The simple answer is, find something that is more interesting to your dog than dirt. To a dog, dirt is pretty interesting though.
The first thing I recommend is, get a toy, like a ball. I recommend the Jolly Ball on a rope.
However, the best thing that you can get for your dog to stop him from digging, is you. Go out there with your dog and play. Play the treat toss game for example. Interact with your dog and your dog will pay more attention to you, than all the other things that are going on in your backyard. Your dog is just looking for things to do. If there is nothing else to do, then the dirt becomes very interesting and once they smell something, the digging is going to start.
If your dog starts digging while you are outside together, then you can use your leash to tell your dog to stop.
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