The role of biota on the genesis and properties of soils was emphasized and thoroughly illustrated in the various models of soil formation published by, among others, Dokuchaev, Joffe, Thorp, Jenny, Wilde, Simonson, Hole, Runge, Johnson, Brimhall, Paton and their collaborators. Indeed, organisms are abundant, with their number reaching up to 1012 per m2 of soil for bacteria, essentially ubiquitous since they are found even under the most extreme climatic conditions on the Earth surface, and highly active; forests alone assimilate up to 67% of all the CO2 that is removed from the atmosphere by terrestrial ecosystems.

With respect to soil bodies, the activity of the biota contributes to the addition, removal, transformation and translocation of matter (sensu Simonson) through a myriad of processes involving organisms such as algae, lichens, mosses, fungi, bacteria, nematodes, acaria, earthworms, termites, ants, beetles, moles, pocket gophers, prairie dogs, birds, badgers, grasses, brushes and trees. The chemical, physical, mineralogical and morphological traits of soil materials are affected by living organisms, either individually or as communities, and key edaphic properties like pH, organic matter quantity and quality, cation exchange capacity, base saturation, nutrient concentration and availability, clay content, colour, porosity, hydraulic conductivity, aggregate formation and stability, oxide content, mineral composition, and horizon diversity have responded to biotic activity since the Carboniferous (Table 12.1).