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).
FIRE STOPPER: One interesting irony is that phosphorus used to make fires (in its elemental form) is used to stop or reduce the hazards of fire (in the form of chemicals derived from it).