The Soil system is dynamic, highly heterogeneous and extremely complex. Soil itself consists of a mineral portion containing mainly silica and a mixture of trace metals, and and organic compounds, as well as water and vast array of different organisms. Soil can exist as a variety of texture; with the texture being a product of changes in the relative proportions of sand, silt and clay. It can contain areas of relative dryness, and includes micropores which are almost always water filled apart from in times of extreme drought. The proportion and type of organic matter varies both with depth, and spatially.
This high level of heteroheneity means that soil contains an extremely large number of ecological niches which have given rise to a staggering array of biodeliversity. Using a taxonomic approach to measure biodiversity, it is often said that more than half the world's estimated 10 million species of plant, animal and insects live in the tropical rainforests. However, when this approach is applied to the soil, the level of diversity is often quoted as being in the range of hundreds of thousands of possibly millions of species living in just 1 handful of soil!