Formation of Soil

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As we know we have many type of soil around us. Soil may be classified in many type. But, today we will see formation of soil.

Soils are formed by weathering of rocks and decomposition of organic matter. So soils are organic or inorganic depends upon source of origin.

The weathering of soil occurs with time will decrease cohesive property of soil within the grain and this results in disintegration of the big rock masses into small particles. We called this small particles as soil. Hence the soil formation is the result of weathering action.

Types of Weathering:

Generally, the soils are formed either by,

     (1)   Mechanical weathering

     (2)   Chemical weathering

(1) Mechanical weathering:

Mechanical weathering occur due to physical disintegration of the parent rock. Physical disintegration can occur due to following reasons:

Temperature changes: 

All types of rocks may not have same thermal coefficient. So they behave differently for the different temperatures values. This will cause the thermal expansion and contraction with change of the temperature. If this process repeated several times, its results in disintegration of and soil are formed.

Wedging action of ice:

All rocks are have some pores that gets filled up with water. During low temperature, this water freeze and convert into ice and little increase in volume occur. This volume change will make area expand in rock pores. This result into cracks and further expansion will propagate the cracks. This will lead to break the rock and convert as soil.

Spreading of roots of plants: 

Sometime there are trees and shrubs grown near the rocks. The roots of these trees and shrubs grow inside the rocks and formed cracks in rock mass. The segment of the rock are forced apart and this action disintegrate the rock and further it will results in formation of soil.

Abrasion: 

Movement of wind, water and glaciers over the rock surface will result in abrasion and scouring. It result in soil formation.

All this physical disintegration of parent rock won’t bring any change in chemical composition of rock. The property of soil will be similar to that of parent rock. The soil formed by physical weathering are sand, gravel and some other coarse-grained soils.

(2)  Chemical Weathering:

Transformation of hard rock into soft and erodible material is conducted through process of chemical weathering. Let’s see some process of chemical weathering:

Hydration: 

In this process, water combines with the rock minerals and results in the change in chemical composition. The chemical reaction causes a change in volume and decomposition of rock. As example of hydrolysis of SiO₂.

Carbonation: 

In carbonation, carbon dioxide in the atmosphere combines with water to form carbonic acid. This carbonic acid reacts chemically with rock and causes their decomposition. Sedimentary rocks which contains calcium carbonates are the formed by chemical reaction of rocks by carbonation.

Oxidation: 

Oxidation occur due to oxygen ions combine with rock minerals. Oxidation results in decomposition of parent rock.

Solution: 

Some rock minerals make solution with water when they get dissolved in water. Some chemical reaction takes places in this solution and because of this soil are formed.

Hydrolysis: 

Hydrolysis is chemical process in which water gets dissociated into H⁺ and OH^-- ions. Hydrogen cation replace some metallic ions such as sodium, calcium and potassium present in the rock minerals and this will result into soil formation.

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