We have had our 10 year old bitch for 10 months (rehomed to us) and in this first Winter with us, she has started scratching up earth and dead leaves on walks in woods and eating something. We can't see anything but earth. A passing dog owner mentioned it might be a need for minerals ??
I had a German Shorthaired Pointer who ate earth and my friend had a Flatcoat Retriever who always ate earth, both lived long lives. They also had the same diet as the other dogs in their households. I think it is just something some dogs do, could be wrong though!
Ripple eats earth - mind you Ripple eats everything - sticks, poo, leaves, seashells, bits of plastic, his blanket (and probably the cats if he could catch them ).