How to Make Hydrangea Blue or Pink

Test the pH level of your soil to determine the amendments needed to alter its pH. Your local extension office provides soil testing services for a small fee and can provide you with detailed instructions for collecting the soil sample. Inexpensive soil-test kits from a garden center or hardware store also provide a quick assessment of the pH of your soil, but do not provide information about the soil texture or organic matter content -- both of which affect altering the pH of the soil.

Adjust the soil pH to between 5 and 5.5 for blue blooms on hydrangeas. Clemson University Extension recommends applying aluminum sulfate to the soil in the spring when new growth appears to lower the pH of your soil.

Sprinkle aluminum sulfate -- available at your local garden center or home improvement store -- over the soil following the recommended application rate on the container. The amount required to lower the pH to the desired level depends on the current pH level of your soil. Work it into the soil with a rake or hand trowel using care to avoid contact with the foliage. Water to saturate the soil to the root level, as aluminum sulfate produces acidity when it dissolves in the soil.

Raise the pH of your soil to 6.0 or above for pink blooms by applying garden lime purchased at the hardware store or garden supply center, or wood ashes from your stove or fireplace to the soil.

Spread garden lime over the soil following the recommended application rate. The amount of lime required depends on the current pH of your soil, as well as its texture and the amount of organic matter it contains. Work the lime into the soil with a rake or other hand tool. Water to saturate the soil to the root level, as lime needs moisture for the lime-soil reaction to occur. Lime may take up to three months to raise the pH of your soil.

If you're using the wood ashes alternative, spread a thin layer of wood ashes over the area during the winter months. It is slower acting than lime, but wood ashes are readily available to those who burn wood, and this makes effective use of the ashes. Work the ashes into the soil in the spring when snow melts and the ground thaws. Water to moisten the soil if the soil is dry. Avoid ash contact with foliage or young roots.