Iron (Fe) is a common element in the soil, but it is primarily found in forms unavailable to the plant. In aerobic conditions, Fe2+ is oxidized to the less-available Fe3+ and forms oxides and hydroxides, which are insoluble. Ferric oxide (or hematite) is an insoluble form of iron that gives many soils their deep red color. In waterlogged soils or other reducing conditions, Fe3+ is reduced to the more bioavailable Fe2+ and solubility increases.
Naturally-occurring soluble iron is typically not enough to meet the needs of the plant. In soil solution at pH 6, Fe2+ and Fe3+ ions are released in extremely low concentrations. As the pH rises to 7, availability further decreases 1000-fold due to greater oxidation. Iron ions become more available at pH < 6, but many other ions increase in availability as well. Thus, below pH 6 iron uptake may actually decrease due to competition with manganese and other metals.
In soils with high organic matter content and/or high cation exchange capacity, Fe2+ may adsorb to negatively-charged sites and be stabilized, increasing its bioavailability. Plants and certain microbes also release compounds known as chelating agents which form soluble complexes with iron, improving plant uptake.
