Comparison of Shoemaker-McLean-Pratt and modified Mehlich buffer tests for lime requirement on Pennsylvania soils

The Shoemaker-McLean-Pratt (SMP) buffer test is commonly used in Pennsylvania and throughout the United States to determine the lime requirement (LR) of acid soils. The buffer contains potassium chromate, a carcinogen, and all waste must be collected for disposal in a hazardous waste facility. An alternative to the SMP buffer is the Mehlich buffer. Although the Mehlich buffer contains barium chloride (BaCl2), also a hazardous and regulated compound, calcium chloride (CaCl2) has been shown to be an effective substitute. The goal of this study was to compare the SMP buffer and the modified Mehlich buffer (CaCl2 substituted for BaCl2) for estimating LR on PA soils and to determine if the modified Mehlich buffer could provide an effective alternative to the SMP test. Twenty-two agriculturally important Pennsylvania soils with pH values ranging from 4.5 to 6.4 were collected, and the actual LR of each soil was determined by incubating soils for 3 months with calcium carbonate. The modified Mehlich buffer was a more accurate predictor of the lime required to raise soils to either pH 6.5 (r2=0.92) or 7.0 (r2=0.87) in comparison to the SMP buffer (r2=0.87 and 0.82, respectively). Comparison of calibration equations for Mehlich buffer versus lime requirement derived in this study were similar to those developed on soils from other states and geographic regions.