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Fluid Journal : Spring 2014
10 The Fluid Journal Spring 2014 The soil suspensions were equilibrated by shaking for 24 h. Thereafter, the suspensions were spiked with carrier-free 32P. The samples were centrifuged, the supernatants filtered through a 0.2-μm membrane filter, and filtrates analyzed for water soluble P by colorimetry. The P activity from the filtrates was measured by scintillation counting. Additionally, spiked solutions (without soil) were prepared and analyzed in parallel to the soil suspensions to determine the initial amount of radioactivity added. Labile P (E-value) was calculated according to Hamon and McLaughlin (2002): Cs V E=R • __ • __ r W where E is the amount of isotopically exchangeable P (mg kg-1), R is the amount of radioactivity added to the soil (Bq), Cs is the concentration of P in the solution (mg L-1), r is the amount of radioactivity (Bq) in solution after 24 h of exchange, V is the solution volume (L), and W the (equivalent dry) mass of the soil (kg). Additional Petri dishes were prepared following the as-described procedure and at the end of the incubation time the residual fertilizer granules were removed from the Petri dishes, individually digested in aqua regia and their total element content analyzed by ICP-AES. The experiment was a completely randomized design with a structure of fertilizer treatment x soil section. The ANOVA procedure was performed for data analysis using Genstat version 14 statistics package. Fisher’s protected least significant difference LSD at 0.05 level was used to compare treatment means. Study overview P recovery. The percentages of P recovered from the granular and fluid fertilizers in the inner soil section are shown in Table 2. For Chile, North, Greenwood, and Pt Kenny soils, approximately more than 70 percent of the added P from the granular fertilizers was recovered within 7.5 mm from the point of application. In the Redvale soil, P from the granular fertilizers was equally recovered between the inner and outer soil sections. In the weak P-sorbing Monarto soil, around 30 percent of added P from the granular fertilizers was recovered in the innermost section. The limited mobility of P from granular fertilizers in acidic and calcareous soils has been previously reported and our results are in agreement with those findings. It has been shown that in non-calcareous soils the key factors controlling the diffusion of P are amorphous Al and Fe oxides. It was likely that the restricted P diffusion in the Andisols and Oxisols was related to the presence of these compounds in the soils. In the calcareous soil, the limited mobility is most likely due to Ca-P precipitation. In contrast to the granular fertilizers, the application of fluid P resulted in more P diffusing to the outer soil section in all the soils. The differences observed between the fluid treatments were likely related to the injection volumes (200 vs. 100). Analyses of the granules extracted from the Petri dishes indicated that the content of Al and Fe in the residual granules did not significantly increase with respect to the control granules in all the soils (data not shown). Furthermore, only a significant increase in the Ca content of MAP and DAP incubated in the Pt Kenny soil was observed. These results indicate that the possibility of precipitation of Al and Fe phosphates in the zone close to the granule in acidic soils is rather unlikely, which contradicts our initial hypothesis and supports the idea that in these soils strong adsorption rather than precipitation reactions limits fertilizer P diffusion and availability. Lability. Using the isotopic dilution method it was found that in all the P-sorbing soils circa 30 percent of P, averaged over all the granular treatments, remained in labile form after the incubation period (Figure Table 2. Percentage of fertilizer P recovered in the inner soil section (<7.5 mm of fertilizer ap- plication site). Treatment‡ % of added P recovered at <7.5 mm Chile North Greenwood Redvale Pt Kenny Monarto SSP 72a 73b 83a 50ab 92a 35a TSP 63a 81a 74ab 50ab 91a 36a MAP 72a 80ab 82a 46b 84ab 26bc DAP 69a 80ab 68bc 54a 76bc 28b TGMAP 200 μL 26c 30e 21e 21d 36e 12d TGMAP 100 μL 45b 41d 34d 17d 48d 14d AAP 47b 56c 60c 35c 72c 22c ‡SSP: single super phosphate TSP: triple super phosphate MAP: monoammonium phosphate DAP: diammonium phosphate TGMAP 200 μL: fluid technical grade MAP, applied in a volume of 200 μL TGMAP 100 μL: fluid technical grade MAP, applied in a volume of 100 μL APP: fluid ammonium polyphosphate, applied in a volume of 58 μL Table 1. Selected soil properties of the six soils used in the study. Soil properties‡ Chile North Greenwood Redvale Pt Kenny Monarto Soil type Andisol Andisol Oxisol Oxisol Calcic Inceptisol Alfisol pH(1:5 in water) 5.3 5.72 5.87 6.4 8.44 7.09 Clay (%) 14 7 13 61 3 8 CaCO3 (%) b.d .l .§ b.d.l. b.d.l. b.d.l. 28 b.d.l. Alox (g kg-1 ) 42.8 42 17.3 2.34 0.241 0.345 Feox (g kg-1 ) 16.7 8.19 4.14 2.22 0.098 0.325 Total P (mg kg-1) 1122 1549 157 128 375 84 Soluble P (mg kg-1 ) 0.22 0.58 0.17 0.02 0.96 0.62 “P Nutrition is Still a Challenge in Acidic High P-fixing Soils.”