Although there are fertilizers made specifically for hydrangeas, Gardening Know How states that any quality general-purpose composition between 12-4-8 and 10-10-10 should suffice. Hydrangeas respond positively to both organic matter and chemical fertilizer.
For easy care, Gardening Know How recommends feeding hydrangeas a slow-release fertilizer for trees and shrubs on an annual basis. A small layer of soil must be applied on top of the fertilizer to activate it. Twice a year, hydrangeas should be given a moderate amount of liquid iron. For natural fertilization, a mix of compost, peat moss and sulfur is ideal.
Hydrangea plants that are given extra sulfur turn blue, and adding more lime to the fertilizer turns hydrangeas pink. White hydrangeas remain white regardless of what kind of fertilizer is used.