Revisiting Double Buffer Methods for Lime Requirement Determination.

Double-buffer methods were developed by Yuan in 1974 and McLean in 1978.  Main advantages of double-buffer methods are pH buffer capacity being determined for each individual soil tested and improved accuracy of lime recommendations.  Double buffer methods rely on pH measurements of soil:solution slurries at varying ionic strength and soil:solution ratios.  In addition, the measurements are calibrated or compared to pH measurement of alkali treated soil at varying ionic strength and soil:solution ratios.  Since ionic strength and soil:solution ratios have a demonstrated affect on soil pH measurement, varying these parameters in the double-buffer methods can cause confusion in data interpretation.  Six soils from the NAPT program were measured for pH with a Sikora double-buffer method and KOH titrations with constant ionic strength and soil:solution ratios.  Compared to the Yuan and McLean double-buffer methods, the Sikora double-buffer method produced less error in measurements of initial soil pH without alkali addition and acidity measured at pH 6.5 with KOH titration.  The double-buffer concept holds promise for measuring soil acidity, but the ionic strength and soil:solution ratios should be held constant, or nearly constant, with all pH measurements.