Utah State University Extension
PH is a measure of the acidity or alkalinity of a substance. The range of pH for most soils is from 4 to 10 (Figure 1). The pH scale is logarithmic, meaning that a change in one numerical pH unit equals a 10-fold change in acidity or alkalinity.
Gale, Jody and Koenig, Rich, "Managing Soil pH in Utah" (2001). All Current Publications. Paper 923.