Alum – How Much To Use To Acidify Soil

Q: I want to acidify the soil around my azaleas and hydrangeas. How much alum do I use?

A: It all depends on what your current pH is and whether your soil is sandy, loamy or clayey. You’ll need more alum to reduce pH in clay soil than to do so in sandy soil. For a quick example, if you have clay soil plus a current pH of 6.0 and you want to drop it down to pH 5.0, you’d need 19 pounds of granulated or powdered alum per 100 square feet. It might be cheaper to use powdered sulfur. In the example above, you’d need only 3 pounds of sulfur. I have a handy chart that makes calculations easy at http://www.walterreeves.com/gardening-q-and-a/soil-ph-lowering-2/.