Power of Hydrogen (pH) is a measurement of the acidity of a substance. The scale runs from 0 pH (extremely acidic) to 14 (extremely basic). Pure water, which is neutral, is 7 on the pH scale. Most plants, including cannabis, have an optimal pH range, and anything outside of that range is harmful to the plant. If the pH of the soil falls outside normal ranges for the plant, nutrient lockout may occur.