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Solubility of twelve iron fertilizer products in alkaline soils

Iron EDDHA has long been regarded as the preferred iron (Fe) chelate for application to calcareous soils. Two newer products, Fe-EDDHSA and Fe-glucoheptonate, have been proposed as chelates for alkaline soils. The objective of this study was to compare the residual solubilities of twelve Fe products when incubated with two calcareous soils. Iron, as ferrous sulfate, Fe-lignosulfonate (2 products), Fe-gluconate, Fe-glucoheptonate (3 products), Fe-EDTA, Fe-DTPA, Fe-EDDHA (2 products), and Fe-EDDHSA were incubated with two soils from 1 day to 8 weeks. Remaining Fe in the soil solution was determined after extraction with 0.01 M CaCl2. Negligible Fe from ferrous sulfate or Fe-lignosulfonate remained in solution after 1 day of incubation. Iron from Fe-gluconate or Fe-glucoheptonate was largely precipitated by 1 week of incubation. Low, but measurable, levels of Fe from Fe-EDTA remained in solution for 4–8 weeks, depending on the soil. Only Fe-DTPA, Fe-EDDHA, and Fe-EDDHSA kept a significant amount of the applied Fe in solution for the duration of the experiment. It was concluded that Fe-EDDHSA should be an effective Fe fertilizer for alkaline soils, but Fe-glucoheptonate is probably best used for foliar, instead of soil application.