The color change is commonly attributed entirely to soil pH, with gardeners often advised to use acidic or alkaline fertilizers and soil amendments. For example, the well-known garden brand Espoma sells a Organic Soil Acidifier, which features an image of a blue hydrangea and a tag like that says, "turns hydrangeas blue." It's easy to conclude that acidity alone creates the color, but a lower pH works indirectly by making aluminum easier for the plant to absorb. A lower pH makes aluminum that is already present in the soil more available to the plant. Once the soil pH drops below 5, though, the aluminum is much more soluble.