Boron phosphates (BPO₄) as a seedling-safe boron fertilizer source

Abstract

Background and aims: Soluble boron (B) sources pose a risk of B toxicity to seedlings just after planting and leaching losses after application and before plant uptake. Boron phosphate (BPO4) has low water solubility and slowly releases B, and hence could be safe for seedlings. Therefore, we investigated the toxicity of several B sources co-granulated with mono-ammonium phosphate (MAP) or co-compacted with potassium chloride referred to as muriate of potash (MOP) on canola seedlings. Methods: Ulexite, borax, colemanite and BPO4 compounds synthesized at 500 or 800 °C for 1 h were co-granulated with MAP or co-compacted with MOP at inclusion rates of 0.5, 1.0 and 2.0 % B. The seedling toxicity of these products was evaluated by placing a fertilizer granule in the centre of a soil-filled Petri dish in which canola was seeded. The area of the non-vegetated zone around the granule application site was evaluated after 7 and 12 days of growth. Results: Application of ulexite, borax and colemanite co-granulated with MAP resulted in toxicity symptoms at the lowest concentration of 0.5 % B, and the area of the affected zone increased with increasing concentrations of B in the granule, whereas no toxicity symptoms were observed with the application of co-granulated BPO4 products even at 2.0 % B content. Similar results were observed for the MOP fertilizers, except for colemanite which showed no toxicity when combined with MOP. Hot water-soluble B concentrations were measured in concentric sections around the granule application site and were in agreement with the toxicity results, with concentration in the toxic range close to the granule for the most soluble B sources. Conclusions: BPO4 is potentially a seedling-safe B fertilizer source.