Prior to 2000, the University of Maine Soil Testing lab used the SMP lime buffer method for routine testing. Chronic problems occurred with reference junction degradation and stability of readings relative to contact time. The SMP buffer also generated large volume of hazardous waste.

In 2000, a recalibration of the lime requirement test was undertaken. Alternative lime buffer methods were explored for compatibility with Maine soils and for their content of hazardous reagents. The Mehlich buffer was chosen for parallel evaluation with SMP during recalibration.

The Mehlich buffer has a greater buffering capacity than the SMP, with a linear pH response over a wider range of acidity. The original Mehlich was modified by substituting Ca for Ba (the only hazardous constituent), duplicating its function as a divalent exchanger. The Modified Mehlich buffer is identical to the original with respect to buffering power and linearity over an extended range of pH (6.6-3.0).

Seven agronomic soils were chosen for calcium carbonate incubations. At the end of 90-day incubations, all study soils were measured for pH in DI water (1+1), SMP buffer (1+1+2,soil+water+buffer), and Modified Mehlich buffer (1+1+1). Regression analysis was performed to predict LR to several discreet target pHs, based on pH measured in buffer alone or in both water and buffer. Slightly better predictability was obtained using multiple regressions for both buffer methods, with r-square values > 0.95 in all cases. In side-by-side evaluations on 900 routine samples, there were no significant differences between the newly calibrated buffers in predicting LR.

Due to its greater buffering capacity, improved stability of readings, improved electrode longevity, and the ability to use the normal waste stream, the Modified Mehlich buffer was chosen as our routine lime buffer test in 2001.