"If you catch your horse grazing on dirt or other debris in his paddock, you might be alarmed. However, in most cases, this behavior is nothing to worry about..."
"Why Do Horses Eat Dirt?... It’s a natural part of their digestive process, and while it may look strange to us, there’s nothing strange about it to them."
"...horses that are lacking in minerals or that are otherwise nutrient-deficient may be eating dirt to fill the gaps in their diets."
"Because soil contains these microbes, horses may nibble on the dirt in their pasture to balance out their digestive systems."
"Soil contains water and salt. A thirsty horse may eat the dirt in his pasture if his trough is empty, and the salt in the dirt can make him even thirstier."