Exchangeability of orthophosphate and pyrophosphate in soils: a double isotopic labelling study

Liquid polyphosphate fertilisers have shown advantages in field experiments as a phosphorus (P) source for crops grown on calcareous soils. Polyphosphate fertilisers contain orthophosphate (oP), pyrophosphate (pP) and other condensed P species. A double labelling technique was developed using ion chromatography for separation of oP and pP, the major P species in polyphosphate fertilisers, in order to measure the isotopically exchangeable oP, pP and hydrolysed pP. Isotopically exchangeable P was measured in soils incubating for zero, three and 7 days after applying oP or pP to simulate a fertiliser band concentration at one g P kg(-1) soil. The data from this incubation study suggest that pP addition initially resulted in less isotopically exchangeable P than oP addition but there was no significant difference in the total isotopically exchangeable P for the two different P sources after 7 days of incubation. The addition of pP to soil resulted in an increase in iron, aluminium and dissolved organic carbon in soil solution, and a decrease in calcium concentration in soil solution. This study has shown that the availability of P added as pP is governed by a complex series of processes that differ from the chemistry of oP reactions in soil.