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Organic Phosphorus Composition and Potential Bioavailability in Semi-Arid Arable Soils of the Western United States

^a USDA–ARS, Northwest Irrigation and Soils Research Lab., 3793 N. 3600 E., Kimberly, ID 83341
^b Dep. of Geological and Environmental Sciences, Stanford Univ., Stanford, CA 94305

View larger version (20K): [in a new window]   Fig. 1. The fractionation scheme used to characterize soil inorganic and organic P.

View larger version (18K): [in a new window]   Fig. 2. Solution 31P NMR spectra of NaOH–EDTA extracts of the Taunton soil (smallest organic C concentration) and Wahpeton soil (largest organic C concentration), indicating the compounds present in the extracts.

View larger version (21K): [in a new window]   Fig. 3. Relationship between inorganic orthophosphate (mg P kg-1 soil) in NaOH–EDTA extracts of the 18 western U.S. soils determined by molybdate colorimetry and solution 31P NMR spectroscopy. The regression model is described by the equation: [Orthophosphate by molybdate colorimetry] = 0.899 ± 0.032[orthophosphate by 31P NMR] -6.023 ± 4.572; R2 = 0.98, F = 815, P < 0.0001, n = 18)

View larger version (19K): [in a new window]   Fig. 4. Relationships between soil properties, climate variables (mean annual precipitation [mm] and temperature [°C]) and organic P determined by bicarbonate extraction and molybdate colorimetry, and by NaOH–EDTA extraction and solution 31P NMR spectroscopy (mg P kg-1 soil). Outlying soils not included in the calculation of correlation coefficients are identified as shaded circles.