i was instructed to use a layer of de 1 cm thick on top of soil to kill fungus gnats. worked so so on the gnats but when i watered it in it made the top layer of my pro mix into clay and f'ed with water absorbtion. ill never use it again but perhaps in smaller amounts it would be ok. ph of de is neutral {7ish} to slightly alkaline or so i have read. there are better less messy ways to deal with gnats in my opinion: yellow sticky traps and letting soil get a little crispy between waterings combined with keeping it free of leaf litter. hope this helps someone.