Snails or empty shells? Snails don't live IN soil, tho they may excavate to lay eggs.

AFAIK, they prefer soft-leaves to tough 1s - I've never experienced snail damage on ferns. What kind of fern is it?

FWIW, I use the iron phosphate pellets they sell @ Home Depot - not technically a poison - just lethal to snails & slugs.

I don't know what kind of fern, a neighbor gave it to me, and he didn't know. There were quite a few snails pretty deep in the pot, well underneath the ferns roots, so about a foot down. The were about the size of a marble. I know snails will eat leaves, but I wasn't sure about roots. The fern wasn't doing well, even though my other ferns are thriving, so I wondered if it could have anything to do with the snails.

Snails make short work of my tomatoes. I killed 15 last night.