Most soil labs run an extractable phosphorus test and measure extractable P. There are a number of extractants but the most common today is the universal Mehlich-3, because it can be used to extract phosphate and the other bases together and then measure simultaneously either colorimetrically or with an ICP (Inductively coupled plasma) atomic emission spectroscopy. Most labs offer all extractant methods, but if you have a preference make sure you ask for it.

Information below is reprinted with permission from SciMax Newsletter (SciMax.com) and Dr. Rick Vanden Heuvel with VH Consulting, Inc. Vanden Heuvel, Ph.D., received his Ph.D. at the University of Illinois.

Question: The Bray P1 and Olsen P (phosphorus) tests are good P tests, and the Bray P2 test is not really needed. So why was the Mehlich-3 P test developed, what is it, and why is it offered by so many laboratories?

Answer: The Mehlich-3 P soil test was developed in the 1980s (the Bray and Olsen tests are from the 1940s and 50s). The extractant for the test is acid based, like the Bray P1, but consists instead of acetic and nitric acids, along with three other components. Historically speaking, one of the goals of soil testing is to provide fast and inexpensive tests. To improve efficiencies in the lab, the testing industry had a strong interest in trying to develop a “universal” extractant, one that can be used for a wider range of nutrients. Usually, each nutrient has an extractant with its own unique chemical composition. Using a universal extractant, the lab must only prepare one soil sample and extract it once. Obviously, this increases the speed of operations, keeping lab costs down, and offering savings to customers. The goal was to have one extractant, as an example, to be used for macronutrients, like K and P, as well as the micros Zn, Cu, Mn, Fe, etc., (normally requiring at least three different extractants and extracting processes).

The effort to find a universal extractant met with measurable success, but it did not satisfy everyone across the Corn Belt. The Mehlich-3 does a very good job of extracting P and K, but it did not provide good enough results for Zn, the most tested micronutrient in the Corn Belt. Usually DTPA is used to extract Zn. Since Zn is so important for corn production, the Mehlich-3 did not satisfy those charged with certifying soil testing procedures. The Mehlich-3 is not recommended for Zn in Iowa by Iowa State University or the NCR 13 Soil Testing and Analysis Committee (a multi-state group). It can be used for P and K in Iowa.

So, while the Mehlich-3 works with P and K (usually requiring two extractants) it did not meet the goal of a truly universal extractant. The old extractants for P and K remain widely used today and are still offered by many laboratories, including Mehlich-3.

Question: Since the Mehlich-3 can be used for P and K, are the test categories for Low, Medium, High, etc., the same as the old test extractants?

Answer: Yes, they are the same for K (ammonium acetate is the normal extractant). They are also the same for Bray P1 when the Mehlich-3 P test is run colorimetrically (color formation, the standard approach). When the Mehlich-3 for P is run with a non-color forming process (called ICP, or Inductively Coupled Plasma), the category interpretations change. The category levels, or breaks, increase with the ICP method. If this method is used, it should be noted on your soil test reports. Both colorimetric and ICP approaches can be used but should be noted in the report.

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.