Modification of the Mehlich lime buffer test

Routine use of the Shoemaker, McLean, and Pratt (SMP) lime buffer method resulted in chronic problems with electrode reference junction degradation, stability of readings relative to contact time, and generation of hazardous waste. During method recalibration, the Mehlich buffer was chosen for parallel evaluation with SMP, with the goal of improving method performance and eliminating of hazardous waste. The Mehlich buffer was modified by substituting calcium (Ca) for barium (Ba). The modified Mehlich (MM) buffer was found to be identical to the original with respect to buffering power and linearity over an extended pH range of 3.0-6.6. Seven agronomic soils were incubated with eight rates of calcium carbonate for 90 days. Regression analysis was performed to predict lime requirement (LR) to several target pH levels, based on pH measured in each buffer alone or in both water and buffer. Slightly better predictability was obtained using multiple regressions, with R(2)>0.95 in all cases. Significant but minor differences occurred between the newly calibrated buffers in extended comparisons. The MM buffer was superior to the SMP during routine usage, with fewer adverse effects on electrodes and the elimination of hazardous waste.