Hi Pagnr,
No one really knows how the origin Terra Preta was made. All added up there are THOUSANDS of square miles of Terra Preta area. Some estimates I have read say all the square area added up is larger than England. I can neither confirm nor deny that. Again, some of the reading I have done claim that the carbon dating covers a span of time greater than 1500 years. Keep in mind that in today's world, you would make charcoal in an oxygen free environment so it burns without being consumed. Now consider the cubic area.....
We were to look at simply ONE ACRE of land. 208' x 208'. Terra Preta is anywhere from 2 feet deep to 6 feet deep, and can be anywhere from 10% to 40% of the volume of that cubic space. So lets say, only 3 feet deep over 1 acre. That breaks down to 4807 cubic yards of material moved, and if we average that at 20% charcoal per volume, that is 960 cubic yards of charcoal, for a 1 square acre plot of land.
SO, you take that 960 cubic yards of charcoal and "charge it" up, however that was done, then mix it back in with dirt at a consistent ratio, and start back filling your hole. Oh, and every 6 inches of depth or so, add a loosely scattered layer of broken, once fired clay. That clay acts just like terra cotta pots do... it will absorb water when abundant and give it off slowly when water is NOT abundant. Now multiply that out by 100 acres. And multiply the effort required to do that, moving all that material BY HAND (no tractors or motorized earth moving equipment....).
There was a journal entry of some spanish ship captain who claims to have sailed quite a way up the amazon river basin, and claims to have seen huge, thriving cities filled with people that were highly civilized and cultured. NOT aborigines. Of course, the sailors do what sailors do.
The ship left and went back to Spain, but it was more than 50 years later (maybe closer to 100 years?) that another ship sailed where this one went to confirm the stories. The place was so different that the second captain thought the first one was a first rate con-man. There were no cities. Small groups of locals were obviously ignorant tribesman.
Looking back now, through the lens of history, suspicion is that the sailors spread diseases that the locals had no defense to and the population plummeted as a result. As the population dwindled, they could no longer maintain the infrastructure of the civilization and the rainforest reabsorbed the cities. Of course, in a rainforest area, the easiest resource to use would have been wood and not stone. It would not take long in that environment for wood structures, without constant maintenance, to decay into oblivion.
It's all very fascinating, but no one really knows how they figured it out, how they charged up the charcoal, or how may people they must have been able to feed if all the Terra Preta acreage were planted at the same time. The estimate is that at any specific location, they could have fed hundreds of thousands of people. This would support the claims of the original explorer to that region.
I just tried to do a quick google search and cannot quickly find documentation on this, but if I do, I will post links. I know I spent months reading about it, since I found it fascinating, along with the science behind the REMINERALIZATION movement, and I know I have seen several posts on this forum about members here who use commercial rock dust in their soils (I think Azomite was mentioned?).
I get interested in something and can spend months searching and absorbing the info. Terra Preta was an interest of mine over a decade ago, but I still feel it is a valid technigue to consider on smaller, individual scales. I guess now I should start experimenting with a process to "charge" the charcoal for use. What's one more project that could last the rest of my lifetime......
Lazarus