There is a medical condition called "Pica" which caused a dog to eat non-food materials.  There are various causes for this condition; some believe the dog is actually suffering from some nutritional deficiency.  Others think this could have a behavioral cause and is done due to stress or boredom.  Please click on the link for more information on this subject:

http://www.petplace.com/dogs/pica-in-dogs/page1.aspx

It would be a good idea to have your dog evaluated by the vet, to find the cause of this symptom and to be treated if necessary.