Co-Pyrolysis of Poultry Litter and Phosphate and Magnesium Generates Alternative Slow-Release Fertilizer Suitable for Tropical Soils

Monoammonium phosphate (MAP), triple superphosphate (TSP), and phosphoric acid (H3PO4), with and without the addition of magnesium oxide (MgO), were mixed with poultry litter (PL) to produce biochar-based fertilizers (BBFs). This is a novel approach aiming to add value to PL residue and to increase P use efficiency in acidic tropical soils. Samples of BBFs were assessed by P-release kinetics in water, Xray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and a bioassay with maize in a greenhouse using a clayey Oxisol. Addition of MgO reduced the kinetics of P released from BBFs impregnated with all P sources. XRD analysis showed the formation of low-solubility Ca2P2O7 and Mg2P2O7 compounds, which is supported by P-O-P or P=O stretching observed in the FTIR spectra. The MgO-treated BBFs increased shoot dry matter production compared to the control, but were inferior when compared to TSP in the short term. The slow release of P and the alkaline reaction of the BBFs show great potential for these materials to be used as enhanced P fertilizers in tropical soils.