Diffusion and phosphorus solubility of biochar-based fertilizer: Visualization, chemical assessment and availability to plants

Developing slow-release phosphate sources more synchronized with the crop cycle is needed to increase phosphorus (P) use efficiency in weathered tropical soils. We hypothesized that impregnation of biomass with phosphate and magnesium prior to biochar production will generate slow-release P fertilizer and increase P use efficiency by plants. In this study, triple superphosphate (TSP) or phosphoric acid (H3PO4) were mixed with magnesium oxide (MgO) and poultry litter (PL) to produce slow-release P biochar-based fertilizers (BBFs). The P fractions of the BBFs soluble in water, citric acid and neutral ammonium citrate + water were analyzed. Phosphorus diffusion in soil was determined using a visualization technique over time and chemical analyses, and the agronomic efficiency of the BBFs was compared with soluble fertilizer (TSP) in both granular and powder form for maize grown in an Oxisol under greenhouse conditions. Results showed that BBFs strongly decreased water-soluble P, which caused a slow-release of P in soil as demonstrated by diffusion visualization technique. When applied as powder mixed through the soil, BBFs improved soil pH and Mg and were able to provide P to plants similarly to TSP. The granular form of PLB-H3PO4-MgO was equivalent to TSP for P release during early growth of maize and preserved higher amounts of P in the granule, which can be used for plants in future harvests. Pyrolysis of biomass with phosphate and magnesium can be an option to enhance P use efficiency from fertilizers, especially for on high P-fixing soils.