Using the Mehlich-1 extract to estimate soil phosphorus saturation for environmental risk assessment

Methods for environmental risk assessment of P loss potential from soils lack uniformity and are generally difficult for routine analysis. Mehlich-1 extractable P (M1-P), an approach that is widely used to assess soil P status for plant growth, was used as a soil test P (STP) estimator of the degree of P saturation (DPS) of a soil. The concept of DPS integrates the dominant properties controlling the P sorption-desorption status of soils. Soil samples from three physiographic regions of Virginia were analyzed for M1-P and a wide range of other extractable P forms and selected chemical and physical soil properties. The DPS determined by ammonium oxalate (NH4-Ox) extractable P (P-ox), Al (Al-ox), and Fe (Fe-ox), ranged from 2 to 155%. Mehlich-1 P, with a range of 1 to 1100 mg kg(-1) was the most suitable single variable for estimating DPS. However, soil type and properties from the three physiographic regions were sufficiently different that regression models to estimate DPS based on M1-P were significantly (P < 0.001) different between regions. Addition of other chemical or physical soil properties yielded insufficient improvements to the regression models over the strong relationships (r(2) = 0.93, 0.98, and 0.75 for the Ridge & Valley, Piedmont, and Coastal Plain regions, respectively) between M1-P and DPS. Interpretations/comparisons between studies are often limited by the numerous methods that are used to calculate DPS. We recommend DPS be determined as mmol kg(-1) of NH4-Ox extractable P, Al and Fe and calculated as 100 (P-ox) (Al-ox + Fe-ox)(-1).