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.
(Ref. No. 11 - Method References)