Formation of Soil

Soil is described as loose, powdery earth in which plants grow. It is made up of small pieces of rock and decayed plant and animal materials. The pieces of rock or particles of rock were earlier parts of bigger rocks and the animal or plant materials originate from dead animal bodies and plants.

The formation of soil from rocks has taken centuries. The rocks get broken up into pieces in several ways. Some of these are given below:

The crumbling and wearing away of rock is known as weathering. The agents responsible for weathering of rocks are running water, moving ice or strong waves. The soil formation started with weathering of rocks by physical, chemical or biological agents. The weathered rocks under the influence of various other pedogenic processes get converted into the so-called matured soil. The physical weathering agents originate from climate. These include temperature, water, ice, gravity and wind. A rock becomes very warm with sun’s heat which makes the rock expand. During night, the rock cools more quickly from outside than from inside. This results in the contraction of rocks from outside and cracks develop on its outer layer. The cracked layers of rock break into small fragments as pieces. Water may run into cracks which freezes with a change in temperature. The freezing of water causes the cracks to become deeper. At times, due to falling of seeds into the cracks, sprouting of seeds takes place and the roots of the plant grow; this makes the cracks bigger and larger. Also, lichens, a type of plants which do not have any roots but have acid present in them, cause chemical changes which make the rocks crumble.

The glaciers push bigger pieces of rocks ahead of them as they move along. This pushing helps the rocks to crumble. Water and chemicals present in it dissolve and wear away some kind of rocks. Once the process starts, the remaining rock crumbles.

Chemical weathering of rocks occurs along with physical weathering. The chemical weathering agents are water, oxygen and carbon dioxide. Water is an important weathering agent due to its solvent action. Water soluble minerals like limestone, gypsum, etc. from the weathered rock increase the presence of carbon dioxide and organic acids, formed as a result of decay of remains of animals and plants. Water is also responsible for hydrolysis along with carbon dioxide. Thus, the silicates in the weathered rock are hydrolysed.

This process releases Ca, Mg, K, Na and silicates into the soil. These materials increase the fertility of resulting soil.

The oxygen reacts with minerals in rocks to produce oxides, which dissolve in water and the resulting hydroxides bring about further weathering. The CO₂ and H₂O combine to form carbonic acid, which is also responsible for the weathering of rocks.

The weathering of rocks involving cracking and crumbling of the rock is the beginning of the soil making. To make the real soil, humus must be added to the sand or fine particles of rock. Humus is an organic matter and is obtained from dead animals and plants. The bodies of almost all animals and plants become

a part of soil through the action of bacteria. Bacteria cause the plants and animals to decay and make the soil fertile. Many kinds of insects and earthworms help to make the soil rich.

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