Field Analysis:Why Composition

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Why record soil composition?
Composition refers to the ‘parent materials’ that make up the deposit. These can be organic or mineral, natural, or anthropogenic. The balance of organic matter and mineral particles in the soil can provide information about the stability of a ground surface and the rate of accumulation of sediments. When a surface is stable, plant colonisation follows and organic matter begins to accumulate in the soil, hence organic rich horizons may potentially indicate a phase of soil formation.

The degree of humification of the organic matter provides information about temperature and wetness. Soil biological activity is inhibited at low temperatures, acid pHs, and in waterlogged anaerobic environments. If the rate of breakdown is lower than the rate of accumulation a peaty deposit may develop.

However, differentiating between naturally accumulated organic matter, and deliberate or accidental anthropogenic organic inputs is often virtually impossible in the field except where other anthropogenic inclusions are also present.

Vertical changes in composition may indicate the former presence of stratigraphy lost to bioturbation. However, in such cases it is important to assess whether this might also be due to changes in source, for example sediments derived from different parts of the catchment for alluvial deposits, or an incidental effect of sorting, for example as currents slow and progressively finer particles are deposited.

Examination of the composition of soil features such as mottles can also be useful in determining whether they have formed in-situ as a result of post-depositional processes such as gleying, whether they are the result of different soil material incompletely mixed together, or whether they are the result of decayed stone or artefacts.

When to record composition
Assessment of soil composition is very quick and hence its recording is recommended particularly when the identification of buried soils, and standstill phases in sedimentary accumulation are of interest. An assessment of organic matter type is slightly more complicated but is very useful for determining formation processes of peaty or very organic deposits.

More information on how to record soil composition can be found here.

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