The Fe pools in soils can generally be divided into: a) ionic and complexed form in solution; b) exchangeable; c) specifically adsorbed; d) organically complexed but water insoluble; e) insoluble inorganic precipitates; and f) held in primary minerals. It is generally acknowledged that organically bound forms are more available to plants than the inorganic insoluble pools. The chemical properties of humic substances, leading to the formation of complexes with Fe and, at least in certain circumstances, to its reduction, can profoundly affect Fe availability in soils. Complexation of Fe at pH values found in most soils helps maintaining Fe in solution and/or in bioavailable forms. Insoluble humic acids can withhold considerable amounts of microelements including Fe (Linehan, 1985), thereby representing a potential reservoir of such element in soil. Furthermore, in the presence of high quantities of Fe, fulvic acids can form inter-chain bonds producing insoluble macromolecules (Sequi et al., 1975).
Humic substances can affect Fe availability also by an indirect mechanism involving the stabilisation of amorphous Fe oxides by high molecular weight humic fractions (Schwertmann, 1991). This process would prevent the re-crystallisation of ferrihydrite to more crystalline oxides, thus keeping Fe in forms more available to plants. The ability of humic substances to complex Fe can also be important for phosphorous nutrition. In fact, phosphate can be bound to humic substances by Fe bridges (Gerke and Hermann, 1992); complexation of Fe by carboxylates such as those released by plant roots (e.g. citrate) can increase both phosphate and Fe solubility (Gerke, 1993). A direct contribution to Fe availability can derive from the formation of water-soluble Fe-humate complexes, which can move in the soil toward the roots (Pandeya et al, 1998).
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