product of physical, chemical and biological processes operating collectively for a very long period of time.

(d) Climate

It is by far the most important factor in the sense that over a long period of time it not only tends to reduce the difference caused by the parent material but also influences biological activities within the soil.

Due to this factor two different parent materials may develop the same type of soil in one type of climatic region. For example, granite and sandstone have developed into sandy soil in dry Rajasthan desert.

On the other hand, two different types of soils may develop from the same parent material in two climatic regions. For example, the crystalline granites have developed laterite soils in monsoon regions and non laterite soils in sub humid regions.

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The process of weathering, its effectiveness and the type of plant and animal organisms in a region are directly linked with the seasonal change of temperature and distribution and nature of precipitation. Hence, climate plays an important role in soil forming processes.

(e) Plant and Animal Organisms

Plants and animals play an active role in transforming parent materials into a mature soil. Dead plants and animals contribute to the organic content of the soil.

The process of decay, added by bacterial action, transforms organic matter into humus.

Humus

  • is responsible for the fertility of the soil.
  • enhances water retention capacity of the soil.
  • helps the soil to support plant life. The plant cover in turn protects rich upper layer of the soil from erosion by increasing the proportion of rainfall entering into the soil rather than running off the surface.
  • prevents greater evaporation of soil moisture by its thick canopy, thus allowing soil to mature and become fertile.

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SOIL HORIZONS

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A layer of soil which lies more or less parallel to the surface and has fairly distinctive soil properties is known as soil horizon.

Soil horizons are distinctive layers found in soils that differ in physical or chemical composition, organic content or structure. The display of horizons on a cross section through the soil is termed as soil profile.

From above figure, there are four main horizons are important – A,E,B and C.

  • The A horizon is the upper most horizon and rich in organic matter.
  • Next is the E horizon. Clay particles and oxides of aluminum and iron are removed from the E horizon by downward seeping water, leaving behind pure grains of sand or coarse silt.
  • The B horizon receives the clay particles, aluminum and iron oxides, as well as organic matter washed down from the A and E horizons.
  • Beneath the B horizon is the C horizon, which is not considered part of the soil. It consists of the parent mineral matter of the soil.

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The climate, plant and animal organisms are the active factors of soil formation.
The parent material, relief and time are the passive factors of soil formation.
Soil profile is the arrangement of the soil into layer like horizons which are physically, chemically and biologically different from each other.

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Bibliography : NIOS Geography Book
Reference : http://www.nios.ac.in

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