The link between acidic soil and hydrangeas is actually forged of aluminum. This element, which occurs naturally in soils, is not necessary for other growth in hydrangeas but impacts strongly on the color of blooms. Hydrangeas grown in soil that allows them to process high levels of available aluminum will have blue blossoms. Those soils in which access to aluminum is restricted will produce pink hydrangeas. Some human intervention can redirect this charming colorful quirk of nature.
All plants have a pH value that best supports their growing. PH describes the acid or alkaline composition of the soil, with pH 9 being extremely alkaline and pH 4 being extremely acidic (both on a scale of 1 to 14). PH stands for: potential of hydrogen ion activity in water. While aluminum is not essential to plant growth, the necessary level of hydrogen ion activity is essential, and it enables plants to take in and process soil chemicals and elements that are necessary for growth
Hydrangeas and most other plants will not grow well at either extreme of the acid/alkaline pH scale. Neutral to mildly acidic soil supports much of what gardeners like to grow. Not all soils come in naturally neutral condition; Clemson University points out that Southern soils have a tendency toward acidity and, given the high natural presence of it in the earth's crust, one of the contributors to acidity is aluminum.
Affecting the color of hydrangea blooms is about doing and not doing at the same time. In fairly neutral soil, hydrangeas whose blossoms turn colors may be a mix of white, blue, pink and purple shades all at the same time. The presence of blue tinges indicates some aluminum; for a more solid blue, add aluminum sulfate around the roots on a regular basis. You are not so much turning pink blossoms blue as making it possible for a higher level of aluminum to do it.
The Paradox: Other Side
To turn blue hydrangea blossoms pink, add dolomite lime to your soil. This is not so the plant will absorb the lime. The lime will make the soil more alkaline, converting some of the hydrogen ions into water and carbon dioxide. Fewer available hydrogen ions restrict a plant's ability to absorb soil minerals, such as aluminum. Limit aluminum uptake and blooms will be pink.
The good news: Human action can change soil pH and therefore the behavior of plants. The bad news: Because of surrounding soil conditions, water runoff and the needs of surrounding plants, you need to be both consistent and persistent in your efforts (some gardeners suggest that your best chance is with hydrangeas in containers). The humbling news: All these gorgeous color changes are merely a byproduct of a growth process affected by but not improved by aluminum in the soil. Then again, gardening is humbling in many ways. Enjoy your color-changing experiments.