Yellow azaleas caused by deficiencies in soil
Why are the leaves on my azaleas yellow?
Azaleas can turn yellow in response to deficiencies of iron or nitrogen. How do you know the difference? Look at the part of the plant affected and the way the leaf looks. Nitrogen deficiencies show up on the older leaves first. The entire leaf turns yellow and the leaf may fall off. Nitrogen deficiencies are more common in the fall and winter.
Iron deficiencies show up on the younger leaves first. The leaf turns yellow but the veins often remain green. The leaf may begin to look white if the deficiency becomes severe.
These problems can be corrected by adding fertilizer containing this nutrient but the problem may be caused by more than a shortage of the nutrient in question. Investigate a little farther to see if there is an underlying problem causing the deficiency.
Either one of these deficiencies can be caused by root injury to the plant. Has the plant been transplanted recently? Has it been kept too wet or too dry? If the roots are not strong, the plant cannot take up nutrients and will be deficient. Find the problem and correct it if you can.
Nitrogen deficiencies are more common in the winter. The plant did not have enough stored nitrogen to last through the winter. Wait until after the plant blooms and then fertilize. Remember to repeat the fertilizer application in August.
Iron deficiencies are often the result of high soil pH. Azaleas prefer a lower pH (5.0 – 5.5) than many other plants (5.5 – 6.0). If the pH is too high and you do not correct the problem, expect the iron deficiency to return. Take a soil sample. The results should tell you how much sulfur or ammonium sulfate to add to the soil to lower the pH. Be patient. These chemicals take a while to work.
Add iron to the plant with foliar iron sprays and soil applied iron products as well as changing the pH. Plants probably respond to iron compounds we spray on the leaves faster than soil applied products. Soil applied products last much longer. Apply the soil applied products first and the sprays if you can afford both.
Some times near buildings we find soils with high pH due to buried mortar or concrete. These problems are difficult to solve. First, dig up the buried mortar. Then soil sample and adjust the pH if needed. One year after planting the shrubs, take another soil sample and adjust pH again if needed. In these situations, you may want to use a plant other than azalea that would not be so sensitive to high pH.