The part of the topsoil which is formed by the decomposition of leaves and other plant material by soil microorganisms is called humus. It is the organic matter that contains essential nutrients for the growth of plants.

Soil constitutes many things in it such as sand, gravel, air, water, humus and microorganisms. Soil containing sand, clay and humus is called loam. This kind of soil is very good for growing plants.

Leaching is the natural process by which water soluble substances are washed out from soil or wastes. These water-soluble substances may be nutrients, fertilizers or even pesticides. These leached out chemicals cause pollution of surface and sub-surface water.