Root. In the computer world, root refers to the top-level directory of a file system. The word is derived from a tree root, since it represents the starting point of a hierarchical tree structure.
Use a fertilizer with high levels of phosphorus. Phosphorus helps to prevent aluminum from creeping into the system of the hydrangea. Choose a fertilizer close to the ratio of 25/10/10 (Phosphorus is the middle number).
Hydrangeas turn blue as a reaction to aluminum in the soil. The soil pH should be slightly acidic (5.2 - 5.5). Do a soil test first, to determine your starting pH. Adding aluminum sulfate to the soil will help lower its pH, if necessary. A word of caution: not all plants can tolerate acidic soil and neighboring plants may be adversely affected.