Answer: Pink and red hydrangeas often turn blue or purple in acid soils. Blue hydrangeas can turn pink in alkaline soils. Plants can be made (or kept) blue by applying aluminum sulfate to the soil; plants can be kept pink by liming the soil or applying superphosphate. Since your soil is alkaline, as evidenced by the change from blue to pink, add aluminum sulfate to the soil this fall and next year's blooms should be blue.
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