Tutorial:Mottling

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Mottling
A mottled appearance in soils can be due to the migration in solution of manganese and iron ions. This leads to accumulations of oxides and hydroxides in spots or patches. It is a charcateristic of gley soils. The gleying may have occurred during a wet season when hydrolysis liberated drab ferrous iron. During the dry season the iron is oxidised and fixed as orange goethite. Usually the lower part of a soil, below the water table, is more gleyed than the upper part. However, in very clayey soils impermeable layers may form a barrier to downward percolation of water, which can lead to greater waterlogging of the upper than the lower part of the profile. Burrows and root traces are more reduced than the soil matrix in surface water gley, but more oxidised in groundwater gley.

Mottling can also occur in soil horizons that are incompletely weathered. For example, anaerobic decay of biological materials in marshes and stagnant lakes produces deposits that are usually quite dark. Green colours can also result in soils from the presence of green minerals such as epidote, chlorite and serpentine.