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)