If the potting soil is just plain old potting soil without any chemical fertilizers then it's not harmful for the dog and they should not get ill because of eating the potting soil.
Still you should try to keep the potting soil away from the dog or keep the dog away from any potting soil because it might cause an upset stomach.
If a dog is eating potting soil they could be lacking minerals and vitamins in their diet and the potting soil contains minerals and vitamins that the dog may be seeking.
If the potting soil has any miracle growth chemicals in it then it will make the dog sick so don't allow them to eat it or if you've added fertilizer to your plants soil the dog could get sick from eating that soil that has the chemical fertilizer on it.