Iron Solubility and Uptake in Fava Bean and Maize as a Function of Iron Chelates under Alkaline Hydroponic Conditions

Iron-EDTA becomes unstable above pH 6.5, resulting in competition among metal ions (Fe, Cu, Zn, and Mn) for EDTA and reduced Fe availability. This study examined nutrient precipitation dynamics and interactions between chelated and sulfate forms of Zn, Mn, and Cu with Fe-EDTA in fava bean and maize under increasing alkalinity (0, 5, and 15 mM NaHCO₃). In fava bean, chelated micronutrients significantly increased shoot Fe, P, and Zn concentrations and improved metabolic activity and oxidative stress tolerance. Maize was more sensitive to alkalinity but showed similar nutrient responses at 5 mM NaHCO₃. Speciation modeling (MINEQL+5.0) indicated Fe displacement from Fe-EDTA at high pH, forming insoluble FePO₄ and Fe(OH)₃. Incubation studies demonstrated that Fe-HBED maintained higher Fe, Ca, and P solubility than Fe-EDTA and Fe-DTPA under alkaline conditions. SEM-EDX analysis linked stable Fe chelates to lower P in precipitates. Results highlight Fe-HBED's effectiveness in ensuring Fe and P availability under alkalinity.