As an effective preventative for soil-based larvae that may be included in my bagged medium of choice, I use cold
pressed orange oil, 2oz./gal water when I pre moisten soil before transplant with great success and reapply at 2-3 week intervals. Organic soil auto grower.
It would be extremely rare to buy a good organic bagged soil that has quality compost in it and it doesn't have fungus gnats.
All you have to do is simply water with BTI (Gnatrol) for 3 weeks and then add Rove Beetles and no more fungus gnats.