Dogs eat soil because of mineral deficiencies, upset stomachs, medical conditions or compulsive behavior. The best way to determine why a particular dog is eating soil is through close monitoring of the behavior and consultation with a veterinarian.
Occasional eating of soil is not necessarily a bad thing for a dog. In some instances, the dog may have an upset stomach; soil can help dilute the cause of the upset stomach. For example, if the dog ate a plant that did not sit well in his stomach, he may eat some dirt to attempt to feel better. In this situation, the dog is likely to eat a small amount of dirt a single time.
More common soil eating may indicate more serious concerns. The dog may be suffering a deficiency of some essential nutrients which causes him to crave soil. Soil contains many nutrients, though it is not a reliable way for a dog to get nutrition. By closely analyzing the nutrients in the dog's diet, it is possible to gauge whether he is receiving the necessary minerals he needs for good health.
If the dog has a healthy diet but still frequently eats soil, more serious conditions are possible. The dog may suffer compulsive behavior to eat soil; if the dog exhibits many other effects of stress, he may be compulsive. Alternatively, the dog may have a medical condition that makes him crave soil or that makes him have frequent stomach issues. A veterinarian can help diagnose these conditions in frequent soil eating dogs.