Compost is a great enrichment for azalea soil. These plants require a light, airy planting medium, and compost can keep your soil from becoming compacted. Compost also adds a little acidity to the soil, which azaleas appreciate. If you live in an area with heavy clay soil, it may be alkaline-the opposite of acidic-a soil acidifier might be right for you. You can find that product at your local garden center.