Although most plants grow best in soil that is slightly acidic, a few, including blueberries and azaleas, prefer soil that is slightly alkaline. Some plants are tolerant of alkaline soils, although they grow better in neutral or acidic soils. These include dwarf baby's breath, carnations, foxtail lilies and oregano.
The pH scale is used to indicate the level of acidity or alkalinity in soil. Soil with a pH below 7.0 is acidic, and soil with a pH of exactly 7.0 is neutral. Soil with a pH above 7.0 is said to be alkaline. Most plants thrive in soil with a pH between 6.0 and 7.0, but will tolerate a pH up to 7.5 as long as they have adequate moisture and sunlight.
A soil test can be used to test soil pH. If the soil is found to be alkaline, the gardener can either choose to grow alkaline-tolerant plants, or to amend the soil to lower its pH and make it more suitable for a greater variety of plants. Generally, calcium sulfate or ground sulfur are added to soil to make it more acidic. If a soil test indicates that soil is too acidic, limestone can be added to bring the pH back up to the 6.0 - 7.0 range preferred by most plants.