Evaluation of the potential impact of Cu competition on the performance of o,o-FeEDDHA in soil applications

Ferric ethylene diamine-N,N-bis(hydroxy phenyl acetic acid) (FeEDDHA)-based iron (Fe) fertilizers are commonly applied to plants grown on calcareous soils and comprise a mixture of FeEDDHA components. Upon application to the soil, the pore water concentrations of the active ingredients racemic and meso o,o-FeEDDHA show a gradual decline unrelated to plant uptake or biodegradation. In the present study, the potential of soil copper (Cu) to reduce the effectiveness of FeEDDHA-based fertilizers in calcareous soils by displacing Fe from o,o-FeEDDHA has been evaluated through modelling and experiments. Predictions with mechanistic multi-surface models show that there is a thermodynamic basis for assuming that under equilibrium conditions a certain fraction of o,o-EDDHA ligands in soil solution can be chelated to Cu, in particular for meso o,o-EDDHA. The large affinity of o,o-CuEDDHA for binding to the soil solid phase, demonstrated in a batch interaction experiment, greatly increases the potential impact of Cu competition on the o,o-FeEDDHA solution concentration; for a given quantity of o,o-CuEDDHA in soil solution, a much larger quantity of o,o-CuEDDHA is adsorbed to the solid phase. Finally, evidence for the actual displacement of Fe from o,o-FeEDDHA by Cu was found in a soil incubation study. With these results, the boundary conditions are met for explaining the observed gradual decline in o,o-FeEDDHA concentration with Cu competition.