Evidence for Struvite in Poultry Litter: Effect of Storage and Drying

The use of spectroscopic techniques (especially phosphorus‐31 nuclear magnetic resonance [ 31 P‐NMR] and X‐ray absorption near edge structure spectroscopy) has recently advanced the analysis of the speciation of P in poultry litter (PL) and greatly enhanced our understanding of changes in P pools in PL that receive alum (aluminum sulfate) to reduce water‐soluble P and control ammonia emissions from poultry houses. Questions remain concerning changes of P species during long‐term storage, drying, or after application of PL to cropland or for other uses, such as turfgrass. In this study, we investigated a set of six PL samples (of which three were alum‐amended and three were unamended) that had been characterized previously. The P speciation was analyzed using solid‐state 31 P‐NMR spectroscopy, and the mineralogy was analyzed by powder X‐ray diffraction (XRD) after storing the samples moist and dried for up to 5 yr under controlled conditions. The magnesium ammonium phosphate mineral struvite was identified in all but one PL samples. Struvite concentrations were generally lower in dried samples (≤14%) than in samples stored moist (23 and 26%). The moist samples also had higher concentrations of phosphate bound to aluminum hydroxides. Solid‐state NMR spectroscopy was in general more sensitive than XRD in detecting and quantifying P species. Although phosphate associated with calcium and aluminum made up a large proportion of P species, they were not detected by XRD.