Evaluation of Mehlich‐3 for determination of cation exchange capacity in Kansas soils

Abstract

AbstractSoil testing laboratories across the United States have implemented the Mehlich‐3 (M3) extraction method for phosphorus (P) and other nutrients. Though M3 is known to work well as a multinutrient extractant, it raises concerns for measuring exchangeable cations, particularly in calcareous soils. The objectives of this study were (a) to evaluate M3 as an extractant for base cations for Kansas soils, (b) to identify a range of soil pH where M3 may be a suitable replacement for ammonium acetate (AA), and (c) to examine the relationship between cation exchange capacity (CEC) summation and CEC displacement. A study was conducted evaluating 308 soils collected from across Kansas and extracted using both the M3 and AA soil extraction methods. Cation exchange capacity was estimated using the summation approach from measurements of both the M3 and AA, and the displacement method. Results indicate a strong correlation among M3 and AA methods for K, Mg, and Na (R2 = .98, .96, .97, respectively). However, these relationships were considerably weaker for extractable Ca (R2 = .78), where extractable Ca was higher in the M3 extracts for high pH soils. Estimated CEC was also affected by soil pH for the M3 method, with an estimated critical pH value of 7.3. Extractable Ca and CEC for soils with a pH below 7.3 showed a strong correlation between M3 and AA methods (R2 = .9). Using M3 as a multinutrient extractant can improve turnaround time for sample analysis in soil testing laboratories. However, the use of the M3 procedure in soils with a pH of 7.3 or above will likely result in overestimation of exchangeable Ca and CEC.