Soil pH – Lowering With Sulfur, Alum or Ferrous Sulfate
Q: I scattered lime around my hydrangeas and got some on my azaleas nearby. The pH of the soil around my azaleas is now 7.0, but azaleas like a more acid soil. How can I correct this?
A: Changing pH is not a short order process…but you can drop the pH one point per year by applying 5 lb of 35% ferrous sulfate per 100 square feet now and once again in six months, followed by a soil test, to see what’s happening.
If I am not mistaken, Lilly Miller Moss Out Granules (click for sources) contains ferrous sulfate.
Sprinkling powdered sulfur (click for sources) on the ground to lower pH is sometimes recommended. Sulfur is problematic because “a little goes a long way!”. I’d rather you not use it for individual plants. The best use would be when you’re preparing a large bed for blueberries or azaleas.
Alum, aluminum sulfate, (click for sources) can also be used.
I think your best bet would be to calculate how much product you need and then shop around to get the best price.