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.