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Fluid Journal : Fluid Journal 2005-2007
Fall 2005 Determining The Yield Potential of APP In Varying Australian Soil Environments T. McBeath and Drs. E. Lombi, M. McLaughlin, R. Holloway, R. Smernik, E. Bunemann Summary: Polyphosphate fertilizers have gained attention in Australian agricultural research due to significant yield increases with the application of ammonium polyphosphate (APP) as compared to granular orthophosphate (OP) fertilizer on highly phosphorus (P) fi xing soils. In order to determine the potential of APP fertilizers to provide yield benefi ts in a range of soil environments, it is necessary to understand the chemistry of their behavior in the soil. Studies have shown that the sorption characteristics of pyrophosphate, the dominant polyphosphate in APP fertilizers, are quite different from that of orthophosphate. A series of experiments was designed to investigate the hydrolysis and sorption of pyrophosphate. Phosphorus sorption was found to be greater where P was added as pyrophosphate than when added as orthophosphate in all five soils, with varying effects on pH and metal concentrations in solution. Researchers design a series of experiments to investigate the hydrolysis and sorption of pyrophosphate. Fluid Journal 1 Polyphosphate fertilizers have recently shown significant yield benefits compared to traditional granular P forms on the calcareous soils of Eyre Peninsula, South Australia. APP fertilizers have unique chemistry compared to other inorganic P fertilizers, as the P in a polyphosphate fertilizer exists as more than one ionic species. Several studies have been conducted to compare hydrolysis of pyrophosphate on a range of non-calcareous soil types. Most previous work on pyrophosphate hydrolysis has been conducted in temperate climates where soils are neutral or acidic and have higher organic matter contents and biological activity, factors known to promote pyrophosphate hydrolysis. Understanding the nature of hydrolysis reactions of pyrophosphate in Australian soil types is necessary in order to elucidate the mechanisms underlying the superior agronomic performance of polyphosphate fertilizer as compared to traditional P fertilizer on calcareous soil types. Owing to the popularity of polyphosphate fertilizers in the U.S., several studies have been conducted to compare the sorption characteristics of pyrophosphate and orthophosphate (the dominant P species in a polyphosphate fertilizer). The results of these studies suggest that pyrophosphate has different affinities for the bonding energies with various soil components as compared to orthophosphate, which could be due to the ability of pyrophosphate to solubilize organic matter, making soil mineral constituents available for sorption. No such study has been conducted on Australian soil types. Hydrolysis Figures 1 A and B show the quantification data for the study of hydrolysis of pyrophosphate in a highly calcareous (Warramboo) soil. Two methods of quantification are shown. The extraction technique using sodium hydroxide (NaOH, Figure 1 B) had a lower recovery than quantification using nuclear magnetic resonance (NMR) data (Figure 1A). There was a loss in recovery or observability over time for both quantification methods. For both quantification methods there was a decrease in the pyrophosphate concentration and an increase in the Table 1. Concentration of P as orthophosphate derived from hydrolysis of pyrophosphate (native P subtracted) and pyrophosphate (pyro-P) as determined by (A) NMR, and (B) 15 minute (1:10) 1 M NaOH extraction. (op=orthophosphate, PyP=pyrophosphate)
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