The chemistry of iron in soils and its availability to plants

Abstract

The solubility of iron in soils is controlled by Fe(OH)3(soil)in well‐oxidized soils, by Fe3(OH)s(ferrosic hydroxide) in moderately oxidized soils, and by FeCO3(siderite) in highly reduced soils. The Fe(III) hydrolysis species Fe(OH)2 +, and Fe(OH)3° are the major solution species of inorganic Fe, but they are maintained too low to supply available iron to plants. Iron is absorbed by plants as Fe2+ and must be in the general range >10–7.7 M to avoid iron deficiency. The redox of soil‐root environments must be <12 to supply adequate Fe2+ for plants. Hawkeye soybeans (Glycine max. L. Merrill) demon‐ strated the ability to reduce their environment to pe + pH 4 to 7 by release of electrons or other reductants into the root medium. Reduction by plant roots and associated microorganism is an important mechanism for solubilizing Fe2+. Iron chelates aid in the movement of iron to plant roots, but they are neither absorbed to any great extent nor do they raise the activity of Fe3+ or Fe2+ in the bulk soil solution. To be effective iron chelates must be stable in soil environments. Chemical equilibrium relationships are useful in predicting iron solubility and availability in soils.