Most plants like their soil to be pH neutral, or around pH 7, which is the range that best allows them to take up and process the nutrients they need. Some plants, however, such as rhododendron, azalea, and blueberries, do better in more acidic soil, or soil that's between 4.5 and 6. Adjusting the pH of your soil is possible, but it should always be done gradually and keep in mind that soil will always try to return to its natural pH.
Contact your county extension service for information about the soil in your area. If your garden is in a place that was formerly a forest, your soil will probably be more acidic and suited to acid-loving plants. Gardens on rock deposits or old ocean beds tend to be more alkaline and will need amendments for plants like azaleas to grow well.
Find out about getting your soil tested. County extension services and universities often do soil testing, and many garden shops will test a sample you bring them. There are also home soil tests available, though these can be less reliable and should be repeated several times for a better reading.
Mix ground rock sulfur into alkaline soil several weeks before planting. Use 1.2 ounces of sulfur per square yard for sandy soil, and 3.6 ounces for other types of soil. This amount of sulfur will quickly lower the soil's pH by one full point. The sulfur will need to be applied every year to maintain a low pH.
Try other organic soil amendments for slower-releasing, more gradual pH adjustment. Organic amendments feed the soil, improve its quality, and are usually more effective in the long run. They can be used directly around the roots of acid-loving plants without changing the pH of your whole garden. Additionally, organic soil amendments do not cause a harmful runoff in wastewater.
Dig in some sphagnum or peat moss around the roots of acid-loving plants, or place these in the bottom of the hole before planting. These mosses will gradually lower the pH of the soil as well as create a healthier root environment.
Use kitchen waste like coffee grounds and used tea leaves around acid-loving plants. This kitchen waste is good food for earthworms, which aerate and fertilize soil. Most households have a regular supply of these, and many coffee shops give their coffee and tea waste to gardeners for free.