Answer: Hydrangea color has a lot to do with the acidity or alkalinity of the soil. Pink and red hydrangeas often turn blue or purple in acid soils. Blue hydrangeas can turn pink or greenish white 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 greenish white, add aluminum sulfate to the soil this fall and next year's blooms should be blue. You can feed with an acidified fertilizer, such as those manufactured for azaleas and rhododendrons or camellias.
Best wishes with your hydrangea!
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