This chapter provides some concepts of the importance of living components that fall in the size range between 0.2 μm and several millimetres on soil ecosystem functioning. The focus is given to soil micro-organisms due to their key role in elemental cycling. Soil fauna and plant inputs via roots and exudates are considered to include major biotic components in soil functioning.

The rate of soil formation with the initial colonization of bare rocks by macro-, meso- and micro-organisms is affected by mineralogical attributes such as the mineralogical composition, the type of cementing components and the porosity and permeability of the parent material. In addition, the variation of environmental conditions such as temperature, humidity and air pollution affect the dissolution of rocks. Calcareous and especially mica- or clay-cemented sandstones favour biocorrosive, chemo-organotrophic bacteria in comparison with siliceous sandstones (Warscheid et al., 1991). Coarse-grained sandstones generally promote, due to their high permeability for water, a temporary microbial colonization, while fine-grained sandstones favour the long-term establishment of micro-organisms because they preserve humidity. If a sandstone contains appreciable amounts of feldspars and clay minerals like illite and chlorite, optimal conditions for the growth and biocorrosive activity of chemo-organotrophic bacteria emerge. This is explained by both the increased amount of extractable nutrients and the enlargement of the inner surface area. Bacteria and other micro-organisms may be abundant and preserved in altered rock fragments (Agnelli et al., 2001).

Soil fauna plays a minor role in the colonization of stones and clastic sediments.