Well you add it to water to dilute it to the pH level you need. Then use that water to water your plants. The plant needs it’s root area to be the proper pH. So you water your plant with it. Estimate where the root zone is depending on the age of your plant. If you set tap water to 5.0 with fresh unused battery acid, which is a 30% solution of sulfuric acid, the other 70% is distilled water. Anyway if you use that water at 5.0 it is just like rainwater. Rainwater contains that much sulfuric acid. It’s a small amount so you have to be careful not to kill your plant. Make sure your pH readings are spot on.
So you are using rainwater, good. You don’t really add acid to water to change the soil pH for long term. It will not work. Just enough so the plant can absorb nutrients. Sulfur fails too unless you use it on a regular basis. We have had acid rain for 10 thousand years and the soil is still basic. One needs to start with acidic soil in the first place and somehow be isolated from the basic soil. A raised bed works. Well you need to eventually adjust pH in the beds too. I’m at that point myself. My beds are 7 years old and starting to increase in pH from the accumulated organic matter. I add sulfur about once every 2 years. I may need to increase that with time. The blueberry comes from areas with acidic clay that never goes away so the soil has a low pH at all times.
Sulfur is the easiest way to try and control soil pH. I suggest though not watering with tap as you are losing ground even faster that way. Use rainwater or water at a pH of 5.0. Do not poor basic water in the root zone of your blueberry plants. Once in awhile is OK, yes the plant can absorb the water and not die, but it cannot absorb nutrition in that environment. Why the leaves turn red.Cold can make leaves red too, but in the summer, the leaves should be green. From light to dark depending on the cultivar.
Here is a photo of Cara’s Choice. It is a deep green, even blue in color. Notice the lighter colored leaves. that is new growth (always a good sign). This plant is in a 20 gallon root pouch, which is a fabric bag. Planted in 2-1-1/2 Pine, peat, DE An example of a plant in fairly good health.
Toro blueberry. It has more rounded leaves, in very good health. In a raised bed 4x4x1 with the same soil mix as the root pouch. Both photos taken 07 18 2020
Why all this effort?