Guest Name? Terra Preta? Mystery soil in South America

Who was this guest?

He talked about his research in earth soil around the world. Specifically I remember discussion about South America. He said the soil, forest floor, almost seems to have been put there intentionally, as the mix of elements/nutrients(?) couldn't have occurred naturally. 

quick search results came up with a term, "terra preta" that I'm unfamiliar with but maybe it was mentioned in the episode and I forgot. 

Hopefully someone remembers his name! 

I appreciate any help 🙂