Gardeners with a penchant for experimentation may enjoy a special characteristic of hydrangea plants. By changing the soil conditions and adding specific chemicals to the soil, a gardener can change certain hydrangea blooms to either blue or pink. The hue that results from the soil amendment depends upon what you add to the soil. Gardeners who desire hydrangeas with blue blossoms must decrease the pH level of the soil and make it more acidic. Hydrangea will turn blue if the pH level is 5.5 or less, according to University of California Extension Master Gardener's Program.
Water the hydrangeas generously several hours prior to adding the aluminum sulfate.
Measure 1 tbsp. of aluminum sulfate into the bucket or watering can and add 1 gallon of cool water. Mix the ingredients well to dissolve the aluminum sulfate.
Pour the aluminum sulfate carefully over the soil near the hydrangeas. Take care not to splash the solution onto the plant foliage. Pour enough solution over the soil to saturate it completely.
Repeat this watering technique once per month during the growing season.
Fertilize the hydrangea by sprinkling the fertilizer over the soil twice during the growing season (in the spring and again in midsummer). Add approximately 1/4 cup of fertilizer over the soil for a small hydrangea, 1 cup of fertilizer for a medium hydrangea, 2 cups of fertilizer for a large hydrangea and as much as 3 cups of fertilizer for a very large hydrangea. Do not allow the fertilizer to contact the hydrangea stem and work the fertilizer into the soil with a hand rake. Water the hydrangea immediately after you fertilize.
Continue these techniques on an on-going basis to keep your hydrangeas blue. If you stop adding the aluminum and potassium to the soil, the soil will revert to its former pH balance and the blossoms will change color in response to the pH change.