Low crystalline apatite in bone char produced at low temperature ameliorates phosphorus-deficient soils

Globally, more than 30% of soils are poor in phosphorus (P) and the productivity of these soils is severely restricted without the addition of P fertiliser. With future P supplies becoming limited, it is becoming increasingly important to identify ways of optimising the use of waste materials as P fertilisers. One technology that has been promoted extensively in recent years to improve quality of degraded soils is the application of biochar. In this context, char produced from recycled animal bone is of special interest because of its high P content (∼15%). This study investigated how production temperature affects chemical P forms in bone char and the impact on soil P availability in different P-deficient soils. The major P form in dried bone meal was poorly crystalline hydroxyapatite. As the pyrolysis temperature increased to 1050 °C, the hydroxyapatite structure measured with X-ray absorption near edge structure (XANES) spectroscopy persisted. Furthermore, crystallinity increased at temperatures above 750 °C, as revealed by X-ray powder diffraction (XRD). Plant availability was highest for bone char produced between 300 °C and 500 °C in three acidic soils from three continents, and declined rapidly above 750 °C. This strongly indicated that crystallinity of hydroxyapatite limits plant availability at high pyrolysis temperatures. In a high pH soil, all materials resulted in low P availability. As pyrolysis increased the P availability in comparison with dried bone, it was concluded that bone char produced at temperatures between 300 °C and 500 °C has the potential to improve fertility of P-poor, low pH soils.