Soil microorganisms play an extensive role in the decomposition of organic matter and production of humus, cycling of nutrients and energy and elemental fixation, soil metabolism, and the production of compounds that cause soil aggregates to form. Many are in symbiotic relationships with plants and animals serving as nitrogen fixers and gut microbes. They function as a substantial part of the food web.
Among the microorganisms found in the soil are bacteria, actinomycetes, fungi, micro-algae, protozoa, nematodes, and other invertebrates (mostly arthropods).
It has been estimated that if you look at one “gram” of soil you will see the following numbers of organisms--bacteria 10 8-9 , actinomycetes 105-8 , fungi 105-6 , micro-algae 103-6 , protozoa 103-5 , nematodes 101-2 , other invertebrates 103-5 . A square meter of soil may contain 30-300 earthworms. There are more organisms in a gram of soil than there are human beings on this Earth!
References: Soil Biology Guide. Dindal (ed.) 1990 John Wiley & Sons and Soil Microbial Ecology, F. Blaine Metting Jr. (ed.) 1993