Formulation, synthesis and characterization of boron phosphate (BPO 4 ) compounds as raw materials to develop slow-release boron fertilizers

We investigated the use of boron phosphate (BPO 4 ) as a slow-release boron (B) source. Boron phosphate compounds were synthesized by mixing boric acid (H 3 BO 3 ) and phosphoric acid (H 3 PO 4 ) and heating at temperatures of 25 to 1000C for 1 or 24 h. X-ray diffraction (XRD) patterns and chemical analysis confirmed the formation of BPO 4 . The crystallinity of these compounds increased with increasing temperature and heating time. The compounds synthesized at 300C or less were hygroscopic and clumped together, while those synthesized at 500 to 1000C 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 BPO 4 compounds decreased with increasing synthesis temperature and with decreasing pH. The solubility and the kinetics of B release from BPO 4 compounds synthesized at 500 and 800C were slower than for most commonly used B sources. Given their slow dissolution, the BPO 4 compounds may have potential to continuously supply B to crops in environments where B leaching is a problem. The compounds synthesized at 500 and 800C show potential for co-granulation with macronutrient fertilizers such as mono-ammonium phosphate to produce slow-release B-enriched granules.