Bigleaf hydrangea flowers (Hydrangea macrophylla) may shift from pink to blue during consecutive seasons, depending on the soil's pH at the time the flower buds form. The soil pH itself, however, does not directly affect the flower color. Aluminum is the responsible party, and alkaline soils lock up the aluminum, making it unavailable to the hydrangea and resulting in pink blooms. Acidic soils don't bind aluminum, and the hydrangea produces blue blooms. Keep in mind that the hydrangea variety must have the pigments available to bloom in shades of blue; white hydrangeas never produce pink or blue blooms. Even pigmented varieties such as Nikko Blue produce pastel blues and pinks, despite high amounts of available aluminum. For intensely blue flowers, choose a variety bred for deep color.
Take two to three samples from different areas of the garden, at least 2 inches below the surface. Place each sample in a separate container and take them to the local extension service. Ask for a pH test. Or, use a home pH test kit.
Treat hydrangeas in soils at or below a pH of 6.5 with 1 oz. of aluminum sulfate in 1 gallon of water. Treat the plants as the buds are forming, six weeks before bloom. If the blooms remain pink, repeat the treatment and wait another year.
Replant hydrangeas into containers if the soil has a pH higher than 6.5. Use a planting mix designed for acid-loving plants such as azaleas. Add 1.5 oz. of aluminum sulfate for every 3 gallons of soil in spring, suggests the North Carolina State University Extension.