Chemical nature of residual phosphorus in Andisols

•High quantities of recalcitrant residual fractions derived from Hedley fractionation are found in Andisols.•Recalcitrant phosphorus in Andisol is associated with Aluminun and iron oxides•Recalcitrant phosphorus is found as inositol phosphates, specifically monoester phosphate and its stereoisomers.

Sequential fractionation has been widely used to study the nature and dynamics of soil P. Residual P – the recalcitrant P fraction remaining after sequential extraction with alkali and acid reagents – often constitutes the majority of the soil P, yet its nature and bioavailability is poorly understood. The objective of this study was to isolate, quantify, and characterize residual P following Hedley fractionation in a range of Andisols under grazed pasture by 31P nuclear magnetic resonance (NMR) spectroscopy. Residual P accounted for 45–63% of the total soil P, of which 53–77% was inorganic orthophosphate. Organic P accounted for 21–42% of the residual P, the majority of which occurred as phosphomonoesters including myo- (16% of the residual P) and scyllo-inositol hexakisphosphate (10% of the residual P). No phosphodiesters were detected in the residual fraction. We conclude that residual P in Andisols consists of a mixture of inorganic P and organic P. Our findings provide the basis for the development of new approaches to improve P use efficiency in agriculture.