Using the Mehlich‐1 Extract to Estimate Soil Phosphorus Saturation for Environmental Risk Assessment

Abstract

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 (Pox), Al (Alox), and Fe (Feox), 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 (r2 = 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 (Pox) (Alox + Feox)−1

Degree of phosphorus saturation threshold for minimizing P losses by runoff in cropland soils of Southern Brazil

“…This further indicates that Mehlich-1 P values can be efficient for calculating the DPS. Similar results were found by Beck et al (2004), using Mehlich-1 P values as an agri-environmental soil indicator of risk in acidic soils of Virginia. However, the authors stressed that the variability among soils can make its implementation difficult with routine soil tests, because a regression model would be needed for each physiographical region of the state.…”

“…Therefore STP has been used to estimate potential P losses or been incorporated as a PI factor because it has shown high correlations with dissolved reactive P in runoff waters (Wang et al, 2009). The constraints of using only this index, however, are due to the fact that it does not account for soil-specific P reactions and for other relevant soil properties, such as soil sorption capacity, indicated by the PSI (Beck et al, 2004). Therefore, the DPS has been recognized as an efficient tool for estimating the potential P released from soil prone to be transported by surface runoff (Beck et al, 2004;Nair et al, 2004).…”

“…The constraints of using only this index, however, are due to the fact that it does not account for soil-specific P reactions and for other relevant soil properties, such as soil sorption capacity, indicated by the PSI (Beck et al, 2004). Therefore, the DPS has been recognized as an efficient tool for estimating the potential P released from soil prone to be transported by surface runoff (Beck et al, 2004;Nair et al, 2004). The first approach using the DPS was proposed by Breeuwsma et al (1995), in which the index was defined as the quotient between P extracted with ammonium oxalate (P OX ) and P sorption capacity (PSC), showing that it was an efficient tool to predict P losses from acid soils.…”

Application of Isothermal Calorimetry to Phosphorus Sorption onto Soils in a Flow-through System

“…The authors claimed that the a value is empirical and needs to be determined for each soil type and experimental conditions. In addition, Beck et al (2004) recommended that the a value be omitted from the DPS calculation.…”

Binding of inorganic sulphate and phosphate in humid-climate soils measured using column leaching, equilibration and extraction methods

“…Further, the Langmuir equation calculation method of determining P saturation was not found to be reliable for many of the soil samples examined in this study. A single point equilibration measurement of a sample with a relatively low P treatment concentration (e.g., 15 to 20 mg P kg -1 ) has been proposed for determining maximum P binding of soils (Bache and Williams 1971;Beck et al 2004) . However, the Langmuir equation functioned well for all the soil samples they studied.…”