How to Read and Interpret Mehlich 3 Test Results
How Mehlich 3 Came to be

Mehlich 3 (M3) is a widely acknowledged third-generation multi-nutrient extraction procedure for determining soil micro-nutrients and cations. The strong acid soil extraction will also measure the readily available minerals and potentially available mineral reserves in the soil. M3 is a modification from Mehlich 2 (M2) soil test, modified from the Mehlich 1 (M1) soil method. The M1 test was considered a double acid method originating in 1954 for use on soils in North Carolina. M1 was not the golden child it was expected to be–the extractant did not meet the expectations when used on neutral or alkaline soils. M2 was formed when Mehlich combined acetic acid, ammonium chloride, ammonium fluoride, and hydrochloric acid. It was noted that the acetic acid was less digestive to apatite when compared to other mineral digestants due to its high buffering action. Copper (Cu) was added to the list of extractants as the demand for soil testing increased; however, the chloride base of Mehlich 2 was too corrosive. This corrosive behavior of the chloride agent was substituted with nitrates. Subsequent changes increased the normality of the ammonium nitrate and nitric acid components, thereby eliminating the chloride salts without taking away from the effectiveness of the extractant. These changes did not lead to the enhanced extraction of Cu. However, adding EDTA to the extractant increased the level of Cu extracted with no harmful side effects and, like this, adopted renaming the method Mehlich 3. 

Balanced Minerals in the Soil

Soil health relies on the balancing of minerals within the soil. These ideal soil ratios fall on three specific sets of information:

  1. Accurately assess the soil’s available minerals (S, P, Ca, Mg, K, Na, B, Fe, Mn, Cu, and Zn) and soil pH.
  2. Measuring the soil cation exchange capacity (CEC).
  3. Measuring the base cations Ca, Mg, K, and Na held at the negative exchange sites within the soil (also known as the base cation saturation ratio or Albrecht’s BCSR). 

Mehlich 3 is one of many soil test methods used to accurately measure the mineral availability within most soils’ pH < 7. However, Mehlich 3 is not used to assess CEC accurately and exchangeable cations in soils pH > 7 or soils with large limestone particles. 

Cation Exchange Capacity (CEC) is the sum of all negative charges presently available to make any electrostatic or ionic bond with any positively charged ion—normal soil tests do not measure for the soil’s total negative charge. 

The total base cations can be used to estimate the CEC of soils pH<7. pH (potenz Hydrogen) is defined as the ratio and concentration of H+ hydrogen ions to OH- hydroxyl ions in an H2O solution—all the + and – ions are equally balanced at pH 7.0.

Further Reading

Astera, M., & Agricola. (2015). The ideal soil V2.0: A handbook for the new agriculture. Soilminerals.com. 

Bayer. (2022, September 16). Reading and interpreting a soil test. DEKALB Asgrow Deltapine. Retrieved September 20, 2022, from https://www.dekalbasgrowdeltapine.com/en-us/agronomy/reading-interpreting-soil-test.html 

CropNuts. (n.d.). Interpreting your soil test results. CROPNUTS Helpdesk. Retrieved September 20, 2022, from https://cropnuts.helpscoutdocs.com/article/829-interpreting-your-soil-test-results 

Interpreting Soil Test Report – Alabama Cooperative Extension System. (n.d.). Retrieved September 21, 2022, from https://www.aces.edu/wp-content/uploads/2018/08/ANR-2481_InterpretingSoilTestReportsFromCommercialLabs_071018.pdf 

Mehlich, A. (1984). Mehlich 3 soil test extractant: A modification of Mehlich 2 extractant. Communications in Soil Science and Plant Analysis, 15(12), 1409–1416. https://doi.org/10.1080/00103628409367568 

Mehlich-3 values for relative level categories. Mehlich-3 Values for Relative Level Categories (Rutgers NJAES). (n.d.). Retrieved September 20, 2022, from https://njaes.rutgers.edu/soil-testing-lab/relative-levels-of-nutrients.php#:~:text=Values%20for%20soil%20test%20categories,is%20above%20optimum%20(excessive). 

Monrawee, F., Satoshi, N., Hisashi, N., Kazuhiro, N., Momade, M. I., Keiichiro, K., & Tetsuji, O. (2018). Applicability of the Mehlich-3 method for the site-specific soil nutrient management in northern Mozambique. Journal of Soil Science and Environmental Management, 9(9), 138–146. https://doi.org/10.5897/jssem2018.0693 

Understanding soil test reports for phosphorus values. AgBMPs. (n.d.). Retrieved September 20, 2022, from https://agbmps.osu.edu/faq/understanding-soil-test-reports-phosphorus-valuesÂ