What's the caloric value of dirt?

David and I were in the front yard on Tuesday morning after going for a walk. I was watering my new plants and I look over and there's David, sitting in the dirt next to one of the plants eating a clump of dirt. I attempted to remove the dirt, but he'd already eaten most of it. Below is a picture of him with the dirt still on his face.

That dirt was a great snack!

Goofy boy.