Soil is the name given to worm out rock particles of different sizes which are mixed with decaying and plant or animal matter. Generally, the layers of materials found above the solid rocks of the earth are soil. This soil covers the land as an almost continuous blanket. The top few centimeters of soil which are occupied by plant roots and other living things is called topsoil. This layer is dark in color, because it contains most of the humus found in the soil.
Humus, as we have already learnt, is the black or brown organic part of soil which is formed by the decay of animal and plant matter. Humans also contain chemical substances which most plants need. Below the region of dark topsoil is a region of lighter soil known as subsoil. The subsoil is made up of large soil particles and stones. A layer of rock lies below the subsoil.There are some useful protozoa that help to make soil more fertile by increasing the nitrate content of the soil. Still other protozoa make the soil less fertile by destroying the nitrogen-fixing bacteria in the soil.
Humus, as we have already learnt, is the black or brown organic part of soil which is formed by the decay of animal and plant matter. Humans also contain chemical substances which most plants need. Below the region of dark topsoil is a region of lighter soil known as subsoil. The subsoil is made up of large soil particles and stones. A layer of rock lies below the subsoil.There are some useful protozoa that help to make soil more fertile by increasing the nitrate content of the soil. Still other protozoa make the soil less fertile by destroying the nitrogen-fixing bacteria in the soil.