Azaleas are not heavy feeders and some cultivars may be intolerant of fertilizer salts. It is best to amend the planting soils with organic matter, phosphorus, and potassium before planting.

Nitrogen should be limited to a low rate - less than one (1) pound per thousand (1,000) square feet per year. Note that this is 1 pound of actual nitrogen, not fertilizer - the nitrogen component will be the first in a series of three numbers on a fertilizer label.  For example, 10-10-10 fertilizer is 10% nitrogen, so it would take 10 pounds of 10-10-10 to get 1 pound of nitrogen. 

Keep in mind that the roots of shrubs might extend 2 to 3 times or more the width of the shrub, and that area should be included in the fertilized area. Iron deficiency can be a problem for azaleas. Use a fertilizer for iron-loving plants or an azalea fertilizer. Lowering the pH of the soil with sulfur to make it more acidic can lessen iron chlorosis (yellowing).

Contact your local Extension office for sulfur rates to lower soil pH.