How old is the dog? - one possibility is that the soil eating is due to a deficiency of a mineral substance in its diet. What is his usual food? If it is a complete diet it should contain all the nutrients required. Another possibility if his diet is not lacking any nutrients, is that he has a problem with his metabolism that is either preventing absorbtion, and preventing the use of or increasing the loss of minerals. I would suggest that you mention it to to your vet. Some useful things to note before you go are the amount that he drinks has it changed at all, does he drink more or less than before he had this habit and is his appetite normal? And also any other changes in behaviour.