How do I convert phosphorus extraction results?

Labs use different phosphorus extraction methods that generate varying results. Recommendation calculators specify the extraction methods used in calculating their nutrient recommendations.

If soil samples were analyzed using a phosphorus extraction method and the desired calculator uses different extraction methods, a conversion is necessary. This article describes how to convert the results from one phosphorus extraction method to another.

Phosphorus extraction methods used by state fertility calculators

Process

• All calculators use Bray-P1 as an available extraction method.
• If Soil Test Extraction Method is not Bray-P1, determine if another method is available that matches (Olsen, Mehlich 3-COL, Mehlich 3-ICP)
• If no other method is found for the calculator, convert the soil test value to Bray-P1 using the calculations in the table.

Sources

Mallarino, A. (2003). Field Calibration for Corn on the Mehlich-3 Soil Phosphorus Test with Colorimetric and Inductively Coupled Plasma Emission Spectroscopy Determination Methods. Soil Sci. Soc. Am. J 68-1928-1934.

Mallarino, A. (1995). Comparison of Mehlich-3, Olsen, and Bray-P1 Procedures for Phosphorus of Calcareous Soils. http://www.agronext.iastate.edu/soilfertility/info/ComparisonofMehlich-3OlsenandBray-P1Procedures.pdf