https://www.archaeology.org/issues/394-2009/digs/8921-digs-amazon-dark-earth
This article says the accumulation of the black soil started 6000 years ago, ~4000 BC. and used consciously as a soil enriching tool from 500 BC by the Pocó and Santarém cultures.
I assume those are two well separable archaeological cultures, named by some locality where they were identified the first time. Pocó predates Santarém, did the latter grow out from the former?