Where I am just west of Minneapolis, our soils already have lots of calcium, and in addition, if you water with municipal water in the Midwest or western US, your water may be coming from limestone aquifers that would also have plenty of calcium dissolve in it. For me, I prefer sulphur; or ammonium sulphate, iron sulphate. You will find sulphur spelled with an "f", too. Use Aluminum sulphate with blue hydrangea, but don't use it with anything else.
You'll find them at nurseries with the fertilizers and pesticides. Magnesium sulphate (Epsom salt) won't do the job. It's not acid enough.
This site gives good advice:
http://www.ipm.iastate.edu/ipm...
