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).
1 Most hydrangeas thrive in rich, porous, somewhat moist soils. Add compost to enrich poor soil.  They prefer full sun in the morning, with some afternoon shade; however, many will grow and bloom in partial shade. This is especially true for the bigleaf hydrangeas (see Recommended Varieties below).