Not sure where the post now is, so I'll respond here.
I have seen my lambs doing it, too.
If they seem overly interested in the soil,they may have pica related to worms and need worming with a broad spectrum anti-helmintic.
When I first saw my ewe eating dirt, I brought it to my neighbor's attention -- this was when she was a lamb or a yearling, so probably 7 or 8 years ago now. He said she was lacking in minerals so we gave her a different mineral and she stopped eating dirt.
At that time she was penned inside the barn and all she ate was hay and had a handful or two of grain everyday as I got her friendly enough to walk up to me when she was put back with the flock.