Answer: The changeable color varieties of hydrangea should be blue in a pH range of about 5.0 to 5.5 and turn pink when the pH is a bit higher, say between 6.0 and 6.5 or so. In your area, the soil may naturally be acidic enough (if it has not been limed) for the hydrangeas to stay blue. Using an acidic mulch such as pine needles or oak leaves, and feeding with an acidified fertilizer such as Miracle Gro should be enough to naturally maintain the pH in the range you need for the blue coloration. If you apply sulfur to lower the pH, you could lower it enough that no plants will be happy, so test your soil before adding anything like sulfur or aluminum sulfate.
Best wishes with your hydrangeas!
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