BACKGROUND: The environmental risk from the application of synthetic chelates has led to the use of biodegradable complexes to correct Fe deficiency in plants. In this article, the Fe oxidation state, the Fe:ligand ratio, and the molecular weight distribution for heptagluconate (G7) and gluconate (G6) are considered as key factors for the efficacy of complexes as fertilizers. Complexes with different Fe:ligand ratios were prepared and analyzed by gel filtration chromatography (GFC). The ability of Fe:ligand ratios to provide Fe to tomato in hydroponics and soybean in calcareous soil was tested and compared with synthetic chelates (Fe 3+ :HBED and Fe 3+ :EDTA).
RESULTS: G7 presented greater capacity to complex both Fe(II) and Fe(III) than G6, but the Fe(II) complexes exhibited poor stability at pH 9 and oxidation in solution.Gel filtration chromatography demonstrated the polynuclear nature of the Fe 3+ :G7 at various ratios. The effectiveness of the Fe fertilizers depend on the Fe 3+ :ligand ratio and the ligand type, the Fe 3+ :G7 (1:1 and 1:2) being the most effective. Fe 3+ :G7 (1:1) also presented a better response for the uptake of other micronutrients. CONCLUSION: Fe 3+ :G7 molar ratios have been shown to be critical for plant Fe uptake under hydroponic conditions and with calcareous soil. Thus, the Fe 3+ :G7 at equimolar ratio and 1:2 molar ratio can be an environmentally friendly alternative to less degradable synthetic chelates to correct Fe chlorosis in strategy I plants.