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.

Key words:

• Iron deficiency
• reduction
• chelation
• absorption
• solubility
• and critical nutrient level