Hello and thanks for researching this very important question!
Trying to retrain a dog to ignore something they find pleasant is quite difficult.
It is a natural dog behavior to dig in the ground for mice, grubs and roots. In this way, the beginning of dirt-eating is "normal". Ingesting large amounts of soil, or being obsessed and fixed on dirt-eating is not normal.
Training a dog to LEAVE IT should be undertaken with a calm attitude, since dogs are "cross" with each other when they are jealous and competitive. You are trying to get the point across that the behavior should just STOP (and that you don't have any particular feelings about it).
Start LEAVE IT training with your pet on-leash at all times when outdoors around temptation. Walk her past her Favorite Holes. When she shows an attraction to the hole/dirt, tug gently and quickly to the side while saying, "LEAVE IT". Your tone should be the same as if you were saying, "SIT" for a treat. Walk away from the dirt and engage in a "More Interesting" behavior of your choice.
Since it has taken about a year for this behavior to reach this level of obssession, you need to be very patient and persistent. Make sure to have distractions with you during training...treats for Good Behavior and toys to play some interactive games that are more interesting than digging/gulping.
Video of LEAVE IT training here.
IT IS IMPORTANT TO KNOW: Since she was an adult when this started...we have to consider medical reasons that the brain might be doing this.
Dogs can develop a number of neurologic and behavioral side effects from illnesses such as hypothyroidism, liver or kidney disease, Cushings Disease, and diabetes. These illnesses are diagnosed by having some simple blood tests run, and should be ruled out right away in any adult pet that has changed its behavior.
Slow-growing brain tumors can also lead to strange changes in behavior and temperament. "Pica", or the ingestion of non-food items, although commonly associated with nutritional problems, can be caused by neurologic deficits, and has been documented in dogs that have one of the lymphoma/leukemia types of cancers.
Make sure that you are feeding her the best-quality food that you can afford, work on formal re-training to LEAVE IT, and have her examined thoroughly for medical problems.
If you need additional support at this time, please click "Reply", otherwise I thank you in advance for your "Accept".