Lindsay 1991

Lindsay WL. 1991. Iron oxide solubilization by organic matter and its effect on iron availability. Plant and Soil 130: 27-34.

This author reviews the chemistry of bioavailable iron in soil solutions. The major controls on the availability of iron in soil solution, which is normally very low, are pH, redox status, and the presence of organic matter and microsites where organic matter is decomposed by microorganisms under oxygen-limited conditions.

The usual concentration of iron in solution in soils is extremely low, and its exact value suggests contributions from multiple solid iron species, including amorphous and a range of crystalline forms of Fe(III) oxides. Organic matter produces transient small organic acids as it is decomposed, which complex with iron and help to bring it into solution. However, these organic acids are in the middle of the soil organic matter decomposition pathway, and do not persist for long in soils. Bringing more iron into solution relies on a combination of local reducing conditions around respiring roots and among microbe-and-SOM microsites, and local pH. Plants and other organisms secrete compounds that are effective at solubilizing iron and making it available to plants. The other major source of iron for organisms is local fluctuations of reducing and oxidizing conditions, which bring iron into solution and precipitate it as metastable, mixed-valence ferrosic hydroxide, which provides iron to solution at much higher concentrations than most other solid forms of iron.

This paper provides useful background information about the chemistry of iron in soils, but it is not clear to me which forms of iron should be targeted and measured in soils if one wishes to learn something about local aerobic and redox conditions.