Answer: Soil acidity or alkalinity will change the color of hydrangea blooms. Pink and red bydrangeas often turn blue or purple in acid soils, and plants can be made red by liming the soil or applying superphosphate. For pink flowers, you may have to add lime to your soil to raise the pH above 7.8). Test your soil to measure the pH range before adding anything - you don't want to upset the balance!
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