Humic substances increase the effectiveness of iron sulfate and Vivianite preventing iron chlorosis in white lupin

The main objective of this work was to study the influence of humic substances (HS) on the effectiveness of Fe sulfate and Vivianite in preventing Fe chlorosis in white lupin with a view of performing cost-effective methods to overcome the problem. Two consecutive crops were performed using calcareous sand treated with different Fe sources (FeSO₄·7H₂O and Vivianite, at three different rates, and Fe-EDDHA) and five HS rates. Vivianite was more effective and persistent than Fe sulfate in increasing Soils and Plant Analysis Development (SPAD) readings and lupin yield expressed as dry matter (DM), probably due to the type of alteration products formed upon Vivianite oxidation. The effect of Fe sulfate and Vivianite in preventing Fe chlorosis was significantly increased by HS, and the best results were obtained with Vivianite and HS applied at 0.32 g Fe kg⁻¹ and 0.06 g kg⁻¹ growing media, respectively. SPAD readings with this treatment accounted for 85% and 70% of those obtained with Fe-EDDHA in the first and second crop, respectively, whereas DM production was greater with the Vivianite + HS treatment than with Fe-EDDHA. These results showed the potential effectiveness of treatments based on the application of Vivianite and HS in overcoming Fe chlorosis. However, HS applied with Fe sources at rates of 0.5 and 1 g kg⁻¹ growing media decreased SPAD readings and plant yield. These positive and negative effects of HS in preventing Fe chlorosis were more evident with Fe sulfate and Vivianite than with chelated Fe, probably due to the effect of HS on the solubility of Fe forms of the growing medium.