Hydrangea 'Preziosa'

Looks like that. How much aluminium sulphate do you add to each bush?

It depends on size and soil pH, my largest hydrangea is Endless Summer and I give that one two cups. My soil pH is 6-6.5 normally and that takes it down somewhere between 5.0 and 5.5. I usually start with 1/2 to 3/4 cup for a 2-3 feet tall plant and if it needs more I add another 1/4 to 1/2 cup. It can be kind of a guessing game without a pH tester. If you add too little the flowers will usually be pinkish-purple, the darker the color the closer you are. If you add too much it can be fatal to the plant so always better to start with less and add more if it needs it.