Iron humatesCharacterization, evaluation and improvement in iron nutrition of strategy i plants grown in calcareous soils

Resumen

This thesis aims to contribute to the knowledge of the influences of the leonardite iron humates character and their behavior in the soil-plant system as a low-cost and an ecofriendly alternative in iron nutrition for the Strategy I plants when they grow in calcareous soils. Moreover, different methods to improve their efficiency were evaluated. Leonardite iron humates are natural iron complexes commonly used in the Mediterranean basin to fertilize cash crops such as citrus, peach trees, and vineyards (Strategy I plants) by foliar application or by fertirrigation. They are less efficient in correcting iron chlorosis than synthetic iron chelates because their structural and physicochemical complexity depends on the diagenetic process which originated them. Currently, the fertilizer market offers only a few iron humates as correctors of iron deficiency because of the strict legal requirements at the European and Spanish regulations. However, due to the new European directives in micronutrient fertilizers and the need to reduce the environmental impact of current agricultural practices, the discussion about the effectiveness of these products is renewed as well as the opportunity to increase their use and commercialization. Therefore, two types of leonardite, one of African origin and another of German origin, were exhaustively characterized. Both products presented ferrihydrite and polynuclear iron compounds in their structures. In particular, the African one presented jarosite in its structure. Moreover, iron humates were also characterized to assess their stability and solubility under calcareous conditions. The kinetic behavior of the iron humate has been studied by ligand competition experiments using chelating agents such as o,oEDDHA, HBED, and BPDS. Iron humate presented high stability. Iron humates exert a long-term effect on the iron fertilization of Strategy I plants in hydroponics (soybean) or soil experiment under controlled conditions as well as in field experiments (tangerines). Deposition and accumulation of humates and iron in the soybean roots were observed in hydroponic assays, and that affected on plant growth as well as in iron nutrition. So, as a first step in improving iron humates, the application mode and the concentration used were studied and corrected. Consequently, iron humates were so efficient at long term as iron synthetic chelates in providing iron ton soybean plants. Furthermore, iron humates from the African leonardite produced jarosite deposits in the soybean roots, which was considered a potentially and bioavailable iron source for the nutritional demands of plants. As a second option in improving the iron humates efficiency, mixtures of iron humates with synthetic iron chelates in different ratios and doses were prepared and applied to soybean plants that grew in calcareous conditions. The mixtures were prepared using stable iron isotopes (56Fe and 57Fe) in order to identify the contribution of each fertilizer to the iron nutrition. The iron chelate shuttle effect was low but improved iron humate efficiency and a slight synergy between iron humates and HBED/Fe3+ was observed because of its lasting effects fit better to the iron humate long-term effect. Finally, leonardite iron humate nanofertilizers were prepared as an improvement option by decreasing particle size. Their contribution to iron nutrition was confirmed and their presence in soybean pods was observed. Although the nanofertilizers did not enhance the iron humate efficiency, they are part of novel technology in line with precision and sustainable agriculture so, further research is needed.