So, can eating dirt help cure depression? I don't know, but there is an intriguing article about "germophobic helicopter parent[s]" obsessed with cleanliness and the impact that has on depression. I don't really know what to think about this article, but it was fascinating to read. Basically, it boils down to the argument that we co-evolved with certain parasites, which we have essentially eliminated from our diet by being cleaner than our evolutionary ancestors.
By the way, I loved the photo of the little girl eating a mouthful of dirt (scroll down to the end of the article to see the photo).