My Tipper would eat the entire Earth, left to his own devices. He's always been that way. I can't turn away for two seconds before he's chowing down on something in the yard. Rocks, twigs, bark, dirt, poop, mulch, grass, dead critters, walnut shells, etc... So he spends his yard roaming time with a stool-guard on, because I don't want to yell at him constantly..
He's been that way for seven years, and it has never been affected by diet changes, he's never really had digestive issues. He's eleven and pretty healthy for his age. He just likes to eat stuff.