Soil

Soil is a complex, life-supporting mixture of organic matter, mineral particles, gases, liquids and living organisms, occupying the zone known as the pedosphere. It originates from parent rock broken down by weathering, a process called pedogenesis, that initially produces immature 'embryonic soil' which then develops further into the layered structure — the soil profile — seen in mature soils. Soil performs four major ecological functions: it is a medium for plant growth, a reservoir and purifier for water, a modifier of the atmosphere immediately above it, and a habitat for a huge range of organisms that, in turn, continue to modify the soil they live in. Several measurable properties define a soil's ecological behaviour: its texture, set by the relative proportions of sand, silt and clay-sized particles; its porosity, the fraction of soil volume occupied by pore spaces between particles; its permeability, how readily water moves through those pore spaces (which rises with pore size but is inversely related to water-holding capacity); its temperature, driven by solar radiation, decomposing organic matter and geothermal heat from below, which directly affects seed germination, root growth and the activity o …