Soil testing. A pH meter showing the alkalinity 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 7-8, meaning that this is an alkaline soil. Some plants are intolerant of alkaline soils, and so corrective procedures may be necessary to decrease 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.