Topsoil is the upper, outermost layer of soil, usually the top 2 inches to 8 inches. It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs. Four elements constitute the composition of soil.
Animals burrowing and plant roots growing down into the soil create the pores in the soil for the air and the water.
The organic matter in the soil comes from the decay of dead plants and animals. Earthworms, bacteria and fungi are just a few of the organisms that live in the soil, feed on the organic matter and decay or recycle plant nutrients. All of the organisms that live in the soil are such an important part of the continuous, natural process of decomposing organic materials and preparing the soil for future plant growth