Azaleas thrive in soil with a pH between 4.5 and 6. Fertilizer is often necessary to keep the soil pH level acidic to keep the foliage green and the flowers blooming.
Alternatively, you can fertilize azaleas in late winter, especially when using a sulfur-based acidifier. These types of applications take more time to affect pH levels in the soil.
Fertilizing in late winter, they have the extra time to work into the soil. You shouldn't need to fertilize two years in a row, but test the soil every two or three years.
Choose an acidic, granular fertilizer explicitly made for azaleas. The current pH of your soil will decide if slow-release is suitable or if you need something more aggressive.