Soil organic matter contributes substantially to the physical, chemical, and biological processes in soils, and is expressed in several distinctive soil characteristics. Soil materials start to form in niches where branches meet, in bark and in fallen trunks. As the ground is approached, the composition of the fauna increasingly resembles that found, at least part of the time, within the soil. Roots are an important source of organic materials in soils, and virtually the only source for many arable soils at the present-day. Polar desert soils may have up to 6 per cent organic matter in the topsoil, as the very slow accumulation rates may locally exceed the even slower decomposition rates. Organic matter content in soils declines rapidly in the profile the distinctively dark organic horizon to which quoted values usually refer. The chemical composition of worm casts, compared with the surface soil of an arable field, usually shows greater proportions of available nutrients.