Please tell me I am not the only one out there with this problem!

I feed Nutro Large Breed (weight Managment), I have an 11yr old female (just lost my little boy at 10). But they both ate dirt....not every day, but quite often. My vet could find no medical reason, he thought they might have found a tasty patch of dirt that they liked. I heard there are vitamin/mineral deficiencies (Pica) that cause a dog to search for them elsewhere, but again my Vet says they are getting everything they need from their food. Anyone have any experience or suggestions? HELP!