I don't use organic fertilizers and have decent results.
For soil, maybe walking around your neighbours' backyards and asking nicely for a bucket or two will work. While at it, ask how they fertilize their yards, that will give you an idea. I presume that a car trip to the nearest wilderness is not possible? Maybe ask a friend?
There are a number of "safe" pesticide recipes on the net (tobacco infusions, oil emulsions, etc.) They tend to suffer from that very same problem - they're safe to the pests as well. Neem oil and its isolated main component, azadirachtin, while expensive over here is very effective. Anyway, you may try sticky traps or maybe a couple of Pinguicula plants with the gnats.