Nature and distribution of soil phosphorus as revealed by a sequential extraction method followed by 31 P nuclear magnetic resonance analysis

SUMMARY Soil samples from long‐term plots annually fertilized with superphosphate since 1952 and also unfertilized (control) plots of an irrigated and intensively grazed pasture in Canterbury, New Zealand, were subjected to a sequential extraction procedure followed by 31 P nuclear magnetic resonance (NMR) analysis of the soil phosphorus (P) forms present. Overall, 80% of the total organic P (as determined by ignition) was removed from the soil by the sequential extraction procedure. Most of the organic P (90%) detected by NMR was in the monoester fraction. Small quantities of diester and pyrophosphate were also found. Choline phosphate constituted a significant proportion (18–25%) of the monoester P in the acetylacetone and 0.5 M NaOH extracts. Long‐term superphosphate additions resulted in almost all of the P accumulated in the monoester P fraction (up to 99%).