Proficiency and Certification of Soil Testing Laboratories in Wisconsin.

The Wisconsin Department of Agriculture, Trade, and Consumer Protection (DATCP) requires all Wisconsin nutrient management plans to be constructed with soil testing results obtained at a DATCP certified laboratory. Tests certified by DATCP are soil pH, Sikora Buffer pH, Bray-1 phosphorus and potassium, and loss on ignition organic matter.  Commercial laboratory proficiency and certification is performed by the University of Wisconsin Soil and Forage Analysis Laboratory (SFAL).  Monthly proficiency is checked with commercial laboratories sending five samples they have performed analysis on to SFAL. Bi-annual proficiency is assessed In February and August, with SFAL sending five soil samples to all commercial laboratories to be reported with one replication.  Historically, monthly samples were judge by side to side comparison. Bi-annual samples were assed using standard deviation of individual sample analysis reported by certified laboratories. Certification is achieved by having at least 66% of the check samples be within one standard deviation of the mean for all analyses.  Currently, all participating soil laboratories meet certification.  A proposed change to the proficiency and certification method is to require laboratories to analyze bi-annual samples in triplicate and having sample results analyzed using median absolute deviation.