Eating dirt is a behavior that horses share with many other species of animals, including humans. Some experts believe a craving for dirt is actually a craving for certain minerals in the dirt. As evidence, researchers in Australia analyzed soil from areas in the country where horses did the most dirt-eating and found that the soil in these areas contained higher levels of iron and copper than the soil in other areas.
Eating dirt is generally considered safe unless the horse lives on sand. Then, owners should use methods to protect the horse from taking in too much sand (and developing sand colic) such as feeding hay on mats and adding psyllium to the diet.