or DE are the remains of fossilized diatomes. These are little creatures which has shells made of a substance which was like glass. DE won't hurt humans or pets but it WILL kill sluggs. I am so sorry I forgot to put it down when I put out my orchids last year because I wound up with sluggs in some root systems.
YUCK!!! 
I hate sluggs!
But DE will take care of them. I will NOT forget to lay down a nice layer of DE under my plants this spring when I put them outside. I will also renew it in July to ensure it remains throughout the summer. No nasty, ugly, smelly, root eating sluggs this year! HA! :twisted:
DE is available on-line from many places and should also be availabe from stores like Home Depot and Lowes.
Randy