For hydrangea blooms to be pink:
Add dolomitic lime several times a year to raise the pH 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).In areas that naturally produce blue hydrangeas (soils with aluminum), consider growing pink hydrangeas in large pots. If hydrangeas are grown in pots, it will be much easier to control the requirements for growing pink hydrangeas.
For hydrangea blooms to be blue:
To ensure that aluminum ispresent, aluminum sulfate may be added to the soil around the hydrangeas. Mix 1 Tablespoon of aluminum sulfate per gallon of water to be applied to plants. Important: Water plants well in advance of application and put solution on cautiously. A fertilizer low in phosphorus and high in potassium is helpful in producing a good blue color(25/5/30 is good.)