I have read that some people have microwaved their potting soil before planting to get rid of any gnat eggs. After planting, some will add sand to the top to deter the gnats. I wonder if diatomaceous earth would work as well, perhaps not as I think it needs to be dry to work as a pesticide.
http://npic.orst.edu/factsheet...
I have used the Parks seed sponges, which work very well. Their only problem is price.
http://parkseed.com/product.as...
http://parkseed.com/80-pack-re...
