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Fluid Journal : Winter 2017
13 The Fluid Journal Winter 2017 ▼ DOWNLOAD Objective The objective of this study was to investigate the chemical mechanisms responsible for the fixation of P in a mildly calcareous western Kansas soil when fluid formulations of MAP, APP, and an 80/20 mixture of MAP and APP, respectively, are applied. Both traditional wet chemical analysis and new-age synchrotron based assessment were employed in an effort to help the agricultural community better understand the fate and transport of P, allowing fertilizer application and formulation recommendations to be improved in the future. Methodology Design. Twenty-four Petri dishes (88 mm diameter and 12.9mm height) were packed to a bulk density of 1.1g cm-1 with a high pH, calcareous silt loam from Finney County, Kansas (see Table 1) pre-wetted to 18% maximum water-holding capacity (MWHC). After packing, the soils were brought to 50% MWHC, the covers were placed on the dishes, the edges were wrapped in Parafilm, dishes were inverted and allowed to equilibrate at room temperature (-24oC) for at least 24 hours. Treatments were then slowly administered using a syringe at the exact center of the dish. A treatment was defined as enough fertilizer to equal 9.2 mg P dissolved in 125μL of E-pure water. This is approximately the same amount of P found in a single fertilizer granule. The four treatments consisted of a water-only control, technical grade monoammonium phosphate (FisherBrand ACS Grade) ammonium polyphosphate (11-37-0 Mosaic formulation) and an 80/20 blend of the previous two treatments, respectively, to account for the required amount of P. Nitrogen was not balanced as this may or may not take place in a field situation. Following treatment administration, Parafilm was again employed to seal dish edges and mitigate moisture loss. The dishes were wrapped in aluminum foil to prevent light exposure, inverted and incubated for four weeks in the dark at 25o C. Following incubation, the dishes were excavated into four concentric circular sections with radii of 0-8mm, 8-15.5 mm, 15.5-27mm and 27mm-dish edge extending from the point of application (POA). The sections were then dried at 40o C, weighed and finely ground with a mortar and pestle. Chemical analysis. Five replicates were used for wet chemical analysis. Plant available P was assessed via the anion exchange resin technique with subsequent colorimetric analysis for the molybdate reactive fraction, and total P was determined via aqua regia digestion followed by ICP-OES analysis (Murphy and Riley 1962, Myers et al. 2005 and Premarathna et al. 2010). Orthophosphate in resin P extractions was quantified soon after extraction and again following digestion of the extraction in sulfuric acid to convert PP and other non-OP species to OP (McBeath 2006 and Murphy and Riley 1962). Oxalate extractable (amorphous) iron (Fe) and oxalate extractable P were analyzed according to Loeppert and Inskeep (1966), and pH was assessed using an electrode in a 1:10 soil:water suspension. Synchrotron investigation. The synchrotron-based investigation was completed on the sixth replicate at Beamline 6B1-1-SXRMB of the Canadian Light Source (Saskatoon, SK). After drying, the 0-8mm and 8-15.5mm sections were very finely ground and thinly spread onto carbon tape before being placed under vacuum for analysis. Three scans were taken of each sample at the P K-edge (Eo = 2149eV). A double crystal indium antimonide/silicon monochromator was used to scan an energy range extending from -33.5 to -9.5eV in 2eV steps, -9.5 to 31.5eV in 0.15eV steps, and 31.5 to 91.5eV in 0.75eV steps. Background baseline manipulation and linear combination fitting of the reported spectra using previously collected standards were completed in Athena (v.0.9.25) according to the concepts set forth by Manceau et al. (2012) and Werner and Prietzel (2015) (Ravel and Newville 2005). Results Affecting soil pH. As has been noted in prior P studies, the addition of APP causes pH to decrease in the fertilized zone of calcareous soils due to the hydrolysis of polyphosphates by phosphatase enzymes. Figure 1 shows that this effect is more or less localized to the area directly surrounding the POA. Phosphorus is considered to be most plant available near neutral pH, so one advantage of using APP is Table 1. Inherent characteristics for a western Kansas, calcareous soil. Texture pH Calcium Carbonate (%) Cation Exchange Capacity (cmol kg-1) Total P (mg kg-1) Silt Loam 8.6 7.7 18.4 744 Figure 1. Soil pH as reported by dish section and treatment.