Measuring the acidity of soil using an electronic pH meter. Soils are generally slightly acidic as rainfall is slightly acidic, due to carbon dioxide in the atmosphere dissolving in water to form carbonic acid (CO2 + H2O --> H2CO3). This forms hydronium ions (H3O+) in the soil, lowering the pH below 7, the threshold for acidity. Rising levels of carbon dioxide can acidify rainwater and surface water further, and industrial atmospheric pollutants such as sulphur and nitrogen oxides can reduce it even more. Very acidic soils are often infertile, as plants cannot take up all the nutrients they require.