Soil acts as an engineering medium, a habitat for soil organisms, a recycling system for nutrients and organic wastes, a regulator of water quality, a modifier of atmospheric composition, and a medium for plant growth. Since soil has a tremendous range of available niches and habitats, it contains most of the Earth’s genetic diversity. A gram of soil can contain billions of organisms, belonging to thousands of species. Soil has a mean prokaryotic density of roughly 1013 organisms per cubic meter, whereas the ocean has a mean prokaryotic density of roughly 108 organisms per cubic meter. The carbon content stored in soil is eventually returned to the atmosphere through the process of respiration, which is carried out by heterotrophic organisms that feed upon the carbonaceous material in the soil. Since plant roots need oxygen, ventilation is an important characteristic of soil. This ventilation can be accomplished via networks of soil pores, which also absorb and hold rainwater making it readily available for plant uptake. Since plants require a nearly continuous supply of water, but most regions receive sporadic rainfall, the water-holding capacity of soils is vital for plant survival.