Formulation, synthesis and characterization of boron phosphate (BPO₄) compounds as raw materials to develop slow-release boron fertilizers

Abstract

We investigated the use of boron phosphate (BPO4) as a slow-release boron (B) source. Boron phosphate compounds were synthesized by mixing boric acid (H3BO3) and phosphoric acid (H3PO4) and heating at temperatures of 25 to 1000°C for 1 or 24 h. X-ray diffraction (XRD) patterns and chemical analysis confirmed the formation of BPO4. The crystallinity of these compounds increased with increasing temperature and heating time. The compounds synthesized at 300°C or less were hygroscopic and clumped together, while those synthesized at 500 to 1000°C were non-hygroscopic and free-flowing. The solubility of these compounds was assessed at different pH and P concentrations, and compared to the solubility of ulexite and colemanite. The solubility of the BPO4 compounds decreased with increasing synthesis temperature and with decreasing pH. The solubility and the kinetics of B release from BPO4 compounds synthesized at 500 and 800°C were slower than for most commonly used B sources. Given their slow dissolution, the BPO4 compounds may have potential to continuously supply B to crops in environments where B leaching is a problem. The compounds synthesized at 500 and 800°C show potential for co-granulation with macronutrient fertilizers such as mono-ammonium phosphate to produce slow-release B-enriched granules.