Page 1 of 12 Tim Mundorf Fall 2016 Introduction In 2014, clients in Iowa and Canada asked Midwest Laboratories to compare Mehlich III extraction of nutrients analyzed by ICP to Midwest Laboratories traditional methods which include P1 & P2 Bray extraction of phosphorus (P) with colorimetric measurement; neutral ammonium acetate (AA) extraction of potassium (K), magnesium (Mg), and calcium (Ca) with ICP measurement; and DTPA extraction of the micronutrients boron (B), copper (Cu), Iron (Fe), manganese (Mn), and Zinc (Zn) with ICP measurement 1. This study consisted of 139 samples: eighty-three from Canada and 56 from Iowa. In 2015, Midwest Laboratories conducted a study across the states of IA, IL, WI, MN, NE, KS, MO, and CO looking at the Haney Soil Health Test. In conjunction with this study we again compared our traditional methods to the Mehlich III (M3) ICP analysis of P, K, Ca, Mg, B, Cu, Fe, Mn, and Zn. This study consisted of 145 samples. In late 2015 and throughout the 2016 growing season we again compared our traditional methods to the Mehlich III ICP analysis of these same nutrients. This study consisted of 380 samples from Nebraska and Western Iowa. This study was done in conjunction with a UNL graduate project looking at the correlation of M3 micronutrient levels to those of DTPA, especially for Zn. In total, we had 664 soil samples representing a wide geography. We compared M3 ICP measurement of P to P1 & P2 Bray colorimetric methods. We compared M3 ICP measurement of K, Mg, and Ca to AA ICP measurement of the same cations. Finally, we compared M3 ICP measurement of B, Cu, Fe, Mn, and Zn with DTPA extraction and ICP measurement of these micronutrients. In each case, we looked at the difference in average values and the correlation between the methods. Mehlich III extract solution as a universal extract solution. Starting in the 1980s and accelerating in the 1990s the use of the Mehlich III extract solution as a universal extract to assess soil fertility levels of phosphorus, potassium, magnesium, calcium, and micronutrients received large amounts of attention. If a laboratory could replace multiple steps of extraction and measurement for individual or groups of nutrients with one extraction followed by one measurement significant cost savings could be realized. The Mehlich III extract seemed to hold this promise. The Mehlich I extract had been developed in the Southeast United States and worked well on acid soils with low cation exchange capacity. In the 1990s the Mehlich III extract largely replaced the Mehlich I extract and seemed to work well on calcareous as well as acid soils and correlated well with the traditional P tests such as Bray/Kurtz and Olson as well as the traditional neutral ammonium acetate extraction used in the analysis of K, Mg, and Ca. 1 Midwest Laboratories adds Sorbitol to the DTPA extract solution to capture boron
Page 2 of 12 In the 1990s, several studies were carried out to evaluate the correlation of the M3 extraction for micronutrients such as B, Cu, Fe, Mn, and Zn with the DTPA extraction. Measurement of these extractions was done with either Atomic Absorption or Inductively Coupled Plasma (ICP) instruments. These studies showed fairly strong correlation of M3 ICP analysis of Zn to DTPA ICP analysis. These correlations were less strong for Fe and Cu and rather weak for Mn and B. Currently, Midwest Laboratories uses four extractions and one dry scoop for our base analysis of a soil sample. These include 1:1 soil/water for ph, P1 and P2 bray extractions measured calorimetrically for phosphorus, ammonium acetate with ICP measurement of cations, and loss on ignition using combustion ovens for organic matter measurement. An additional test for micronutrients B, Cu, Fe, Mn, and Zn can be requested using DTPA extraction and ICP measurement. To dramatically reduce costs, a laboratory could use the Mehlich III extract as a universal extract solution and measure phosphorus, cations, and micronutrients with an ICP instrument. If