Tutorial:Soil Biology

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Biological processes
Biological processes are essential in soil development, both for the addition and breakdown of organic matter and the mixing (bioturbation) of the topsoil.

Organic matter accumulation
Organic debris at the surface of the soil in the absence of biological activity, either because of waterlogging or cold temperatures, will accumulate as an organic or histic layer, which if it continues to accumulate to a sufficient depth (typically 30 cm) will be classed as a peat.

In most situations, however, organic matter is broken down and incorporated in the topsoil (A horizon). Organic matter is broken down by a succession of soil organisms and micro-organisms, the most resistant compounds form the dark coloured humus found in top soils. Three different types of humus may be formed: mull, moder, or mor.

Effects of soil organisms
Soil organisms play a number of key roles in soils. Burrowing animals and worms mix the upper soil material in a process known as bioturbation. Bioturbation is a very important process archaeologically as it causes the vertical and lateral movement of artefacts, and it destroys depositional stratigraphy (and hence the context for these artefacts). The depth of bioturbation is one of the determinants of topsoil depth and may be marked by a worm sorted layer of gravel sized stones.

Soil organisms are also very important in the formation of soil structure. Earthworm excrements form the granular structure typical of topsoils (A horizons). Roots, fungal strands (mycelia) and organic exudates from roots, soil fauna, and micro-organisms are important in binding soil particles together into the aggregates known as peds that define soil structure.

Soil organisms are also instrumental in decomposition and many weathering processes, with archaeological consequences for the preservation of organic material and artefacts.

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