Isotopic Exchangeability, Hydrolysis and Mobilization Reactions of Pyrophosphate in Soil

Abstract

Liquid polyphosphate fertilizers have shown significant yield increases compared with granular orthophosphate fertilizer on highly P‐fixing soils in Australian cropping trials. Polyphosphate fertilizers contain mixed P species (predominantly pyrophosphate and orthophosphate) and are therefore chemically different from most other inorganic P fertilizers, which contain only orthophosphate. To determine the mechanisms causing the yield responses to polyphosphate fertilizer, a method for investigating the isotopic exchangeability (lability) of P added as pyrophosphate was developed and tested. This method enabled the quantification of the hydrolysis reaction of pyrophosphate to orthophosphate in soil solution. As the rate of pyrophosphate applied increased, the proportion of pyrophosphate added that was isotopically exchangeable increased, and the percentage of pyrophosphate added that hydrolyzed to orthophosphate decreased. The method was further developed to test the hypothesis that pyrophosphate addition to soil may cause the mobilization of native orthophosphate reserves. The results suggested that there was no mobilization of native P after 7 d of incubation in two calcareous soils.