Soil is formed when large rocks of the earth are broken into small pieces by natural factors like rain, storms, tree roots etc. Soil has not always been on the earth. Millions of billions of years ago the surface of the earth was in the form of hard and stony solid mountains. Volcanoes bring molten rock from the Earth's interior to the surface.
These pieces of molten rocks, which flowed to the surface in the form of hot lava, cooled down and converted into igneous rocks. As time passed, these rocks kept breaking into small stones due to earthquakes. In this way, the process of breaking of large rocks into small pieces by natural forces such as sun, water, wind, iceberg, root of trees, earthquake etc. is called fault. This process is very slow and goes on continuously. Several thousand years ago, these small stones were converted into soil by the effect of water, wind, roots of trees and plants and mutual friction.
Water, air, roots of plants and mutual friction have an important role in making soil, the details of which are as follows-
Water
- Water plays an important role in breaking down small pieces of rocks.
- During rains, water enters the cracks of the rocks.
- When the temperature is low in winter, the water freezes into ice. When water freezes it expands i.e. solid ice takes up more space than liquid water and this is because the internal molecular structure of ice is open.
- This expansion outwards breaks the rocks into smaller pieces by exerting pressure in the cracks.
- These small pieces of stone start flowing downwards with wind or water and while flowing down they rub with each other and get converted into fine particles.
- These fine particles are brought to the low-lying plains by rain and river water and combine with nutrients and organic matter to form soil.
Plant Roots
- The roots of plants penetrate the crevices of rocks in search of water. These penetrating roots exert outward pressure on the stones and break them into small pieces. These small pieces are eventually converted into soil due to weathering.
The process of breaking down of large rocks into smaller pieces due to natural causes like water, wind, roots of plants and mutual friction is called Weathering. The process of weathering is very slow. It takes hundreds of years to build up a few centimeters of soil layer.
Temperature Variation
- Weathering of rocks also takes place due to difference in day and night temperature. The stones expand during the day and contract during the night. This gradual process of expansion and contraction causes the rocks to break down and these small pieces eventually get converted into soil.
Chemical Weathering
- Rocks contain many types of mineral substances. In the presence of moisture and oxygen in the air, these minerals become oxygenated due to which the stones become brittle and disintegrate in the soil.
Animals and Microorganisms
- Organic acids produced by living organisms cause biological degradation in rocks. Mineral substances produced by erosion and decomposed organic matter mix to form soil. Atmospheric corrosion occurs due to change of temperature and effect of frost.
"Atmospheric gases and acids which are exposed to water cause chemical corrosion."
The weathering of rocks takes a long time. It takes many years to build up a few centimeters of soil. Microscopic plants like mosses, lichens and ferns create cracks in the rocks and when these plants die, their remains mix with the soil to form humus. Humus is the nutrient-rich part of the soil that makes it fertile.
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