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

Summary 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.