Waste products as alternative phosphorus fertilisers part I: inorganic P species affect fertilisation effects depending on soil pH

One of the bottlenecks to efficient phosphorus (P) recycling is limited understanding of the relationships between inorganic P species in waste products and their P fertilisation effects. In this study, we characterised inorganic P species in seven waste products (two biomass ashes, meat bone meal, fish sludge, catering waste and two food waste-based digestate products) and two manure products (dairy and chicken manure) by: (1) Sequential chemical fractionation, (2) X-ray powder diffraction and (3) solid-state P-31 MAS-NMR spectroscopy. We then used the characterisation data to explain the results of a bioassay studying the fertilisation effects of waste and manure products after application to a nutrient-deficient model soil that was limed to two pH levels (approximately pH 5.5 and 6.9 at pH level 1 and 2), with ryegrass (Lolium multiflorum) as the experimental crop. The P in waste products was mainly present as a complex mixture of inorganic P species, predominantly Ca phosphates with differing solubility. Fertilisation effects were largely explained by sequential fractionation data, with a positive relationship between apparent P use efficiency and the H2O-soluble inorganic P fraction at pH level 1 (R-2 = 0.52) and a negative relationship between apparent P use efficiency and the HCl-soluble inorganic P fraction at pH level 2 (R-2 = 0.66). X-ray powder diffraction and solid-state P-31 MAS-NMR spectroscopy confirmed the sequential fractionation data, but provided little additional information.