Measuring agronomic and environmental soil phosphorus saturation and predicting phosphorus leaching with Mehlich 3

The role that soil testing can play in identifying agricultural soils with an increased potential for P loss is an important topic. Our research compared the Mehlich 3 P saturation ratio (M3-PSR) with the ammonium oxalate degree of P saturation (DPSox), and the M3-PSR was then evaluated for predicting agronomic and environmental soil P saturation thresholds. Intact soil columns (15-cm diam, 20 cm deep) and soil samples were collected from five soil series that ranged in soil texture, chemical properties, and Mehlich 3 P. The soils were analyzed for pH, organic matter (OM) and oxalate and Mehlich 3 extractable P, Al, and Fe. Each intact column was leached with the equivalent of 5 mm of rainfall and resulting leachate analyzed for P. Mehlich 3 extractable Al, Fe, and P were closely related to oxalate extractable Al, Fe, and P, although Mehlich 3 extracted only a small amount of Fe compared with oxalate. The M3-PSRs, calculated as the molar ratios of Mehlich 3 extractable P/[Al + Fe] (ratio I) and P/Al (ratio II), were well correlated to each other and to DPSox. All three P saturation measurements showed a threshold or change point above which the concentration of P in column leachate increased rapidly. Both the agronomic optimum M3-PSRs and the environmental limit suggested in the Netherlands for DPSox (25%) were below the observed change point. The M3-PSR measured in a single Mehlich 3 extraction shows excellent promise for identifying soils that represent an increased risk for P leaching losses.