It is now attested that a large part of the Amazonian rain forest has been cultivated during Pre-Colombian times, using charcoal as an amendment. The incorporation of charcoal to the soil is a starting point for the formation of fertile Amazonian Dark Earths, still selected by Indian people for shifting cultivation. We showed that finely separated charcoal was commonly incorporated in the topsoil by Pontoscolex corethrurus, a tropical earthworm which thrives after burning and clearing of the rain forest, and that this natural process could be used to improve tropical soil fertility. Our paper is a contribution to the present debate about (i) the origin of black carbon in fertile Dark Earths, (ii) the detrimental vs favourable role of Pontoscolex corethrurus in tropical agriculture, (iii) natural processes which might be used to increase tropical soil fertility.