Does the dog do this any place it is or just
in certain areas
in particular? If so, there may be something
in or on the dirt that is appealing to the dog. (cat urine and feces is popular). aHem.........
Could just be out of boredom and/or behavioral. You might try giving the dog something else to chew on such as a large, raw knuckle marrow bone (I used to get them at the butcher and have them cut to a larger size). Maybe try a Kong you can stuff with kibble, peanut butter, cheese, etc.