Double-Buffer Methods Revisited with Focus on Ionic Strength and Soil/Solution Ratio

Double-buffer (DB) methods for lime requirement determination construct a pH titration curve of soil from two pH measurements, which defines the pH buffer capacity of the soil. The Yuan and SMP DB methods rely on pH measurements that vary in ionic strength and soil/solution ratios. Because ionic strength and soil/solution ratios have an effect on pH measurements, the effect of these parameters on data interpretation was analyzed. For comparison, a Sikora DB method was developed. Soil/solution ratios had little effect and ionic strength had significant effects on pH. Extrapolation of the lime response curves generated by the SMP and Sikora DBs to the y axis resulted in a pH that was less than the soil-water pH because of high ionic strength in the buffers. For the Sikora DB, the y axis pH was similar to 1 mol L-1 KCl soil pH, and DB lime response curves agreed well with KOH titration of the soil in 1 mol L-1 KCl. For determining lime requirement, the Yuan and SMP DB methods extrapolate titration curves derived from two soil-buffer pH values to a desired soil-water pH. This approach is misleading because soil-buffer pH is determined in a high ionic strength >0.2 mol L-1 while soil-water pH is determined at a much lower ionic strength. The target pH should be a lower pH at the ionic strength of the buffer solution that is comparable to the desired soil-water pH. A lime requirement method that holds promise is an initial measure of soil pH in 1 mol L-1 KCl with subsequent measurement of soil-buffer pH using Sikora buffer containing 1 mol L-1 KCl. The method requires only two pH measurements and offers the advantage of defining the pH buffer capacity of each soil tested.