Answer: There are several possible causes of your azalea problem. First, let me say that you are on the right track with the Miracid fertilizer, you may want to use it at the low constant feeding rate to help keep the pH low.
Brown leaves may be due to lack of water. Azaleas are not tolerant of dry soil, even for a short time. In a container, their root system is very restricted making them even more dependent on regular watering. Terra cotta containers dry out faster, complicating the problem. Your plants did fine when temps were cooler, but now the hot weather is dramatically increasing the demands on them.
If your container is on the small side, this could be a contributing factor. I have grown acid loving plants in south Texas and had success by using 1/2 whiskey barrels as planters and a soil mix of 2/3 peat moss and 1/3 sand to make sure it was acidic and held water well.
A final consideration is the quality of your water. If your water is soft (high sodium) or high in pH, you may need to switch to rainwater or pond water (if available). Your County Extension office in Bexar County can instruct you in how to take and submit a water sample to Texas A&M for analysis. There is a small charge for this service, but it's less than the cost of replacing plants.
Miracid can be used on many plants in your area. Your soils are naturally a bit on the alkaline side. Miracid will gradually lower the pH a bit as you use it. Those that respond best to this acidic formula include azaleas, blueberries, gardenias, camelias, many ferns, and dogwoods.
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