Variation in environmentally- and agronomically-significant soil phosphorus concentrations with time since stopping the application of phosphorus fertilisers

• Soils enriched in Olsen phosphorus (P) may lose much P from soil to water.
• Indicators of potential P loss from soil decreased when P fertiliser applications stopped.
• The rate of decline did not speed up when N was applied at a rate of 50 kg/ha/year.
• The decline occurred in bioavailable inorganic P pools.
• The decline in potential P loss could be monitored with an Olsen P test.

Following the cessation of P fertiliser to a P enriched pasture soil, indicators of potential P loss (WEP and CaCl2-P) decreased exponentially with time. Soil P fractionation revealed a similar pattern occurred in bioavailable inorganic P fractions (principally NH4Cl, HCO3 and NaOH extractable pools). However, the addition of 50 kg N ha− 1 year− 1 to one treatment was not great enough to speed up the rate of decline in P fractions or P loss indicators compared to the same treatment without N applied. 31P NMR data indicates that while the frequency of detecting orthophosphate diesters was greater in the N treated soils, the concentrations of organic P species stayed relatively stable with time. Our data suggest that the depletion of high soil P concentrations to decrease P loss in a pastoral soil in New Zealand operated under cut and carry, while maintaining agronomic potential, can be monitored with the use of a soil Olsen P test.