How do I lower the soil pH for azaleas and rhododendrons?
Home gardeners can lower their soil pH by adding sphagnum peat moss to the soil. (The pH of sphagnum peat moss generally ranges from 3.0 to 4.5.) When planting azaleas and rhododendrons, dig a wide, shallow hole. Backfill with a mixture that is half soil and half moistened sphagnum peat moss.
After planting, mulch azaleas and rhododendrons with wood chips, pine needles, or shredded oak leaves. These materials are mildly acidic and help maintain the desired soil pH. Additionally, sprinkle a small amount of sulfur around plants on an annual basis.