If you are trying to acidify your soil for growing acid-loving plants such as blueberries and rhododendrons, it has been traditional to recommend adding either sulfur or acid–based organic material such as pine needles or oak leaves.
Recent research has shown that adding sulfur will drop pH over the course of a year, but that combining the sulfur and the organic material drops the pH much faster and much more.
If you want to grow blueberries or other acid–loving plants, add both the sulfur and the organic matter for best results.
Suitable organic matter would include ground-up lemons, pine needles, oak leaves or peat moss among other things.
How Much To Add?
I know I’m going to get this question and the answer is, “It depends.” It depends on the soil test you’re going to do and then you’ll have to repeat this test yearly to judge how your soil pH is progressing as you add material to it
Remember one really important point. This is not a set-it-and-forget-it thing. Modifying soil pH is an ongoing garden chore requiring ongoing testing and modifications because the underlying soil parent material will want to revert to its natural state.
“Organic Amendments and Sulfur in Combination Reduce Soil pH,” Maren J. Mochizuki (University of California Extension, Ventura, CA), A. James Downer, and Ben Faber.