they do not include OM in the base test, they could take two soil scoops per sample and use only a ph probe and an ICP instrument for analysis. The total investment is the labor for two scoops and two analyses as well as one extract solution, a ph meter and an ICP instrument. For comparison, Midwest Laboratories will take six scoops of soil and use four different extract solutions (P1 & P2 Bray extracts, ammonium acetate, & DTPA with Sorbitol) as well as running two separate extracts (P1 & P2 Bray) through a colorimeter and two separate extracts (AA & DTPA) through an ICP. They will use a similar ph measurement and add the combustion of a sample in an industrial oven to measure OM through loss of weight. The total investment is the labor for six scoops and six analyses, four extract solutions, a colorimetric analyzer, two ICP instruments (twice as many analysis to run), a ph meter, and an industrial combustion oven. Obviously there is a large opportunity to reduce cost if the laboratory views the two methods and philosophies as being equal or nearly so. Accepted Soil Test Methods by State An important consideration when choosing a soil test method to use for assessing a nutrient for possible fertilizer application is the test method used in correlation and calibration for your land grant university. Below is a table showing the methods used by most land grant universities in the Midwest. Many of these universities do not have critical levels established for micronutrients so an NA is given. Most land grant universities have a P1 Bray based critical level. Some also have a M3 based value but these are usually for the colorimetric measurement not the ICP measurement. K and Mg values are normally neutral ammonium acetate based. The extraction basis for micronutrient critical values varies but DTPA is the most common. All critical levels are for corn unless otherwise noted. Ammonium Acetate (AA) Mehlich III (M3). All units are ppm unless otherwise noted.
Page 3 of 12 University of Nebraska Nutrient Management for Agronomic Crops in Nebraska Revised 2014 EC155 Critical Level 20 125 Na 8 0.8 Na Na 4.5 0.25 Soil Test Method Bray 1 AA (H 2 PO 4 ) 2 DTPA DTPA Hot Water Iowa State University A General Guide for Crop Nutrient and Limestone Recommendations in Iowa Revised Oct 2013 PM 1688 Critical Level 20 170 Na Na 0.8 Na Na Na Na Soil Test Method P1 Bray or M3 Color AA or M3 Dry DTPA Critical Level 13 120 Soil Test Method Olson P AA or M3 Field Moist Critical Level 35 Soil Test Method M3 ICP Kansas State University Soil Test Interpretations and Fertilizer Recommendations MF- 2586 Sep 2003 Critical Level 20 130 Na *** 1.0 Na Na Na 1.0 Soil Test Method Sufficiency Bray P1 AA Ca(H 2 PO 4 ) 2 DTPA DTPA DTPA Critical Level 30 160 Soil Test Method Bray P1 AA Build -Maintenance *** Depends on Yield and OM Michigan State University, The Ohio State University, & Purdue University Tri-State Fertilizer Recommendations E-2567 Revised July 1995 Critical Level 20 165* 50 Na 1-12** 1-20 *** 2-24** Na Na Soil Test Method Bray P1 AA HCl HCl * At 20 CEC ** Depending on soil ph *** Only on organic soils University of Missouri Soil test Interpretations and Recommendations handbook Revised May 2004 Critical Level 22 110 100 7.5 1.0 0.2 1.0 4.5 Na Soil Test Method Bray P1 AA AA Not given DTPA DTPA DTPA DTPA
Page 4 of 12 South Dakota State University South Dakota Fertilizer Recommendations Guide Sep 2005 Critical Level 15 160 30 29 0.75 0.2 1.0 4.5 0.5 Soil Test Method Bray P1 AA AA 500 ppm P DTPA DTPA DTPA DTPA Hot Water Critical Level 11 Soil Test Method Olson North Dakota State University North Dakota Fertilizer Recommendation Tables and Equations Critical Level 11 130 Na Na 0.75 0.3 Na Na Na Soil Test Method Olson AA DTPA DTPA DTPA DTPA Hot Water University of Minnesota Fertilizer Guidelines for Agronomic Crops in Minnesota Critical Level 15 120 100 Na 0.75 Na Na Na Na Soil Test Method Bray P1 Not Given Not Given DTPA Critical Level 11 Soil Test Method Olson University of Illinois Illinois Agronomy Handbook Critical Level 20* 200* 100* 11* 4.5* Na 1* 2* 0.5* Soil Test Method Bray P1 or M3 AA or M3 Not Given 0.1 HCl DTPA * Converted from lbs/acre 0.5* 5* DTPA H 3 PO 4 University of Wisconsin Nutrient Application Guidelines for Field, Vegetable, and Fruit Crops in Wisconsin A2809 2012 Critical Level 20 130 50 Na 3.0 Na 10 Na 0.9 Soil Test Method P1 Bray Bray 1 AA 0.1 HCl 0.1 HCl Hot Water DTPA Hot Water
