Deriving Cornell Morgan equivalents from Mehlich-3, and Morgan and Modified Morgan Soil Tests from other Laboratories

Phosphorus Soil Testing and Nutrient Management Planning in New York. "What's Cropping Up?" 12 (4): 1-3.

Conversion Equations Part 1: Do Modified Morgan and Mehlich III P Have a Morgan Equivalent? "What's Cropping Up?" 11(3): 2-3.

Conversion Equations Part 2: Do Mehlich-III K, Ca and Mg Have Morgan Equivalents? "What's Cropping Up?" 11(4): 4-5

Ketterings, Q.M., K.J. Czymmek, W.S. Reid and R.F. Wildman(2002). Conversion of modified Morgan and Mehlich-III soil tests to Morgan soil test values. Soil Science 167(12): 830-837.

NOTE: Spectrum Anatytic now (as of July 2005) has two reporting systems in place. Cornell conversion equations are based on Spectrum's original reporting system (prior to July 2005) that converted Mehlich-3 analyses back to Bray equivalents and ammoniumacetate equivalents (for K, Mg etc.) and reported results in lbs/acre. The new system no longer converts data (i.e. now reports straightforward Mehlich-3 data) and reports those data in ppm. To use the Cornell conversion equations, if the report has data in lbs/acre, you can plug the data directly into the Cornell conversion tools. If you have data in ppm, those need to be convert back to Bray-1 and ammoniumacetate equivalents first, and those equivalent values should then be used in the Cornell conversion equations.

The conversions from Mehlich-3 data in ppm back to their original reporting system are:

(M3-P in ppm * 0.70) x 2 = Bray P1 in lbs/acre
(M3-K in ppm * 0.84) x 2 = Ammoniumacetate K in lbs/acre
(M3-Ca in ppm * 0.75) x 2 = Ammoniumacetate Ca in lbs/acre
(M3-Mg in ppm * 0.88) x 2 = Ammoniumacetate Mg in lbs/acre

Cornell University publishes the "Cornell Guide" which includes recommendations for N, P, K, Ca, and Mg and micronutrients for a large number of field crops in New York. The recommendations are based on decades of field research in NY showing soil nutrients extracted using the Morgan solution are correlated well with nutrient response for the vast array of soil types in NY. Several private soil-testing laboratories that serve NY producers use the Mehlich-III and/or modified Morgan extraction solution. In the past, Cornell’s fertilizer recommendation software did not allow for the use of extractants other than Morgan's solution because prior research conducted in NY demonstrated a poor relationship between Morgan and Mehlich-III extractable P. However, comparisons within similar soil types, pH and textural class or Al content suggested it might be possible to derive better conversion equations (models) by including specific soil (chemical) characteristics in the equations. In 1999, Cornell University faculty and staff, agribusiness and state and federal agencies joined resources in a statewide study aimed at deriving such conversion equations for NY soils. Personnel from Agway Inc., Agricultural Consulting Services Inc., ConsulAgr Inc., Cooks Consulting Services and the Miner Institute collected 232 soil samples (0-6 or 8 inches) in NY. These samples represented 27 soil types and eight major agricultural soil groups from across NY. The soil samples were initially analyzed at Cornell’s Nutrient Analysis Laboratory, A&L Laboratories Inc., Brookside Laboratories Inc., and Spectrum Analytic Laboratories. A&L analyzed the samples for Modified Morgan and Mehlich-III P. Brookside determined Mehlich-III P, K, Mg, Ca and Al while Spectrum generated pH and Mehlich-III P, K, Ca, Mg and Al data. At Cornell, soils were analyzed for pH and Morgan extractable P, K, Ca, Mg, and Al. In 2002/2003 the soil test laboratories of New Hampshire (Mehlich-III), the University of Massachusetts (Morgan), and the University of Vermont and University of Maine (Modified Morgan) joined the project. A&L Canada Laboratories (Mehlich-III) joined in 2003 and in December 2005, the University of Connecticut (modified Morgan) and the DairyOne laboratory (Mehlich-III) were added.

