Characterization of Fe–Leonardite Complexes as Novel Natural Iron Fertilizers

Water-soluble humic substances (denoted by LN) extracted at alkaline pH from leonardite are proposed to be used as complexing agents to overcome micronutrient deficiencies in plants such as iron chlorosis. LN presents oxidized functional groups that can bind Fe2+ and Fe3+. The knowledge of the environment of Fe in the Fe–LN complexes is a key point in the studies on their efficacy as Fe fertilizers. The aim of this work was to study the Fe2+/Fe3+ species formed in Fe–LN complexes with 57Fe Mössbauer spectroscopy under different experimental conditions in relation to the Fe-complexing capacities, chemical characteristics, and efficiency to provide iron in hydroponics. A high oxidation rate of Fe2+ to Fe3+ was found when samples were prepared with Fe2+, although no well-crystalline magnetically ordered ferric oxide formation could be observed in slightly acidic or neutral media. It seems to be the case that the formation of Fe3+–LN compounds is favored over Fe2+–LN compounds, although at acidic pH no complex formation between Fe3+ and LN occurred. The Fe2+/Fe3+ speciation provided by the Mössbauer data showed that Fe2+–LN could be efficient in hydroponics while Fe3+–LN is suggested to be used more effectively under calcareous soil conditions. However, according to the biological assay, Fe3+–LN proved to be effective as a chlorosis corrector applied to iron-deficient cucumber in nutrient solution.