The Indians were able to produce powdered charcoal while consuming a lot of low grade pottery in the process. This is many tons of charcoal per acre. The manufactured soil retains fertility without significant assistance in an environment were its only competitor is low productivity slash and burn. High density settlement resulted and was almost certainly responsible for the legends of El Dorado. The Spaniards were about a generation too late and the knowledge was lost.]
Reconstructing the production protocol was tricky but is is really very simple.
It was and still is impossible to use wood economically to produce the powdered charcoal. I say impossible because the direct costs of harvesting wood is well known and the cost of producing charcoal is also well known. That implies that wood charcoal which also has to be fine ground must have a cost base approaching that of sawn wood. The sunk cost is far too high to ever use as a soil additive. This is borne out even in Africa were we see charcoal been made to take advantage of its direct cash value as fuel.
That leaves us with dry crop residue as a source material and a very productive one to boot. In the time and place, and this is almost still true today, the only crop that fitted the volume need to make the process practical is and was corn. Today bagasse could also be used. The important factor is tonnage per acre. Corn is good for ten tons per acre. Most other crops simply fail to produce enough plant material. Additionally, corn waste or stover must be removed and burned regardless.
Since it must be gathered and burned in any event, the question is how to convert this feedstock into a ton or two of powdered charcoal or more reasonably into biochar retaining both the charcoal content and some remaining plant material.
Here, the nature of the corn root itself helps out hugely. It form a flat disc, not unlike the base of a floor lamp. This dirt ball can be treated almost like a brick. It permits the building of tightly packed stacks whose outer wall is formed be tightly packed root discs loaded with mud. It is no big trick to build a vertical wall of these root discs to act as the outer shell of what is a temporary earthen kiln. It was actually a brilliant innovation by some Indian a couple of thousands of years ago.
This earthen kiln is then fired by the process of dumping a charge of glowing wood coals on the top of the stack, directly into the packed dry corn stalks, and covering it immediately with the sun dried earthen platter that carried the coals. You would then cover the top with additional dirt to maintain the integrity of the earthen kiln and let the coals do their work.
The coals will drive a chimney into the stack and all the combustion will take place inside the covered chimney. This nicely minimizes any unnecessary energy loss and maximizes combustion which goes into reducing the balance of the stover. The earthen wall even filters out any errant heavy gases as they try to escape. I suspect that it is only with the recycled gas systems of today that we can do better.
This task would be done after the corn had fully ripened and dehydrated which occurs just after harvest. The corn stalks dry out then and are still pretty impervious to wetting by rain.
Once the burn is complete the next day, one would rake out any unburned roots to throw into the next kiln and then take baskets of the soil - charcoal mixture back into the field to produce the hills for the next crop. The only tool used would be the earthen ware pottery and a strong back. Today I would use a metal garbage can lid.
This process produces enough material to salvage the field in tropical conditions for an immediate crop during the next season. Once this was understood, it became practice and was intensively employed long past its actual necessity for many thousands of acres in the Amazon.
When I first made this hypothesis on the likely protocol, I did a literature search of the Archeological data on the Terra Preta soils looking for the pollen data. Remember that corn is not your first choice of a crop plant on a rain forest soil. I was gratified to discover that the two principal crops were corn and cassava which also produces a lot of biomass but no usable root ball. This confirmed that the protocol had legs.
I am quite prepared to work with someone who wishes to run field tests at no charge since I personally think that this will revolutionize all subsistence farming generally as they can be the first adopters. Larger acreages will need kiln equipment at the least and this will be capital intensive.
And it would be great to get this going where the crop cycle is currently multiple years through slash and burn. I think particularly of the Philippines were I have had fifteen year fallow periods reported. The same must be true for a lot of land in Africa and elsewhere. The more interesting question is the fertility increases in soils now been exploited.