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

摘要

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 gtoreq 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-3icp 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.