Process for formation of soil
The process involving the origin, development and formation of soil is known as pedogenesis. For millions of years there was continuous breaking of rocks into small particles due to heat, rain and wind.
These small particles were carried away by wind and water and ultimately got deposited over other rocks. In this way, thick layer of clay, sand and small pieces of rocks are formed. This is known as soil.
The process of soil formation is known as weathering. Weathering is the process of disintegration of parent rocks into smaller particles (regolith). The weathering basis of agents can be classified as
(a) Physical weathering
(b) Chemical weathering
(c) Biological weathering
(a) Physical weathering:
The physical agents responsible for such weathering are temperature, wind, water, ice, gravity, etc.
Temperature fluctuation leads to simultaneous expansion and contraction or vice versa and the enormous stress thus, generated leads to fragmentation of big rocks to smaller ones.
The freezing of water inside the rock crevices generates pressure due to volume expansion and causes the rock to disintegrate.
Fast-flowing rivers or streams exert enough external pressure on the Parent rocks to disintegrate.
High wind carrying small pieces of rocks lead to collision with big rocks forming smaller rocks.
Earthquake can cause rock slippage from higher altitude to the plains, disintegrating rocks to smaller parts.
Glaciers moving at a high speed from a high altitude cause rocks to break down into smaller pieces.
(b) Chemical weathering:
When different chemical processes are involved in weathering it is known as chemical weathering. The important chemical weathering agents are moisture, water and air. The chemical weathering is temperature sensitive and rate of this weathering is enhanced with the increase in temperature.
The physical weathering form smaller pieces of large number of rocks and thus, large surface area of rocks is now exposed and the rate of chemical weathering is enhanced.
The chemical processes involved in chemical weathering are:
Water soluble minerals are leached through hydration and hydrolysis by water. Some minerals are converted to oxides by oxygen and to carbonates or bicarbonates by carbonic acid formed through the reaction of water with dissolved carbon dioxide (H20 + C02 = H2C03).
The oxides or carbonate or bicarbonates are either dissolved in water or washed away from the surface. These lead to further penetration of water molecule inside the rock matrix and the rock becomes more unstable.
(c) Biological weathering:
Various types of microorganisms extract minerals from rocks as their energy source. This ultimately leads to change in the physical structures and mineral composition of the rocks. As a result these rocks become weak and break down into a number of pieces. This is known as weathering and since it is caused by biological means it is also known as biological weathering.