Soils are the fundamental resource supporting agriculture and forestry, as well as contributing to the aesthetics of a green planet. They are also a base from which minerals are extracted and to which solid wastes are disposed. In addition, soils act as a medium and filter for collection and movement of water. By supporting plant growth, soil becomes a major determinant of atmospheric composition and therefore earth’s climate.
ORIGIN OF SOILS
Soils are formed by weathering of rocks due to mechanical disintegration or chemical decomposition. When a rock surface gets exposed to atmosphere for an appreciable time, it disintegrates or decomposes into small particles and thus the soils are formed.
FORMATION OF SOILS
Soils are formed either by (A) Physical Disintegration or (B) Chemical decomposition of rocks.
A. PHYSICAL DISINTEGRATION
Physical disintegration or mechanical weathering of rocks occurs due to the following physical processes:
1. Temperature changes
Different minerals of rocks have different coefficients of thermal expansion. Unequal expansion and contraction of these minerals occur due to temperature changes. When the stresses induced due to such changes are repeated many times, the particles get detached from the rocks and the soils are formed.
2. Wedging action of ice
Water in the pores and minute cracks of rocks gets frozen in very cold climates. As the volume of ice formed is more than that of water, expansion occurs. Rocks get broken into pieces when large stresses develop in the cracks due to wedging action of the ice formed.
3. Spreading of roots of plants
As the roots of trees and shrubs grow in the cracks and fissures of the rocks, forces act on the rocks. The segments of the rock are forced apart and disintegration of rocks occurs.
As water, wind and glaciers move over the surface of rock, abrasion and scouring takes place. It results in the formation of soils.
Note: In all the processes of physical disintegration, there is no change in the chemical composition. The soil formed has the properties of the parent rock. Coarse grained soils, such as gravel and sand, are formed by the process of physical disintegration.
B. CHEMICAL DECOMPOSITION
When chemical decomposition or chemical weathering of rocks takes place, original rock minerals are transformed into new minerals by chemical reactions. The soils formed do not have the properties of the parent rock. The following chemical processes generally occur in nature:
In hydration, water combines with rock minerals and results in the formation of a new chemical compound. The chemical reaction causes a change in volume and decomposition of rock into small particles.
An example of hydration reaction that is taking place in soils is the hydrolysis of SiO2
SiO2+ 2H2O Si(OH)4
It is a type of chemical decomposition in which carbon dioxide in the atmosphere combines with water to form carbonic acid. The carbonic acid reacts chemically with rocks and causes their decomposition.
The example for this type of is, that is taking place in sedimentary rocks which contain calcium carbonate.
Oxidation occurs when oxygen ions combine with minerals in rock. Oxidation results in decomposition of rocks. Oxidation of rocks is somewhat similar to rusting of steel.
Some of the rock minerals form a solution with water when they get dissolved in water. Chemical reaction takes place in the solution and the soils are formed.
It is a chemical process in which water gets dissociated into H+ and OH– ions. The hydrogen cations replace the metallic ions such as calcium, sodium and potassium in rock minerals and soils are formed with a new chemical composition.
Note: Chemical decomposition of rocks result in the formation of clay minerals. The clay minerals impart plastic properties of soils. Clayey soils are formed by chemical decomposition.
TRANSPORTATION OF SOILS
The soils formed at a place may be transported to other places by agents of transportation, such as water, ice, wind and gravity.
1. Water transported soils
Flowing water is one of the most important agents of transportation of soils. the size of the soil particles carried by water depends upon the velocity. The swift water can carry the particles of large size such as boulders and gravels. With a decrease in velocity, the coarser particles get deposited. The finer particles are carried further downstream and deposited when the velocity reduces. A delta is formed when the velocity slows down to almost zero at the confluence with a receiving body of still water such as lake, a sea or an ocean.
All types of soils carried and deposited by water are known as alluvial deposits. Deposits made in lakes are called lacustrine deposits. Marine deposits are formed when the following water carries soils to ocean or sea.
2. Wind transported soils
Soil particles are transported by winds. the particle size of the soil depends on the velocity of wind. The finer particles are carried far away from the place of the formation. Soil deposits by wind are known as Aeolian deposits.
Large sand dunes are formed by winds. Sand dunes occur in arid regions and on the lee ward side of the sea with sandy beaches.
Loess is a silt deposit made by wind. These deposits have low density and high compressibility. The bearing capacity of such soils is very low. The permeability in vertical direction is large.
3. Glacier- deposited soils
Glaciers are large masses of ice formed by the compaction of snow. As the glaciers grow and move, they carry with them soils varying in size from fine grained to huge boulders. Soils get mixed with ice and are transported far away from their original position.