Tutorial:Soil Gleying

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Gleying/hydromorphism
Gleying is essentially the process of waterlogging and reduction in soils. In waterlogged soils where water replaces air in pores, oxygen is quickly used up by microbes feeding on soil organic matter. This creates anaerobic conditions and leads to the reduction of iron from its oxidised (ferric) Fe3+ state to its reduced (ferrous) Fe2+ state, which is soluble in the soil water. The removal of iron leaves the soil a grey or bluish colour.

For more information on redoximorphic colour patterning click here.

Where some oxidation can take place for example around roots, aggregate surfaces and pores, and towards the top of the watertable, relatively insoluble oxidised (ferric) iron is deposited as rust coloured iron oxides leading to the formation of reddish mottles and concretions.

A soil whose properties are dominated by this process is called a gley (UK), gleysol (World Reference Soils Base) or ultisol (USDA).

Waterlogging can occur because of submersion of the sub-soil below the watertable (a ground water gley), this tends to occur in low-lying areas such as floodplains and hollows.

Where the soil contains an impermeable layer, a perched watertable may develop above the main watertable. This can lead to gleying in surface and shallow sub-surface horizons. Such a soil is called a surface-water gley.

Gleying is very common in archaeological deposits as burial results in deposits being submerged within the watertable, and organic deposits can speed the development of anaerobic conditions. Waterlogging slows organic decay processes, aiding the survival of organic matter and deposit stratigraphy.

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