Iowa State University (ISU) has provided soil-test interpretations for the Bray-1, Olsen, and Mehlich-3 (M3) soil phosphorus (P) tests since 1996. These tests use a colorimetric analysis method to measure the P extracted from the soil. This method is based on the intensity of a blue color that develops after adding appropriate chemicals to the soil extract. The M3 and Bray-1 P tests measure approximately the same amount of P in acid or neutral soils. However, the Bray-1 test measures less P than it should from some high-pH (calcareous) soils and is not recommended for soils with soil pH 7.4 or higher.
Iowa State University
Mallarino, Antonio P., "Interpreting results of the Mehlich-3 ICP Soil Phosphorus Test" (2003). Integrated Crop Management News. 1667.
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