Thanks for writing. You'll first need to test the soil in your containers to know what the pH is --- potting mixes like that can vary from batch to batch. Look for a pH-testing kit at your local nursery or garden center. If your potting mix has an acidic pH already (as many do), you should be fine and don't need to do anything to amend the potting mix. If the pH is neutral or alkaline, then you'll want to add some soil sulfur to help decrease the pH. You can find instructions on how much sulfur to use on the back of the sulfur's packaging.
---Justin, Senior Garden Editor, BHG.com