Regression analyses between modified Morgan or Morgan and Mehlich-III extractable P showed results similar to those reported by Klausner and Reid (1996); a very poor correlation (identified by a low r² value) existed when analyses were compared across all soil types and chemical characteristics. However, including pH, Mehlich-III extractable Al, and Ca in the analysis resulted in greatly improved predictions. Because of differences in reported values between the different commercial laboratories, conversion equations were developed for each of the laboratories. The Conversion Spreadsheet has all conversion equations summarized in an Excel-based conversion tool for New York State.

The P, K, Ca and Mg conversion models derived for the different laboratories are programmed into Cropware (Cornell’s nutrient management software) and used to determine the NY P index for fields that have soil test data for laboratories that joined the NY conversion study. Separate studies are needed to address conversions for other extractants or laboratories other than those mentioned above. Separate studies are also needed if laboratory procedures are changed. When using conversion equations, make sure to use the equation developed specifically for your laboratory. The use of the wrong equation will lead to inaccurate predictions.

To properly conduct the conversion, in addition to Mehlich-III soil phosphorus, required tests include: pH, Mehlich-III calcium and aluminum; with aluminum being especially important. Unfortunately, in the past, the demand for Al analysis was such that it was not part of the basic soil test package offered by two of laboratories identified above. Many planners find they need to develop a CNMP with Mehlich-III soil test data that are less than three years old and do not include aluminum. This presents a challenge when a conversion of reasonable accuracy requires aluminum, yet the planner is unable to collect and analyze new soil samples right away.

If a CNMP planner chooses to use a soil test other than the Morgan soil test (at this time, Morgan provides lowest risk of an inaccurate recommendation because New York calibration studies are based on this soil test), all Mehlich-III derived soil test data should include a test for aluminum. This is for the planner's protection: using the actual aluminum level significantly improves accuracy and therefore reduces the planner's risk of providing an incorrect recommendation. As plans are revised with new soil tests, planners will be expected to add aluminum to the Mehlich-III analysis for use in the conversion process. Again, use of the Mehlich-III to Morgan conversion or any modifications of the conversion process discussed here or in other forums must be born completely by the user.

If a planner has soil tests less than three years old without an aluminum analysis, AND if (s)he is willing to fully bear the risk of making an inaccurate recommendation, (s)he may elect to enter an estimated Al value into Cropware or the conversion spreadsheet. In a database of 232 soil samples, Mehlich-III aluminum values ranged from 380 ppm to 1576 ppm. In order to get a sense for the risk involved in using an estimated Al value, try plugging soil test P and pH values into the conversion spreadsheet using 380 ppm aluminum and again using 1576 ppm aluminum. Using an estimated Al value that is higher than the actual value will produce a Morgan equivalent that is lower than reality. This may induce a higher P2O5 recommendation, reducing agronomic risk while potentially increasing environmental risk. Estimating a Mehlich-III Al value that is lower than the actual field value will result in higher Morgan equivalents possibly leading to P recommendations that are too low. This is also what happens if the user forgets to enter a value for Al. The agronomic risk of performing this procedure can be somewhat reduced if fields are to receive moderate rates of manure. To reduce agronomic risk on fields that are not receiving any manure, be sure to review past fertilizer practices to inform current decisions. The average Al soil test in the Cornell dataset was 780 ppm, and about 65% of the samples were between 550 and 850 ppm while about 83% of the samples were between 450 and 950 ppm.

Bottom line: unless soil P levels are really low, one cannot make a reasonable conversion without an aluminum entry in the Cropware or the conversion spreadsheet. For the sake of expediency, yet with the potential for increased inaccuracy, a planner may choose to estimate aluminum (use e.g. the average value from the Cornell database: 780 ppm) and enter that figure for Mehlich-III soil tests that are presently less than three years old and that do not have aluminum test results. Both NRCS and Cornell consider soil samples greater than three years old to be stale and are therefore not appropriate for use in a CNMP. Be sure to add aluminum to any future Mehlich-III soil test analyses that will be converted to a Morgan equivalent.