What do you add to the soil to make a pink Hydrangea turn blue?

I am in zone 5 and I add Aluminum Sulphate to the plant in April . Just buy it in a bag and mix to the directions and water your Hydrangea with it. I usually fertilize my Hydrangea a couple times in April to make sure it gets to the roots and then in the summer when it blooms it is a beautiful Blue.You need to do this every Spring to have the blue color or you will have a pink Hydrangea.

What do you add to the soil to make a pink Hydrangea turn blue?
water with blue die, although that may turn it purple
Reply:iron sulphate...or put a hand full off rusty iron nails just under the soil by the roots
Reply:Aluminum Sulfate is added.It's best if you "spike it into the root system.

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