Soil testing. A pH meter testing the acidity of soil. Farmers and gardeners use pH meters to determine the level of acidity in 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 of acidity on a pH scale. A pH of 7 is neutral, above 7 is alkali and below 7 is acid. This meter displays a pH of 4, meaning that this is an acid soil. Some plants are intolerant of acid soils and so corrective procedures, such as the application of chalk or lime, may be necessary to increase the soil pH to make it more suitable. For normal and alkaline soils, see E770/1092 and E770/1156.