Earth worm castings. I raise my own worms now but, I had good luck with 'Wiggle Worm" brand on Amazon. It won't burn your plants even it is in full contact with your plant. If you don't use a mulch to cover it, it will get a bit crusty.
There are insects in with the castings {Hypoaspis Miles} and the love to eat Gnat larvae. It is much safer than chemical remedies! Any other questions, just ask!