Earth’s crust is subjected to a continuous battering of wind, rains and changes of temperature which decompose rocks into small fragments.
The modification of earth’s crust as a result of its interaction with atmosphere and the hydrosphere is called weathering, the final products of which are gravel, sand, silt and clays. Three main types of weathering can be distinguished on the basis of the nature of agencies which cause it:
1. Physical weathering:
Physical weathering of rocks is a mechanical process which is brought about by changes in temperature, water and wind. Differential thermal expansion and contraction due to changes in temperature produces internal tensions in the body of rocks which develops cracks and splits apart. Freezing waters in cracks and fissures expand and create intense pressure, much more than the tensile strength of rocks which tear rock fragments apart. Abrasions produced by wind, rapidly flowing waters and glaciers also cause slow disintegration of rocks.
2. Chemical weathering:
Chemical weathering of rocks involves relatively slow and simple chemical reaction like dissolution, hydration, hydrolysis, carbonation, oxidation and reduction etc. Principal agents of this type of weathering are water, air, oxygen, carbon dioxide and other materials carried by these agencies which act and bring about gradual transformation of rock fragments into gravel, sand, silt and clay. Mineral constituents of the rocks may also be altered in the process.
3. Biological weathering:
A number of organisms plays an important part in weathering of rocks. Lichens, bacteria, a number of fungi and algae etc. in presence of plenty of moisture, produce metabolites which aid in the process of disintegration of rocks. Root of plants growing in cracks and fissures creates considerable pressure which tears rock fragments apart.
As a result of weathering, rocks are broken down into smaller particles amidst which algae, ‘ichen, mosses and other small organisms grow. These slowly contribute organic matter when they die and decay. With time various other types of plants and animals appear which add more and more organic matter. Different types of bacteria, fungi, actinomycetes etc. decompose various kinds of polysaccharides, proteins, fats, lignins, waxes, resins and their derivatives.
The residual, incompletely decomposed organic matter left after the microbial action is called Humus. It gives the soil a loose texture, ensures proper aeriation and has a great capacity for imbibing and retaining water and other nutrients.
The process of organic enrichment, the decomposition of organic matter and the accumulation of humus are referred to as Humification. The generation of plant nutrients from dead and decaying organic matter is called Mineralization. Weathering, mineralization and humification, the three processes taken collectively are termed as Pedogenesis or the soil formation.