Evaluating the Bray P1 Test on Alkaline, Calcareous Soils

Soil test phosphorus (STP) measurements are important for predicting crop P needs and for P loss risk assessments. Previous work indicates that the Bray P1 method extracts less P and does not correlate as well with Mehlich 3 or Olsen STP methods in alkaline or highly calcareous soils. This research was conducted to determine if the Bray P1 test is an appropriate method for the eastern red soil (ERS) region of Wisconsin, where pH typically ranges from 7 to 8 and soils are often calcareous. Soil samples (n = 113) from the ERS region and comparison soils (CS) from Iowa and Kansas (n = 38) with high pH and carbonate content were analyzed for P using the Bray P1, Mehlich 3, and Olsen methods and for soil pH and carbonate content. Results indicate that Bray P1 is strongly correlated with Olsen and Mehlich 3 for all samples in ERS regardless of carbonate content (R2 = 0.83 and 0.98, respectively), but only weakly correlated to Olsen and Mehlich 3 for Iowa and Kansas soils with carbonate contents >or= 5 g kg-1 (R2 = 0.01 and 0.08, respectively). Further investigation indicated that carbonate in CS is calcitic while in ERS it is dolomitic. Because dolomite reacts much more slowly than calcite, Bray P1 is not neutralized in ERS during the 5 min extraction time. For soil with unknown carbonate content, a pH measurement on the Bray P1 filtrate can identify samples where Bray P1 is neutralized and an alternative P extractant should be used.