As long as your soil itself (The Mix itself) isnt a ph of 5.6 then yes flushing with bring it up. Never adjust higher tho, the 7.8 would be a very unwise move, exspecially from a 5.5 to a 7.8, might as well pour vinegar on them you know, just flush and wait, try to bring her back up to close to 6.8 ish. 5.7 isnt super low, thats what hydro is usually dialed at. I personally keep my soil grow around 6.2 so that gives you an idea, and my soil girls look pretty healthy
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