Dig a hole near your plant 12 inches deep with a clean shovel. Collect a sample from the wall of the hole as deeply as possible into a clean, plastic cup. Test the sample according the method of your pH test kit, either testing in water or inserting a probe, to determine the starting pH of your plants. Refill the hole.
Fill a gallon watering can with 1 gallon of clean water. Add 1 tbsp. of aluminum sulfate to the water. Stir the mixture slowly to mix the sulfate into a dilute solution.
Water around the base of your plant with the sulfate mixture, working in a 1 to 2 foot diameter. Avoid pouring the sulfate water directly against the stems, branches and leaves of the plants as you water.
Leave the soil alone for two to three weeks as the sulfate has a chance to get into the soil and lower the pH. Repeat Step 1 to test the soil to note how far the pH dropped and whether or not it still needs to be lowered. Repeat Steps 2 through 4 as often as needed until you reach the desired pH.