Now that's a strange subject.
The "practice of eating earthy or soil-like substances such as clay", etc
It's good because it absorbs toxics, aids digestion, reduces risk of Crohn's Disease in children (adding healthy bacteria within the digestive tract) and is beneficial for foetal development (presumable due to trace elements).
Despite it's historical widespread occurrence in a lot of cultures, it's gone out of favour.
Maybe because it is associated with being "dirt poor"(poor is not fashionable-an't that a comment on society :)), also due to the assendance of companies advertising dirt removers and germ killers.
I suspect that along with pottery classes, it's on it's way back.
I'll be looking for dirt/clay cookies on the shelf at the supermarket to see when it comes around again.....