How Do Hydrangeas Indicate pH?

Hydrangea Color Variation

• Soil acidity affects the color of hydrangea flowers. In acidic soil, blooms are blue, while in neutral to alkaline soil, they appear pink. Sometimes a pH that's only slightly acidic, with a pH between 5.5 and 6.5, produces flowers that are purple in color, while other times the blossoms are either pink or blue -- even the ones right next to each other in the same cluster. However, a few hydrangea varieties never turn blue; in acidic soil, their flowers are a dull maroon color, while alkaline soil makes them deep pink.

How It Works

• Soil pH makes certain nutrients available -- or not -- to plants. One of these nutrients is aluminum, which is available to hydrangeas when the soil is acidic with a pH lower than 5.5. Specifically, the aluminum turns hydrangea flowers blue. If the soil is alkaline with a pH above 6.5, the roots are unable to absorb aluminum, which results in pink flowers.

Assessing Soil pH with Hydrangeas

• This color variation means that noticing the color of your hydrangea flowers is an easy way to estimate your soil's pH. If your soil seems too acidic or alkaline for healthy plant growth in the rest of your garden, you may need to perform a soil test to determine the exact pH balance -- something hydrangea color alone can't do with accuracy. However, once you plant hydrangeas and determine your soil's pH, the flowers can help you reassess the soil's acidity each year in case it changes. For instance, if your pink hydrangeas start turning blue after a few years, you'll know your soil is becoming more acidic than it was originally.

Adjusting Soil pH

• Changing your soil's pH lets you play with the coloring of your hydrangeas, although you should be careful if other plants are in the area that may need a certain soil pH to thrive. Adding limestone to soil makes it more alkaline, while sulfur and iron sulfate make it more acidic. And if you want your hydrangeas to be blue, keep phosphorus levels low in fertilizers -- such as the amount in a 12-4-8 fertilizer -- because phosphorus binds to aluminum, preventing it from being absorbed by plant roots.