Page 5 of 12 Summary of soil test methods defined in university recommendations. Here is a summary of the number of listed Land Grant University with guidelines based on each method for each nutrient: Phosphorus: P1 Bray: 10 Olson: 4 Mehlich 3 colorimetric: 2 Mehlich 3 ICP: 1 Potassium: Ammonium Acetate: 8 Mehlich III: 2 Wet Method AA or M3: 1 Bray 1: 1 Magnesium Ammonium Acetate: 3 Zinc: DTPA: 8 HCl: 3 Copper DTPA: 3 HCl: 1 Manganese DTPA: 4 HCl: 2 H 3 PO 4 : 1 Iron DTPA: 6 Boron Hot Water: 5 DTPA: 1 P1 Bray is the most common method indicated for phosphorus. Ammonium Acetate is the most common method indicated for potassium and magnesium. DTPA is the most common method indicated for the micronutrients zinc, copper, manganese, and iron. Hot water is the most common method indicated for boron.
Soil Test Data Characteristics Page 6 of 12 The ph of the soil samples included in this study ranged from 5.0 to 8.5. 127 of the 664 (19%) samples could be considered calcareous with ph values of 7.5 to 8.5. 182 (27%) of the samples could be considered strongly acidic with ph values from 5.0 to 5.9. The average ph was 6.6. Phosphorus Across the over 650 samples in our study the average P1 Bray colormetric value was 32 ppm. The average M3 ICP value was 56 ppm. The average M3 ICP value was approximately 1.8 times the P1 Bray value. The r-squared was 0.701. 2 (Figure 1) If the 127 high ph (7.5 & above) soil samples are removed from the data set the values do change. The average P1 Bray value increases to 34 while the average M3 ICP value drops to 55. Now the average M3 ICP value is approximately 1.6 times that of the P1 Bray. The r squared is 0.873. Since Midwest Laboratories adds an Olson bicarbonate extraction when the soil ph increases to 7.5 and above, the comparison without high ph soils is probably more accurate. The correlation is fairly strong but the difference in values is quite large. M3 ICP values should not be used in recommendations based on the P1 Bray test. Using a multiplier to adjust the values will bring them closer together but will bring in additional uncertainty because the r squared while strong, is still significantly less than 1. Potassium and other Cations Often the M3 ICP cation values are considered to be nearly equal to the neutral ammonium acetate ICP values. In our studies we observed significant variation. The K values by AA extraction and ICP analysis averaged 267 ppm while the M3 extraction with ICP analysis averaged 316 ppm. The M3 values averaged 119% of the AA values for K. The correlation was actually a little worse than the phosphorus numbers with an r squared value of 0.784. This variation and lack of correlation makes us unwilling to call the M3 method of K extraction equal to the neutral ammonium acetate method. (Figure 2) Magnesium average 446 ppm with AA extraction while averaging 548 with M3. This result meant the M3 K value was 1.2 times higher than the AA value on average. The r squared between AA Mg and M3 Mg was 0.857. (Data not shown) Calcium averaged 2634 ppm using ammonium acetate extraction and 3612 using M3 extraction. The M3 extraction average was approximately 37% higher than the AA extraction average value. The r squared was very poor at only 0.156. (Data not shown) Another challenge for Midwest Laboratories is our standard practice of calculating CEC and base saturation of cations based off the cation values measured by neutral ammonium acetate 2 r 2 is the coefficient of determination. It shows the amount of variation in a dependent variable that is explained by variation in the independent variable.
