Soil Requirements Generally, Rhododendrons and Azaleas prefer an acidic soil rich in organic matter. Many books will dwell at some length about the proper pH levels and how to acidify the soil. Addition of sulfur powder, aluminum sulfate or ferrous sulfate are often recommended. Under no circumstances should you use aluminum sulfate around Rhododendrons as it is toxic to them. Perhaps more important than the proper pH, is proper drainage.
Poor drainage kills more Rhododendrons than anything else.
Before you plant, dig a hole and fill it with water. If the water does not drain within a few minutes, select another planting area. Regular garden soil in Atlantic Canada is often poor in organic material and may be too heavy in texture. Such a soil would be moisture retentive yet well drained. Rhododendrons and Azaleas have small, fine root systems.