I have about 50 azaleas, Formosas and Gardenia Azaleas. They are in soil with a ph from 7.5 to 8.0. I fed them azalea food about a month ago and measured the ph and it shows little change. The azaleas bloom wonderfully but the foliage is yellowing on the Formosas and almost non-existent on the Gardenia Azaleas. They have been in the ground about 2 years and have shown very little growth as well. I am guessing I need to acidify the soil. How often and what should I add to the soil to get it to the desirable ph which I believe is somewhere around 5 to 6.5. I live in Montgomery Texas north of Houston.
While a range of 5.5 to 6.5 is desirable, it is also difficult to maintain since alkaline soil tends to loose acidity when you stop amending the soil. Products that you can add to the soil range from: liquid iron chelated products available at most nurseries; garden sulphur; ferrous sulphate; green sand (organic product), etc
Liquid products appear to correct pH imbalances more quickly than some of the others but even then, they do not correct the problem overnight. It will take weeks for the leaves to darken. But that is fine. You want it to happen slowly. As far as when to reapply, read the label directions of the product.
Azaleas tolerate the alkalinity of the Dallas/Fort Worth Metroplex but I always have to add compost and one of those products in March/April. I basically write down on the calendar when the problem happened and try to amend the soil a few weeks before that time next year.
Since our water is also alkaline, I use mostly rain barrel water in winter and early Spring. However, you have waaaay to many plants to water this way. But there are some irrigation systems that allow people to amend the water. You could add some liquid acidifying products if your irrigation system permits you to do that.