Page 7 of 12 extraction. The variation between M3 and AA extraction values for cation analysis would lower our confidence in calculated CEC and base saturation. Micronutrients Zinc showed a fairly strong correlation between M3 and DTPA extractions with an r-squared value of 0.904 but there was significant difference in the actual values. DTPA extraction values averaged 2.1 ppm across all samples while M3 values average 4.0 ppm. If a multiplier were used to convert values between DTPA and M3 zinc a factor of 1.9 would be appropriate. If unadjusted M3 Zn values are used in DTPA based zinc fertilizer recommendations very little Zn will be applied. An adjustment to the M3 value should be made in this case. (Figure 3) Iron showed a rather low correlation between M3 and DTPA extractions. The R2 was 0.418. Actual values were very different. M3 extractions averaged 151 ppm while DTPA extractions averaged 49 ppm. An adjustment factor is probably not appropriate give the poor correlation so none will be given. (Figure 4) Copper showed a similar correlation value between M3 and DTPA as iron. The r-squared was 0.498. The actual values were off by a factor of 2. Mehlich III extraction averaged 2.4 ppm copper while DTPA averaged 1.2. Adjustments between the two again seem inappropriate given the low correlation. (Figure 5) Manganese showed extremely low correlation between M3 and DTPA extractions. The R2 was 0.054. The values were nearly unrelated and the range of values was completely different. On average the M3 extraction of Mn give readings of 81 ppm while the same samples analyzed using DTPA extraction had an average concentration of 9 ppm. No factor for adjustment between values give by these two methods would be appropriate. (Figure 6) Boron is a little different. The extract solution we use is DTPA with Sorbitol added to extract the boron. The correlation between this extraction and the M3 extract had an r-squared value of 0.512. Average values were 0.6 for DTPA plus Sorbitol and 0.8 for Mehlich III. (Figure 7) Other Research Servi-Tech Laboratories did a similar study focused on micronutrients in the early 1990s using 471 samples. In their study r-squared values for 0.922 for zinc, 0.855 for iron, 0.563 for copper, and 0.123 for manganese were reported. These values seem to support Midwest Laboratories conclusions with the exception of iron where Servi-Tech showed higher correlation values. They concluded that a multiplier of 1.5 to 2 depending on soil ph would be appropriate for comparing DTPA based zinc concentration to that of Mehlich III. Servi-Tech did not compare boron values between DTPA and M3 but DTPA without Sorbitol would be inappropriate for boron extraction. North Dakota State University also compared M3 extraction to their standard methods in the late 1990s using 100 samples. Potassium was analyzed using M3 and ammonium acetate extractions with an r-squared of 0.94. Several micronutrients were analyzed. R-squared values for Zn, Fe, Mn, and Cu were 0.96, 0.63, 0.15, and 0.86 respectively.
