Answer: Flower color is determined indirectly by the soil pH, which affects the availability of aluminum in the soil. In acid soils the flowers will be blue, in alkaline soils the flowers will be pink. Aluminum is available to the plant in acid soils. For blue flowers, maintain a soil pH between 5 and 5.5. If your soil is alkaline, you'll need to apply sulfur to reduce the pH to this range when you see new growth emerging in April. You only need to apply sulfur once; in the spring. Continuous application will upset the pH balance in your soil and stress your plant. Yellowing leaves can be a sign that you've over-applied sulfur (or aluminum sulfate, if that's the chemical you are using).
At this point I would stop applying the acidifying agent and concentrate on watering the plant deeply once each week. Pull off the yellowing leaves to encourage the development of healthy green leaves. You can feed with a liquid fertilizer such as Miracle Gro, which can be used as a foliar feeding as well as root food. Dilute according to label directions and spray the leaves as well as the soil beneath the hydrangea. You should see some green-up within a week.
Best wishes with your hydrangea!
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