A dog that eats dirt and other things such as potting soil or just the regular dirt is usually lacking vitamins and nutrients in it's diet.
The dirt and potting soil contain those vitamins and nutrients so you may need to change the dogs diet to make sure they get a balanced diet of vitamins and nutrients that they need.
The dirt also contains iron content which could also be the reason the dog is eating dirt to increase their intake of the iron.
Dirt contains nutrients as well and is why some kids even eat dirt as they may be lacking the vitamins and nutrients and even when kids eat dirt it's perfectly fine and safe for them.
I was one of them kids that ate dirt as a 2 and 3 and 4 year old and I lived to tell about it.
Your dog will be fine eating dirt as well and they will just poop it out after it passes through their body.