Page 8 of 12 A more recent study conducted in 2012 and 2013 by Iowa State University resulted in r-squared values of 0.31, 0.03, and 0.95 for Cu, Mn, and Zn respectively. This study again shows a strong correlation for Zn analysis between M3 and DTPA extractions but poor correlations for Mn and Cu. Conclusion A soil test laboratory uses approved methods and most will participate in some proficiencytesting program to prove their ability to produce accurate results with the methods they use. The two most widely used proficiency-testing programs in the United States are the Agricultural Laboratory Proficiency (ALP) testing program and the North American Laboratory Proficiency (NALP) testing program. These programs normally allow variation up to approximately 10 percent 3 from expected values of proficiency samples for a laboratory to receive their accreditation. When a laboratory uses one test method but then takes those results and applies a mathematical adjustment based off an average value comparison between methods additional variation is brought into the process unless the r-squared between those methods is 1. In our findings, most of the comparisons in soil test values found by Mehlich 3 ICP versus P1 Bray colorimetric, Neutral Ammonium Acetate ICP, or DTPA ICP had r-squared values less than 0.90. Even though a laboratory has passed ALP or NALP proficiency tests with their approved method the additional variation brought in by a mathematical adjustment to make the value equivalent to another method (for example P1 Bray) may bring in enough variation that the value now would fall outside the range acceptable for proficiency testing accreditation. In our discussions of laboratory methods so far we have left out an important factor. That factor is the intended use of laboratory soil testing results. There are two schools of thought for the use of soil test analysis and fertilizer application. These are sufficiency and build/maintenance. In the case of sufficiency, the fertilizer applied is the rate that should maximize economic return for the next one to few crop years before another sample is taken and the same decision is made again. In the case of build/maintenance, fertilizer application is made to build soil test values to critical levels and to maintain them in a range to maximize long term yield. In the sufficiency theory, the soil test is more of a snapshot for a fertilizer recommendation based on that year s data. In the build/maintenance theory the trend line of soil test levels and its relationship to fertilizer application vs crop removal is very important. Many of our clients have decades of data based on our traditional methods. This difference in intended use of soil test data is critical in Midwest Laboratories decision to continue with our traditional methods of soil nutrient assessment. Midwest Laboratories makes our in house nutrient recommendations based on build/maintenance theory. Most of our clients do as well. This underlying philosophy compels us to only change soil test methods if absolutely necessary. Using trend lines to assess the impact of fertilizer application and crop removal on soil test levels is an important part of build/maintenance philosophy. If we were to change methods we would impose artificial variation in these trend lines and significantly decrease their value. We use our traditional 3 NALP and ALP deviation is actually calculated by MAD (Median Absolute Deviation) Warning limits are at 2.5 x MAD. This measurement is similar to standard deviation and limits are at approximately 2 times standard deviation. The 10% figure is only an approximation for simplicity of calculation.
Page 9 of 12 methods for our in house recommendations. Most of clients do as well. Changing our base methods would make many of our clients fertilizer recommendation equations invalid. As we have seen in the summaries for each nutrient, the Mehlich III method brings significant variation in both expected nutrient levels and correlation to Midwest Laboratories traditional methods. There is nothing wrong with the method per se. The problem with the method for Midwest Laboratories is the decision to use most of our traditional methods was made when we opened in 1975. We have over 40 years of data and process improvement based on these methods. We believe our clients see significant value in this. The other issue is the preponderance of land grant universities critical levels that are based on the methods we use. We are happy to offer other methods to be used upon client request but our in house recommendations and quality controls are still based on our traditional methods. Figure 1 Figure 2 Potassium
Page 10 of 12 Figure 3 Figure 4
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Page 12 of 12 Figure 7 Sources: Soil micronutrient extraction by Mehlich-3 compared to CaCl 2 -DTPA, F. F. Vocasek & J. B. Friedericks Communications in Soil Science and Plant Analysis, 1994 Soybean Yield Response to Foliar- Applied Micronutrients and Relationships among Soil and Tissue Tests. Joshua T. Enderson, Antonio P.Mallarino, and Mazhar U. Haq, Agronomy Journal 2015 Relationships between the Mehlich-III soil test extraction procedure and standard soil test methods in North Dakota, M.E. Schmisek, L.J. Cihacek, & L. J. Swensen Communications in Soil Science and Plant Analysis 1998 Conversion equations for soil test extractants: Mehlich-1 and Mehlich-3, University of Kentucky Southern Regional Fact Sheet, January 2005