here's a good article on this topic:
I guess my question on neem meal in the soil would be, is there any azadirachtin in Neem meal? If so, some of it will end up in the plants, you're basically using systemic Neem, right?
btw, the old advice "let the soil dry out" is bullshit! The gnats seem to thrive just as well if you wait longer between waterings. The soil must be kept wet or the plant dies, I've never seen any helpful effect from letting the soil get dry, except that it lets you apply DE and have it work for a day or two.