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Fluid Journal : Winter 2017
14 The Fluid Journal Winter 2017 the acidulation effect that increases P availability. Dealing with fixation. Resin extractable P results were somewhat surprising. Figure 2 reveals that the Figure 2. Resin extractable P as a percentage of total soil phosphorus. Figure 3. Resin extractable P as a percent of total P after sulfuric acid digestion of extract solution in the 0-8mm dish section. 80/20 blend of MAP and APP produced the greatest proportion of labile OP as compared to the total P present in the soil (native P + P fertilizer added) and MAP produced the least. Superior performance of APP relative to MAP was expected due to the extra hydrolysis step that polyphosphates require prior to their becoming available not only for plant uptake, but for chemical fixation as well; however, the superior performance by 80/20 was a curious finding. Previous studies, such as Lombi et al. (2006) and Pierzynski and Hettiarachchi (2016) confirm that the values obtained for MAP and APP are reasonable. Digestion of resin P extraction reveals potential presence of polyphosphates. The increased lability found using the 80/20 blend was not anticipated but could be explained if calcium pyrophosphate or similar polyphosphate precipitates formed upon the addition of the fertilizer treatments. According to Lindsay (1979), OP solution activity is maintained at a higher concentration when more soluble calcium pyrophosphate is controlling solution activity instead of apatite-like P solids that would commonly be found in calcareous soils. Often the assumption is that PPs do not persist in soils very long but their presence in an incubation study such as this after four weeks is not unreasonable. To investigate whether soluble PPs were still present and were missed during the resin P analysis, the initial resin P extracts were digested to convert all extracted non-OP to OP that could then be measured colorimetrically. A limitation of the molybdate reactive P determined via Murphy and Riley (1962) is that the procedure only assesses OP concentration. Important to note is that the non-OP fraction includes PP as well as any organic forms of P that may be present. The results outlined in Figure 3 support the above explanation. The increase in OP in the APP and 80/20 treatments following digestion suggests that PPs are present in the soil solutions. A soluble PP mineral may be controlling P solution activity thus contributing to the significant increase in “plant available” P relative to the MAP treatment. The seemingly reduced performance of APP was confounded by the PP fraction remaining in solution that was overlooked by the initial round of analysis. Treatment effects. The oxalate in the dark extraction procedure developed by Loeppert and Inskeep (1996) is designed to assess amorphous iron