Soil pH is determined on a 1:1 (V/V) soil/water mixture composed of a 10 gram NCR-13 volumetric soil scoop and 10 mL double-deionized water. Samples are stirred both before and after a 15 minute equilibration period. pH is measured on a Mettler Toledo Seven-Multi pH meter with an InLab Routine Pro combination electrode, calibrated to pH buffers 4, 7, and 10. Mineral soils with pH values of less than 6.0 are analyzed further for the following lime requirement test.
The Sikora Buffer Index (lime requirement test) is determined by adding 10 milliliters of buffer solution to the above 1:1 sample. The Buffer Index of the suspension is determined with the pH meter, after the sample has been stirred intermittently for 15 minutes.