How is soil formed?

Explanation

Soil is a mixture of small particles of rocks and humus (i.e., organic mtter obtained from decaying of living organisms or their wastes). Temperature variations due to radiatioins of the sun, rain water, winds and living organisms influence the formation of soil from the rocks involving two processes: weathering and paedogenesis.
Brakdown of bigger rocks into small, fine soil particles is called weathering. It may occur due to physical, chemical or biological means. Under the influence of solar radiations, rocks heat up and expand. At night, these rocks cool down and contract. Since all the parts of rocks do not expand and contract at the same rate, cracks appear in the rocks and ultimately the large rocks breakdown into smaller pieces. Flow of water through or over the rocks make the cracks bigger. Flowing/falling water also has a erasing effect on the rocks. On freezing the water expands in rock crevices and break the rocks. Similarly, strong winds continue to rub agaist hard rocks and erode them. Growth of lichens, mosses and other plants also influence the formation of soil by eroding the rocks over which they are growing.
Paedogenesis in involves the decomposition of organic materials by bacteria and fungi and humification and mineralization of decomposed organic matter. Earthworms also play an important role in the soil formation.