Phosphorus Chemistry in Bone and Wood Char.

Phosphorus (P) deficiency is one of the major factors limiting plant growth in agricultural ecosystems. Bones are rich in P, but are currently an underutilized waste product. In this study, we investigated the P chemistry in bone and wood char in order to determine their potential as P fertilizer. Bone char has been used as a soil amendment for centuries. However, there is little knowledge regarding the effect of charring temperature and feedstock mixtures on the chemical characteristics and quantity of plant available P in bone char. We hypothesized that raising the pyrolysis temperature would increase the crystallinity and concentration of calcium phosphates in the char, thereby affecting its solubility. Moreover, we expected the addition of wood char to increase plant available P. Oven-dried bones were pyrolyzed at different temperatures (220, 350, 550 and 750 °C), either by themselves or mixed with hardwood. Chemical structure and crystallinity of P in the chars were determined by XANES-spectroscopy. Formic acid extractions were used to assess the amount of plant available P in each char type. Modified dry ashing technique was used to measure total P content in the chars and additional elemental characteristics of interest, such as total Ca, Mg, Fe and Al. Chars were also analyzed for calcium phosphate solubility. As a means of comparison, the same measurements were obtained for triple super phosphate fertilizer and rock-phosphate. These analyses allow us to evaluate the value of bone char as a P fertilizer, and to determine which processing conditions can maximize P availability in bone char.