Chemical reactions of polyphosphate fertilisers in soils and solutions.

Polyphosphates have been shown to offer substantial agronomic benefits over traditional granular phosphorus (P) fertilisers in highly calcareous soils of southern Australia. With ongoing field investigations into the efficiency of polyphosphate fertilisers compared to fluid and granular orthophosphate (OP) fertiliser products, a need developed for detailed study of the mechanisms responsible for the enhanced efficiency of polyphosphate fertilisers in Australian soil types. Polyphosphates provide an analytical challenge as they contain chemically different forms of phosphate compared to most fertilisers, where P occurs entirely as OP. An investigation was conducted into the most suitable method for the speciation, quantification and separation of the P species supplied in polyphosphate fertilisers. While the conventionally used colorimetric technique was comparable to ion chromatography (IC) for quantification of OP, it did not provide the speciation and separation capabilities of IC. Polyphosphate fertilisers are thermodynamically unstable and hydrolyse to more simple forms of phosphate: this can be induced both chemically and biologically. A study was undertaken using IC to ascertain the effects of time, temperature and acidity on the stability of polyphosphate fertiliser formulations. All of these factors affected the stability of polyphosphate fertilisers and recommendations on storage of the product and mixtures with other fluid fertilisers can now be developed. Stability of PP in soils was assessed using solid-state speciation by solid-state ³¹P-nuclear magnetic resonance (NMR). Hydrolysis of both solution and solid-phase PP could be quantified using the NMR technique, and results using this method were compared to conventional techniques, which extract OP and PP into aqueous phases and use IC to assess the extent of PP hydrolysis. The concentrations of OP and PP determined by the extraction technique were lower than those determined by NMR, and consequently the proportion of undetected P was greater for IC than for NMR. There is disagreement in the literature as to the differences in partitioning behaviour (sorption/precipitation) of OP and PP in soil. A partitioning study was undertaken using the IC technique for aqueous P speciation. Retention of PP to soil solid phases was much stronger than OP, and addition of PP to soil resulted in greater concentrations of OP in the equilibrating solutions, indicating a possible competition for P sorption sites between PP and OP. A double labelling technique was developed where OP was labelled with ³³OP and PP was labelled with ³²PP. This technique was used to investigate the effects of time and concentration on the lability and partitioning of OP and PP in soils. Using this dual labeling technique it was possible to determine the hydrolysis of added PP, and to distinguish OP derived from hydrolysis of PP from native OP in soil. The dissolution of dissolved organic carbon, iron and aluminium and the sorption/precipitation of calcium as a result of addition of PP were assessed and related to changes in the lability of P supplied as PP and OP. This double labelling technique was further developed to assess the possibility of mobilisation of native OP by addition of PP to soil. The findings of this thesis indicate that the hydrolysis reaction is pivotal to the behaviour of the P species that constitute a polyphosphate fertiliser in soils. Investigations of isotopic exchangeability showed that while native P mobilisation was not detected, slow reactions of PP in soil including sorption, potentially desorption, and hydrolysis underpin the potential availability of PP in soil.Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2006