Biochar Phosphorus Release Is Limited by High pH and Excess Calcium

Aside from its use for improving soil properties, biochar is increasingly promoted as a direct nutrient provider for sustainable recycling of waste materials. However, incomplete understanding of the interacting factors that determine P release from biochar may limit the efficiency of P recycling from biochar to soil. In particular, the contrasting pH of biochar and soil need to be considered. In this study, soil‐free biochar (rice [ Oryza sativa L.] husk, 700°C) extractions were performed under different pH (4.6–9.9) and extractant conditions to test how solution composition affects biochar P release. When solution pH was in the range of 7.6 to 8.6 and excess Ca was present in the solution or in biochar, P release was low—only 1 to 7% of the total P was released compared with ∼20% under most other conditions. Importantly, we demonstrate that biochar total Ca concentration is closely related to P availability ( R 2 = 0.76) and could be used to predict biochar P release. The results suggest that for maximum P release, low Ca concentrations in biochar and (soil) solution are needed and/or a pH <7.5 at the soil‐biochar interface. This novel understanding will help engineer sustainable biochar fertilizers optimized for P provision. Core Ideas Phosphorus release from rice husk biochar was investigated. Soil‐free extractions were performed at different pH and solution compositions. The lowest P release occurred at pH 7.6 to 8.6 and in the presence of excess Ca. A conceptual model was developed of biochar P release in different soil conditions. Biochars with Ca <1% and/or pH <7.5 are optimal for maximum P release to soil.