Soil testing. A pH meter showing a neutral pH of soil. Farmers and gardeners use pH meters to determine the pH level of soil to assess its suitability for different plants. A probe placed in the soil measures the amount of free hydrogen ions present, giving a value on the pH scale. A pH of 7 is neutral, above 7 is alkali and below 7 is acid. This meter displays a pH of 6-7, meaning that this is a neutral soil. Some plants are intolerant of acid or alkaline soils, and the pH has to be corrected towards this normal level. For a sequence of pH tests for acid, neutral and alkaline soils, see images E770/1157-1159.