Soil testing. A pH meter showing the acidity 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 4-5, meaning that this is an acid soil. Some plants are intolerant of acid soils, and so corrective procedures may be necessary to increase the soil pH to make it more suitable. For a sequence of pH tests for acid, neutral and alkaline soils, see images E770/1157-1159.