Phosphate Rock Efficiency Enhanced By Granulation with Water-Soluble P Fertilizer and Elemental Sulfur.

Phosphorus (P) is the most limiting nutrient for crop yields in tropical soils. More than 90% of the phosphorus sources used in agriculture worldwide is fully soluble. These sources may not be the most appropriated for highly weathered soils, as P from soluble fertilizers can quickly and strongly be adsorbed by soil colloids and/or transformed into low solubility compounds through precipitation. Theoretically, the ideal phosphate fertilizer should provide a slow P release during the entire crop cycle, which would enables the synchronization between the dissolution of the P source and the P plant demand. The manufacture of a fertilizer by mixing phosphate rock (PR) with fully acidulated phosphate (AP) and elemental sulfur (S⁰) can increase the solubility of the PR, obtaining a fertilizer with "synchronized" P releasing behavior. We evaluated the agronomic performance of phosphate fertilizers produced by mixture of PR+AP+S⁰ in the same granule. A Typic Haplustox was used to grow maize in a pot trial, evaluating: monoammonium phosphate (MAP) (standard), MAP+S⁰, Bayóvar phosphate rock (PR), PR+S⁰, MAP(50%)+PR(50%), MAP(50%)+PR(50%)+S⁰, MAP(25%)+PR(75%) and MAP(25%)+PR(75%)+S⁰. The fertilizers were granulated based on the total P and four P rates were applied (0, 15, 30 and 60 mg kg^-1 P). The amount of S used in the granulation was applied aiming a P:S relation in the granule corresponding to 2:1. The fertilizers PR and PR+S⁰ showed very low efficiency, followed by MAP(25%)+PR(75%) and MAP (25%)+PR(75%)+S⁰. The formulation MAP(50%)+PR(50%)+S⁰ showed efficiency near the MAP, therefore, can be an alternative for conventional P fertilizer, fact that  suggest additional experiments under field conditions. The presence of S⁰ in the fertilizers improved the P recovery from the PR. The oxidation and consequently benefits of S⁰ on PR solubilization increases with time, thus, the residual effect of the fertilizers should be also evaluated.