pH is the negative logarithm to the base of 10 of H-ion activity (i.e. apparent or effective concentration of H-ion in solution). It represents the the intensity factor (index) of soil acidity. SOURCE Soil Survey Laboratory Information Manual.Soil Survey Investigations Report No.45. Version 2. February 2011. United States Department of Agriculture, Natural Resources Conservation Service, National Soil Survey Center, Lincoln, Nebraska, USA. 530 pages. NOTE: the pH of the soil is potentiometrically measured in the supernatant suspension of a soil/liquid mixture. This liquid can be made using several reagents (KCl, CaCl2, H2O, NaF). The reagent used must be given in the process parameter linked to the observable property as well as its concentration and the ratio used between soil and liquid. NOTE: Generally, the soil derived object will give the value of pH for a certain depth interval, layer or horizon that must be precised using the SoilDerivedObjectDescriptiveParameter.