The availability and degradation of pyrophosphate fertiliser in Australian soil types

摘要

Polyphosphate fertilisers have recently gained attention in Australian agricultural research due to the observation of significant yield increases with the application of ammonium polyphosphate (APP) as compared to granular orthophosphate (OP) fertilisers on highly phosphorus (P) fixing soils. In order to determine the potential of APP fertilisers to provide yield benefits in a range of soil environments it is necessary to understand the chemistry of their behaviour in soil. Polyphosphates are chemically different to most other forms of phosphate fertiliser in which the P occurs entirely as OP. At the point of sale APP commonly contains around 30%-40% OP, 40%-50% pyrophosphate and the remainder tripolyphosphate. Polyphosphates are thermodynamically unstable and hydrolysis to more simple forms of phosphate can be induced both chemically and biologically. A series of experiments were designed to investigate the availability and degradation or hydrolysis of pyrophosphate, the dominant polyphosphate in APP in a range of soils with varying pH, organic matter and P buffering capacity. Availability reactions were investigated using a double labelling E-value technique where orthophosphate was labelled with ~(33)OP and pyrophosphate was labelled with ~(32)pyrophosphate. To investigate the degradation or hydrolysis reaction of pyrophosphate in soil we used the non-invasive solid-state ~(31)P nuclear magnetic resonance (NMR) technique.Using the double-labelling technique we were able to compare the contribution of OP and pyrophosphate to the labile P pool. Solid-state ~(31)P NMR was found to be a useful technique to follow in situ the hydrolysis of pyrophosphate to OP in soils. The results of the degradation study suggests that pyrophosphate is able persist for several weeks in a highly P-fixing soil forming a series of reaction products.