Aluminum sulphate will make your soil acidic, lime will make it alkaline. Susan, your soil is naturally acidic in this area, especially if you have a lot of pine litter like I do. You could have it tested to find out exactly how much lime to add, or just start with a light amendment and add a bit more at a time until you start getting pink blooms - too alkaline can kill plants, just like too acidic can, so it's best to start slowly.
(I don't know a thing about hydrangeas, but I'm pretty decent at chemistry
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