Slow-release boron fertilisers: co-granulation of boron sources with mono-ammonium phosphate (MAP)

Abstract

The application of pure boron (B) fertilisers, independent of other macronutrients, is impractical due to the increased cost of dual handling and spreading. Bulk blending of B with other micronutrients is also an unattractive option as the relatively low rates of B required results in poor nutrient distribution in the field. Co-granulating B with other macronutrients such as mono-ammonium phosphate (MAP) may overcome these problems. Five B sources (boron phosphate (BPO4) synthesised at 500 and 800°C for 1 h, colemanite, ulexite and borax) were co-granulated with MAP to targeted B contents of 0.5, 1.0 and 2.0%. The co-granulated BPO4 had lower water solubility than co-granulated colemanite, ulexite and borax. Boron released from co-granulated borax, ulexite and colemanite was remarkably greater than co-granulated BPO4 products. Over a 4-week soil incubation period with weekly leaching of one pore volume of water, the cumulative B release from co-granulated ulexite, borax and colemanite was 97, 75 and 58%, respectively, but only 16 and 4% for the co-granulated BPO4 synthesised at 500 and 800°C, respectively. Co-granulated BPO4 products have potential as a high quality fertiliser to increase crop yield with slow B release.