I was able to find "Amazonian Dark Earths, by Lehmann et al. available at a1books.com for $74.50, but that's still pretty pricey. Some of the papers published in the book are available online, however. Do a google search on Amazonian Dark Earths Lehman and look for the PDF files.
http://ccat.sas.upenn.edu/fishweir/a...ricksonADE.pdf
Nobody seems to be doing much research on developing this process outside of the tropics yet, but there are some hints that the addition of bio-char to any soils increases fertility and may aid in the prevention of nutrient leaching. Lehmann, for instance, sited two studies: 1) Soil fertility increases have been observed in North American Appalachian sites around remnants of charcoal hearths. 2) Higher nutrient availability has also been shown in clayey and sandy soils in the Western United States after the addition of charcoal produced from conifer and hardwood.
A note about the pottery fragments: Most of the fragments were of undecorated kitchen pots and bowls of daily use that had been fired at low temperatures. Clay materials were often mixed with sand, crushed older pottery, and/or plant material. The fragments seem to be high in phosphates, which has been attributed to the process of cooking foods such as grains, meat and fish, and roots. I don't know how this pottery compares to terra cota, but the unglazed rough texture and low firing temps would suggest that it might be similar. I have yet to find any reference as to what effect the fragments may have had on plant growth.
