Have you also found your dog eating soil at one time or another? There are several possible reasons for him to do this. It can be due to a deficiency of a mineral substance in his diet. Any diet that is labelled as a complete dog food should contain all the nutrients required by a dog.
Another possibility is that, although his diet is not lacking any nutrients, your dog has a problem with his metabolism that is preventing absorption, preventing the use, or increasing the loss of these minerals.
Although not an emergency, I would suggest that he is taken to your veterinarian. Some useful information to find out before you go to the vet are, for example, the amount that he drinks – has it changed at all (increased or decreased)? Is his appetite normal? Also look for any changes in behaviour. Note these and take the info along to your vet.
