Does the use of urea as a nitrogen source pose any special problems for "sun" azaleas? Where I am located most of the fertilizer formulations 100% of the nitrogen is in the form of urea.
Azaleas don't need much nitrogen. Too much nitrogen will keep them from blooming. The best formulation is something like our Holly-tone which is:
Urea formaldehyde and ammonium sulfate are both good sources of nitrogen for azaleas. They are both slow release which is good. Nitrates are NOT as good for azaleas. It can cause nitrogen toxicity.
Lack of nitrogen leads to slow growth and an overall yellowing of the foliage that affects the old growth first. Nitrogen is most effectively added by using urea. However, urea is strong; use it at no more than 1 oz per 1 1/2 gallons (25 g per 5 liters) of water or severe burning may result. Milder sources of nitrogen include ammonium sulfate. Make sure these fertilizers are thoroughly watered in. To use nitrogen a plant needs magnesium, so magnesium sulfate (Epsom salts) is often used in conjunction with nitrogen if the plants look pale green.
Here is a link that might be useful: Espoma's Holly-tone
My experience backs up rhodyman's first statement: "Azaleas don't need much nitrogen." I've been growing rhodies for 30 years in several locations and have never fertilized them, just given them a nice coating of organic mulch over the roots. They grow, they bloom well, and they look healthy. So I'd not fertilize them unless there seems to be a need.
Rhododendrons and azaleas in nature are generally characteristic of high precipitation environments where soils tend to be leached and low in certain nutrients like nitrogen. It is easy to damage them with the wrong fertilizer.
Other than that, fertilizing should be based on a soil test, same as with all kinds of garden plants. On some sites, azaleas will benefit from applications of suitable formulations of deficient nutrients.
Uniformly yellowish-green leaves is often just the need for more nitrogen or magnesium or both. This will be more noticeable in the full sun. Some less sun tolerant varieties will always be light green in full sun.
nitrogen deficiency (photo courtesy of Harold Greer)