The soil is the top surface layer of the earth’s crust. It is formed by the breakdown of rocks over a long period of time by various physical, chemical and biological processes. The fine particles thus obtained is called soil. Soil is mainly composed of air, water, humus, microorganisms and minerals. These enhance the quality and fertility of the soil and thereby, provide nourishment to the plants.
Formation of Soil
The soil is formed by the following processes:
The rocks expand by the heat of the sun. These rocks cool down and contract at night. This expansion and contraction of rocks result in the formation of cracks in the rocks. As a result, the huge rocks break into smaller pieces.
The water enters into the cracks formed due to the heating by the sun. This water when freezes widen the cracks. When rocks are in contact with flowing water for a longer period of time, it results in wearing of the rocks. Also, the rock particles present in the flowing water rub against the rocks and break it into smaller particles. These particles are carried by water and deposited as soil on its way.
Strong winds erode the rocks and carry the sand from one place to another.
The chemical processes include hydration, hydrolysis, oxidation, and reduction. The acidic substances derived from the decomposition of organic matter break down the complex compounds into simpler substances.
The lichens growing on the rocks release certain chemicals that break down the rock into a fine powder and form a thin layer of soil. The mosses also grow on the rock surface and further break the rock. The roots of the trees get into the cracks of the rocks and force them to widen, thereby, breaking the rocks.
The soil contains the remains of dead plants and animals called humus. It is home for a variety of microorganisms. The mineral nutrients present in the soil are determined by the rock it was formed from. These make the soil porous and aerate the soil. As a result, the quality of the soil and the soil structure are improved.
Soil Forming Minerals
The minerals comprise of 40 to 45% of the total components of soil. The most common minerals include sodium, potassium and nitrogen. The other important minerals present in soil are mentioned below:
- Hematite- Red to blackish red in colour. Swells up on absorbing water to form hydrated iron oxide.
- Limonite- Hydrated ferric oxide, yellow to brown in colour. Important for colouring and cementing agent in soil.
- Goethite- Limonite with some adsorbed water are goethite.
- Gibbsite- Most common aluminium compound found in soil. Found in highly weathered soil.
- Carbonate Group- Magnesium hydroxide and calcium hydroxide combine with carbon dioxide to form carbonates.
- Calcite- White or colourless, major constituent of sedimentary rocks, consisting of calcium carbonate.
- Dolomite- Chief source of magnesium in the soil.
- Siderite- Important mineral found in water-logged soil. Produced by the alteration of other iron-bearing minerals.
- Sulphate group- Formed by the combination of sulphur and oxygen ions. It further reacts with calcium ions to form calcium sulphate.
- Gypsum- Found in sedimentary rocks and desert soils. It is water soluble and can be leached easily.
These minerals assist in plant growth. Plants absorb these minerals from the soil through the roots. Nitrate, another important mineral resource is rich in amino acids, whereas, magnesium helps in the production of chlorophyll necessary for photosynthesis.
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