Converting between soil test extraction methods
The MLSN guidelines were developed using a database of soil test results from good performing turf. The Mehlich 3 extractant was used to obtain these results.1 That leads to a common question, one that goes something like “I’d like to use MLSN but my soil test was done with a different extractant. Can I still use MLSN? How should I proceed?”
I’ve answered this question in a number of ways. And in this blog post, I share an excellent resource from Ohio State University.
ANR-75: Converting between Mehlich-3, Bray P, and Ammonium Acetate Soil Test Values.
You can read that document to see conversion values for the Ohio and Indiana region for converting Mehlich 3 potassium, calcium, and magnesium to their expected equivalent amount with an ammonium acetate extraction—or vice versa. And there are also conversions provided between Mehlich 3 and the Bray 1 extraction.
Based on my research, I’ve recommended in the MLSN cheat sheet to use a factor of 1.2 to convert ammonium acetate K, Ca, and Mg to expected results with a Mehlich 3 extractant. The OSU document (see Table 2) recommends for their regions’ soils to use factors of 1.14 for K, 1.15 for Ca, and 1.24 for Mg
There are a couple other options besides trying to convert test results from a different extractant to the expected value had the test been done using Mehlich 3. One option, if you have enough data, is to use your regional data from good-performing turf and the common extraction method in your region to find an MLSN value for soils in your region. Another option is to do soil tests with Mehlich 3.
The MLSN database does include Olsen (0.5 M sodium bicarbonate) and Bray 2 data for phosphorus. You can see these values (6 mg/kg for Olsen, 30 mg/kg for Bray 2) in the MLSN cheat sheet or you can analyze the MLSN data for even more details. ↩︎