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).
For instance, growing a hydrangea in soil with a pH level of about 6.0 - 6.2 will produce a plant with pink flowers, while lowering the pH level to about 5.2 - 5.5 will produce a plant with purple/blue flowers.