These are the oxidation-reduction reaction which reduced ferric iron to ferrous iron and oxidised ferrous iron to ferric iron. The ferric (Fe2+) and ferrous (Fe3+) irons possess highly different solubility properties. The ferric form of iron precipitates in alkaline conditions as ferric hydroxide. However, the ferric iron may be reduced under anaerobic conditions into more soluble ferrous iron. If the conditions are anaerobic sufficient H2S may be evolved to lead the precipitation of iron as ferrous sulphide. In aerobic habitats in the well drained soils the iron mainly exists in ferric form.

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