Azaleas are flowering shrubs that favor relatively acidic and low-nutrient soils when compared to most other shrubs. They possess a shallow, fine root system that is easily damaged by excessive fertilization. You should always test your soil for nutrient deficiencies before fertilizing. The ideal fertilizer for azaleas should contain only those nutrients your soil lacks, should not reduce the acidity of the soil, and should be applied in a slow-release granular form to avoid burning the plant's root system.
Most soils that contain enough organic material to provide proper drainage already contain enough nutrients to sustain azaleas. Before planting, you should test the soil to determine whether it is deficient in phosphorous, nitrogen, potassium, magnesium, iron, or calcium. Most of these nutrients can be effectively measured using a home testing kit available at garden centers and nurseries. A more precise and comprehensive soil test can be obtained from a county or university extension office. If your soil does not show a marked lack in any essential nutrient, fertilizer can easily do more harm than good.
Any effective fertilizer for azaleas should either add acid to the soil or at least avoid reducing acid levels. Many common fertilizers contain alkaline ingredients that will raise the pH of the soil, while most calcium supplements neutralize acid. If you must apply one of these fertilizers, use an acid-forming soil additive at the same time. Azaleas are acid-loving plants, and will not grow well in soil with a high pH. The best type of fertilizer for azaleas is specifically formulated for acid-loving plants, and will generally be marked as such on the package.
When choosing a fertilizer for azaleas, it is important to take the plant's fragile root system into account. Fertilizer should be applied in the form of slow-release granules to avoid exposing the roots to excessive levels of chemicals, which could burn the roots. Most fertilizers that come in powder or liquid form expose the roots to too much fertilizer at once and should be avoided. The need for chemical fertilizer can be reduced altogether by mulching with natural materials, which will provide the plant with nutrients as they work into the soil and slowly decay.
In general, the best fertilizer will be the most expensive, off-brand product that contains the needed balance of nutrients. Name-brand fertilizers often have inflated prices, but the cheapest off-brand products usually contain a lower proportion of active ingredients to filler material. To find the highest quality fertilizer, compare the store brands and generics available and choose the one that is most expensive by volume in the largest package available.