What is it? Hair. Human hair. And lots of it ... Dan Grech of American Public Media's Marketplace has the interview:
BLACKER: There is a slight nutritional difference between blond hair and black hair, but as far as a plant is concerned, they wouldn't notice the difference.
GRECH: Which is more nutritional?
BLACKER: Blond tends to be slightly less nutritional. Slightly. I'm not going to say a word.