Gleying (also gleization), is the process whereby a soil changes colour from an earthy reddish/yellow to a bluish/grey, due to waterlogging, and the resultant loss of iron compounds and oxygen. Tundra, meadow, and podsol soils are often affected by gleying, leaving a mottled appearance, because of the lack of drainage. Soil so affected may be called gleysol.

In gleying, the iron is removed from the silt and sand particles. These iron compounds are either leached from the profile in drainage, or are preserved in the soil by stagnant water, but in a reduced state. If the soil is grey, there is no iron present, if the soil is bluish-green, there still remains some iron in the soil.

The oxygen in saturated soil is consumed by the bacteria present.