Soil is a mixture of broken rocks and minerals, living organisms, and decaying organic matter called humus. Humus is dark, soft and rich in nutrients. Soil also includes air and water.
Soil is formed by the following actions:
(a) Action of sun: The sun causes the heating of rocks during the day which causes them to expand. Then, these rocks cool down during night time leading to their contraction. Since, all parts of the rocks do not expand and contract at the same rate; it results in the formation of cracks on the rocks. Finally, these huge rocks break down into smaller pieces.
(b) Action of wind: Strong winds and storms also erode the rocks. The strong wind carries small rock pieces and sand from one place to another like water.
(c) Action of living organism: Roots of plants break down the rocks. Similarly, Lichens grow on the surface of rocks. These lichens release certain chemical that causes the rock surface to powder down to form a thin layer of soil.
(d) Action of water:
(i) Water gets into the cracks in the rocks, when this water freezes, it causes the cracks to widen.
(ii) Flowing water wears away even hard rocks over long periods of time. Fast flowing water often carries big and small particles of rock downstream. These rocks rub against other rocks and the resultant abrasion causes the rocks to wear down into smaller and smaller particles, resulting in the formation of soil.
The type of soil is decided by the average size of particles found in it and the quality of the soil is decided by the amount of humus and the microscopic organisms found in it.
The nutrient content of a soil, the amount of humus present in it and the depth of the soil are some of the factors that decide which plants will thrive on that soil. The quality of the topsoil is an important factor that decides biodiversity in that area.