PLEASE HELP! Need PH advice ASAP!

Ok this is my first grow, I'm using FFOF soil in 7gal pots. I've been noticing slow growth and decided to check my PH Runoff for the first time. The plant is 3 weeks old, and the runoff read at 4.0! The water i put in was 7.0 ...I'm not sure how to correct this & need prof advise plz. Thnx

you have buffers in the soil... probably sulfur 2S + 3O2 + 2H2O -> 2H2SO4 or Al2(SO4)3+6H2O --> 2Al(OH)3 + H2SO4. Your accidentally making sulfuric acid in your soil

And the solution?

add some dolomite (lime) or flush with alot of basic water. baking soda works, tap water works. anything with calcium