You have seen them blooming in many stunning shades of blues and pinks and sometimes with pink and blue blossoms on one plant! How does this happen? It is the pH levels of the soil which affect the blossom color. A pH above 7 indicates alkaline soil and numbers below 7 represent increasing degrees of acidity. Nantucket soils tend to be more acidic so we have more blue hydrangeas. Pink hydrangeas are produced in the more alkaline soils. The colors can be manipulated to create the purple/blue by adding aluminum sulfate to the soil in the very early spring. Or you can change your blue hydrangeas to a lavender or pink color by adding lime to the soil in the fall. Be patient, the color change may take a season or two to adjust.