Biochar as composite of phosphate fertilizer: Characterization and agronomic effectiveness

Organomineral phosphate fertilizers (OMP) may reduce phosphate release rate and its direct contact to the soil solid phase, increasing the effectiveness of phosphorus (P) fertilization. This study aimed to evaluate the effect of granulating biochar (BC) with triple superphosphate (TSP) in two forms (blend or coated) and three proportions (5, 15 and 25%, w/w) on the P release kinetics and plant growth. A successive plant trial using two soils of contrasting P buffering capacities and five P doses (0, 20, 40, 80 and 120 mg kg^-1) was set to investigate the agronomic effectiveness of OMP that presented the slowest P release kinetic. The kinetic test showed that within the first 1.5 h, TSP, OMP blend and OMP coated fertilizers released 92, 82 and 36% of total P, respectively. Thereby, BC addition to TSP reduced the P release rate, mainly due to coating. The fertilizers coated with 15% and 25% BC (C15 and C25, respectively) presented the slowest P release rate. For the plant trial, C15 was chosen because it requires less BC when compared with C25 fertilizer. In the first crop, C15 provided more P to plants, especially in the soil with high P buffering capacity, which increased by 10% and 20% the P uptake and the P recovered by the plant when compared with TSP, respectively. In the sandy soil, fertilizers C15 and TSP showed the same performances regarding yield, P uptake and P recovery rate. At consecutive cultivation, regardless of the soil type, P sources (C15 and TSP) did not differ in yield, P uptake and P recovery. Therefore, biochar-based organomineral phosphate fertilizer can enhance P use efficiency in high P-fixing tropical soils, increasing P recovery and uptake when compared with TSP.