If azaleas, blueberries, or other acid-loving plants are thriving, chances are the soil is acid, with a pH roughly between 5.0 and 6.0, well below the optimum 7.0 level.
The relative acidity or alkalinity of soils is important, because it affects the availability of soluble salts supplying plant nutrients. If the pH is not correct for the plant, nutrients may be unavailable. Plants that can’t tolerate acid soils appear stunted and have sparse roots.
Acid soils are common in high-rainfall areas, such as the eastern United States. The frequent rain, especially during the warm part of the year, over centuries leaches the alkaline minerals (calcium, potassium, and magnesium) from the soil and replaces them with hydrogen, which makes soil acid.
The difficulty with acid soil is that many of the nutrients needed for plant growth become less available because they are converted into insoluble salts. Phosphorus especially is tied up in insoluble compounds. Phosphorus deficiency causes plants to grow very slowly and take on a dark, dull, color.
Other elements such as aluminum and manganese can become water soluble in acid soil, often in amounts that are toxic to plants. The microorganism activity that causes decay is less in acid soils; therefore, organic matter will not break down as fast.
Acid soils can be sandy or clay, but sandy soils lose their minerals more quickly than clay soils, so are more likely to become acid. Soil might also have been made acid by adding too much peat moss, which is extremely acid and decays slowly.
Test your soil to determine the pH level. You can test pH successfully with an inexpensive kit from the garden center. The type that adds a liquid to dry soil and shows acidity by color change is the most accurate of the kits available to home gardeners. The litmus paper type and probes that you stick in the soil are more likely to be influenced by other factors than just acidity.
You can also have your soil tested by a laboratory. The lab will tell you exactly how acid the soil is and give recommendations for correcting this and any other soil problems.
Handling Acid Soil
There are two main ways to deal with acid soil. The first is to treat the soil chemically to raise its pH; the second is to grow plants that are adapted to acid soils.
Treating the soil chemically to raise the pH is done by adding ground limestone. In regions with naturally acid soil, it is usually necessary to add limestone every two or three years. Test the soil annually and treat it when necessary. For more information, see Raising Soil pH with Limestone .
The other way to solve this soil problem is to use plants that are adapted to acid soils. Plants native to your region are always a good bet. Ask at your local nursery for acid-soil plants.