You always start with a soil test and move forward from there
So You Want to Grow Blueberries and Rhododendrons.
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
A soil test will give you the exact numbers you’ll need to know.
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.
And Yes, You Can Mess About Without A Test
Different websites will tell you to add 1 pound or x ounces per 1000 square feet of garden soil but I’m sure you’ll understand that if your soil is a bit more alkaline than mine, that 1 pound will have a different effect for each of our gardens.
Get a soil test – even if it’s one you purchase from a garden center – do one.
“Organic Amendments and Sulfur in Combination Reduce Soil pH,” Maren J. Mochizuki (University of California Extension, Ventura, CA), A. James Downer, and Ben Faber.