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.