Slow-Release Humic Acid-Based Zinc Fertilizers Improve Growth and Nutrition of Maize and Brachiaria Grass Successively Grown in Oxisols

Abstract

Zinc (Zn) fertilization efficiency is often low in Oxisols due to the interaction of Zn with soil constituents and phosphorus (P). When Zn is supplied in soil fertilization as a complex via slow-release organomineral fertilizers (OMFs), it is less prone to react with soil constituents and P, thus increasing the efficiency of Zn supply to crops. However, obtaining effective synthesis routes for the production of enhanced-efficiency Zn-OMFs for Oxisols is still a challenge. In this study, we synthesized Zn-OMFs based on humic acid (HA) components in order to assess features concerning Zn complexation and release kinetics, as well as the agronomic performance of such products on biomass production, and Zn and P nutrition of maize and Brachiaria grass successively grown in two Oxisols with contrasting texture and soil organic matter (OM) content. Infrared analyses indicated that Zn was complexed in OMFs by sulfur and carbon groups, with kinetics studies showing that Zn is gradually released to soil from Zn-OMFs. Consequently, Zn diffusion and initial content in soil solution were reduced in Oxisols. At least one of the OMFs overperformed ZnSO4 in terms of maize biomass production and Zn shoot accumulation. Moreover, residual effects of OMFs were greater than those of ZnSO4 in Oxisols for Brachiaria grass cultivated after maize, with increases in biomass production and shoot Zn content varying depending on OMFs and Oxisol properties. A secondary effect of OMFs increasing P availability in the soil and P uptake by maize and Brachiaria grass plants was also noteworthy. Our findings showed that the overall agronomic performance was greater for OMFs when compared with ZnSO4, which is likely related to the slow release of nutrients by the synthetized OMFs due to Zn complexation by HA.