Kinetics of Sikora Buffer Reacting with Soil Acidity.

Buffer methods developed for assessing lime requirement use a short reaction time of buffer with soil due to the large volume of samples tested.  The current SMP buffer method calls for mixing soil and buffer for 10 minutes and letting set for 30 minutes, or mixing for 15 minutes and letting set for 15 minutes.  There is a report that SMP buffer will continue to react with soil acidity beyond 30 minutes and a more accurate determination of soil acidity is obtained after a 24 h reaction.  A study was conducted to assess the kinetics of reaction of Sikora buffer with soil acidity.  Five soils were used from the NAPT program with soil-water pH ranging from 4.6 to 7.1 and CEC ranging from 5 to 19 cmol_c/kg.  Sikora buffer, diluted Sikora buffer, water, 1 M KCl, and 0.01 M CaCl₂ were added to soil and shaken for 0.17, 0.5, 1, 4, 12, and 21 h before measuring pH on a LabFit instrument.  The pH obtained from Sikora buffer, diluted Sikora buffer, and 1 M KCl at the different shaking times produced a linear relationship between pH and soil acidity neutralized which could be extrapolated to pH 7 to obtain exchangeable acidity.  Exchangeable acidity at pH 7 plotted versus time of shaking indicated equilibrium had been reached between soil acidity and buffer after 21 h with about 87% of the acidity reacted with the buffer after 30 minutes.  The kinetics of buffer pH changes will be instructive to assess the potential of using 1 M KCl and Sikora buffer a) to more accurately quantify soil acidity for lime requirement recommendation and b) to quantify 1 M NH₄ acetate pH 7 exchangeable acidity that can be added to exchangeable bases for a quicker analysis of CEC.