I use sulfuric acid to help acidity my soil for blueberry plants that I have growing in pots. It just takes a few drops per gallon. I measure with a ph meter. I lower the ph until I get about to a ph of 5 then I water.
I've read that the sulfuric acid reacts with calcium to create gypsum. Blueberries hate calcium so this is beneficial for them. I do not know if it would be a negative for tomatoes. I do know this will definitely acidity your soil.
You could also try fertilizing with ammonium sulfate.
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