Comparison of Bray-1 and Mehlich-3 tests in high phosphorus soils

Soil testing forms the basis for both agronomic recommendations and phosphorus (P) runoff risk assessment. Although several studies point to good linear correlations between Bray-1 and Mehlich-3 P in the agronomic soil test range, it is not known how these tests compare for soils that test very high in P and are of environmental concern. Our objectives were to determine the relationship between Mehlich-3 and Bray-1 P soil tests as affected by 1) P determination method: colorimetric or inductively coupled plasma atomic emission spectroscopy; 2) soil type and pH (pH < 6.8 and pH >= 6.8); and 3) soil test P level. Soils were collected from 43 New York fields ranging from acidic to high pH soils with free CaCO3 (study 1) and 126 fields of a single farm with a long-term manure history (study 2). Bray-1 and Mehlich-3 P were linearly related independent of soil pH and detection method for soils with Mehlich-3(icp) less than 200 mg P kg(-1) soil. For soils higher in P, concentrations in both the Bray-1 and Mehlich-3 extracts exceeded the linear portion of the standard curves of the, colorimetric method (1:9 dilution). Without further dilution of the extract, the colorimetric method failed to identify an increase in environmentally available dissolved inorganic P. States that currently use colorimetric analyses for P runoff risk assessment will need to evaluate its performance on high P sites and ensure that dilution protocols are in place to accurately analyze soils with greater than 200 mg P kg(-1) soil.