Rhizoferrin, a siderophore produced by Rhizopus arrhizus, has been shown in previous studies to be an outstanding Fe carrier to plants. Yet, calculations based on stability constants and thermodynamic equilibrium lead to contradicting conclusions. In this study a kinetic approach was employed to elucidate apparent contradictions and to determine the behavior of rhizoferrin under conditions representing soil and nutrient solutions. Stability of Fe3+ complexes in nutrient solution, rate of metal exchange with Ca, and rate of Fe extraction by the free ligand were monitored for rhizoferrin and other chelating agents by 55Fe labeling. Ferric complexes of rhizoferrin, desferri-ferrioxamine-B (DFOB) and ethylenediamine-di(o-hydroxyphenylacetic acid) (EDDHA) were found to be stable in nutrient solution at pH 7.5 for 31 d, while ferric complexes of ethylenediaminetetraacetic acid (EDTA) and mugineic acid (MA) lost 50% of the chelated Fe within 2 d. Iron-calcium exchange in Ca solutions at pH 8.7 revealed rhizoferrin to hold Fe at nonequilibrium state for 3 to 4 wk at 3.3 mM Ca and for longer periods at lower Ca concentrations. Ethylenediaminetetraacetic acid lost the ferric ion at a faster rate under the same conditions. Iron extraction from freshly prepared Fe hydroxide at pH 8.7 and with 3.2 mM Ca was slow and followed the order:. DFOB>EDDHA>MA≥rhizoferrin>EDTA. Based on these results we suggest that a kinetic rather than equilibrium approach should be the basis for predictions of Fe chelates' efficiency. We conclude that the nonequilibrium state of rhizoferrin is of crucial importance for its behavior as an Fe carrier to plants.