Soil is a mixture of organic matter, minerals, liquids, gases and organisms that together support life. It covers much of the earth's surface and is a major component of its ecosystem. Soils continuously undergo changes as a result of numerous physical, chemical and biological processes. A typical soil consists of roughly fifty percent solids, of which about 45 percent is mineral matter and five percent is organic matter, and fifty percent voids, or pores. These pore spaces allow the infiltration and movement of water and air, both of which are essential for the life existing in the soil. Compaction, a common problem for soils, reduces the void spaces, hindering air and water from reaching soil-dwelling organisms and plant roots.
A gram of soil can contain thousands of species, with billions of individual organisms, mostly microbial and still largely unstudied. Bacteria, the smallest and most numerous microorganisms in the soil, play an essential role in the decomposition of organic matter, nitrogen fixation and the transformation of nitrogen and sulfur. Fungi and actinomycetes in the soil also contribute to organic matter decomposition. Larger soil animals, including earthworms, add organic matter into the soil and also improve aeration and drainage by means of their burrows.
Topsoil is the uppermost layer of soil, usually with a thickness of 13 to 25 centimeters. It has the greatest concentration of organic matter and microorganisms, and it is where most of soil's biological activity occurs. It also generally has a high concentration of roots because this is where plants obtain most of their vital nutrients.