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The kinetics of the reaction of FeEDTA with seven soils having a wide range of CaCO3 content was studied. The reaction between 2 to 72h, following the instantaneous chelation of competing cations, conformed to first-order kinetics. A kinetic model for the reaction of iron chelates in soils is proposed, which treats the overall changes in concentrations of competing cations in soil solution as a set of parallel pseudo-first-order reactions. The model predicts that the overall kinetic constant, for the loss of iron from soil solution, should be equal to the sum of the rate constants of formation of the chelates of competing cations. An excellent agreement between theoretical prediction and the experimental results was observed. The overall rate constant of FeEDTA reaction in these soils ranged between 5.64 × 10-3, and 20.6 × 10-3 hr-1. The rate constants of the formation of chelates of the competing cations followed the order: CaEDTA > ZnEDTA > CuEDTA. The reaction of FeEDTA with four soils beyond 72 h seemed to involve a slow release of Ca2+ and Cu2+ from their solid phase into the soil solution. The implications of metal cation interactions during the reaction of iron chelates with calcareous soils in the micronutrient nutrition of crops are indicated.

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