Blueberry bushes and azalea shrubs have something important in common. Both crave the pH range from 4.5 to 5.5, and in most areas of the country, soil must be amended to achieve this range of acidity to please these popular plants. Amend soil in new beds three months or more prior to planting to ensure the proper range has been achieved, or expect three months to see change in your existing bedding plants.
Test your soil for the current pH level. A home soil-test kit can be purchased from a garden center or specialty store. The kit will include testing materials, result analysis and amendment recommendations. Another way to get accurate results with expert tips for soil acidification is to obtain a soil-sample kit from a local university extension office. For a small fee, the lab will assess your soil and provide helpful tips for altering the pH level.
Determine whether your soil is primarily loam-, clay- or sand-based. The rate of amendment will depend on the composition of your bedding soil.
Apply aluminum sulfate at rates suggested by the Clemson University Cooperative Extension horticulture experts for best results. To lower pH levels in loamy soil to 4.5, apply 2.1 pounds aluminum sulfate per 10 square feet of bedding soil currently at 6.0 pH. Cut the rate by a third in sandy soil and increase it by half in clay soil. Amounts of aluminum sulfate increase as the current bedding soil pH number goes up. Excess soluble salts, which can kill plants if allowed to build up in soil over time, are created by adding more than 9 pounds of aluminum sulfate in a single application. The Washington State University Extension experts recommend splitting larger amounts over several applications for best results.
Work the aluminum sulfate into the soil with a rototiller or shovel, providing maximum soil to amendment contact for best results. In a bed with established plants, carefully work the amendment into the soil around the roots through the entire planting bed.
Wipe leaves of established shrubs and plants clean of aluminum sulfate that may have fallen on them during application, or foliage burn may result.