Soil and plant relationships associated with iron deficiency with emphasis on nutrient interactions

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Abstract
The major cause of iron deficiency in plants is the insolubility of Fe (III) oxides in soils. Minimum solubility occurs in the pH range of 7.4 to 8.5 where the dominant solution species Fe(OH)3° is present at approximately 10‐10.4 M. The critical level of soluble iron for plants is approximately 10‐8 M, so the ferric oxides are too insoluble to supply adequate iron to plants unless modifying factors intervene. Major factors that modify iron solubility relationships in soils include oxidation‐reduction processes and the presence of chelating agents. Reduction processes near respiring plant roots enable ferric chelates to dissociate and Fe3+ to reduce to Fe2+. It is necessary for Fe2+ to increase above the critical level to supply adequate iron to plants. Competition of metal ions for chelating agents gives rise to many nutrient interactions. Redox relationships also affects metal solubility relationships in soils. When combined these factors greatly affect iron availability to plants. Important nutrient relationships that modify iron availability include interactions of iron with zinc, copper, manganese, phosphate, and nitrogen. Examples of these interactions are given to illustrate how the puzzle of iron deficiency is gradually being resolved.