Hi Jean –
Espoma’s “Soil Acidifier” comes in a 6-lb bag and the active pH lowering ingredient is elemental sulfur. (For container application use 1 tbsp for every 4 inches of pot diameter.)
Sulfur, when incorporated into the soil and “digested” by microbes, will produce sulfuric acid which releases hydrogen ions that in turn will lower the soil pH. We also have elemental sulfur granules that when incorporated into the soil, will do the same thing.
Also, there are some fertilizers that will lower the pH. The fertilizers that do this contain nitrogen in certain forms, that when they are broken down in the soil, release hydrogen ions and lower the pH. The main types of nitrogen in these fertilizers are urea, monoammonium phosphate (MAP), diammonium phosphate (DAP), ammonium polyphosphate, ammonium sulfate and urea ammonium nitrate. Watch out though, fertilizers with nitrates (NO3) will raise the pH. (Typically, fertilizers have a balance of nitrogen in the forms that will lower (ammonium) and raise (nitrates) pH so that the fertilizer won’t effect the pH).
Gypsum is another option, although it is mostly used in extreme cases of high pH and sodic soils. This will make the sodium more leach-able, but will leave calcium in the soil.
Another route would be to add “pH down” or another pH lowering amendment to your water to reach the desired 5-5.5 pH range.
Espoma Soil Acidifier:
Raw Nutrients (nitrogen – ammonium sulfate):