I have seen no alternative to the corn culture earthen kiln approach that I have proposed a few months back.
Since then we have seen film on the production of subsistence charcoal in Africa and it is very instructive. Firstly, in the modern world, everyone can get their hands on an axe and a simple saw. This makes it easy to hack everything down and to cut it up. Making this woody waste into charcoal is quite another matter.
It fails to pack well but the charcoalers are still able to create pits and to throw dirt on the burning pile to suppress the flames. This obviously will produce some charcoal, but the yield must be terrible. what is clear though is that the produced wood charcoal is poorly charcoaled at best. We see people carrying bundles of charred sticks and bulky bags of char. It makes great fuel. It is almost impossible to use as a soil additive.
Whatever lingering thoughts that I may have had in support of the charcoaling of wood for soil remediation can be laid to rest. Only a modern industrial grade charcoaler might be able to produce suitable material.
Subsistence farmers could not even begin to make wood waste work for them. They needed a helper crop. That was provided in the form of corn to the Amazon Indians.
I also think that wood charcoal was always too valuable as a fuel as is true today in Africa, to ever be crushed and folded into the seedbed. In fact a man load of charcoal probably weighs a hundred pounds and needs be carried miles back to town. That one hundred pounds needed about one ton of source material to be cut down and stacked and covered with dirt while burning. Maybe they did twice as good in terms of yield. However it worked, that man load of charcoal took two days of labor input at the least.
There is simply no way that such a production model could be used to produce terra preta. And the Indians did not have steel tools.