The scientific mystery of the Amazonian anthropogenic dark earth (anthrosols) referred to variously in Portuguese as Terra Mulata and Terra Preta do Indio (Indian Dark Earth) is closer to being solved. The intricate recipe used for producing these fertile soils has been the object of longstanding scientific curiosity.
An important factor, highlighted in the latest news release on the subject, is the use of slash-and-char techniques versus slash-and-burn:
[Update: Back40 (Muck and Mystery) pulls in some quite interesting notes and comments on soil+char.]
Whereas slash-and-burn methods use open fires to reduce biomass to ash, slash-and-char uses low-intensity smoldering fires covered with dirt and straw, for example, which partially exclude oxygen.