I proceeded by developing my understanding of the constraints under which the farmers operated and investigating possible solutions. This approach should also inform other researchers looking at alternative solutions which may be out there.
It is fairly trivial to determine the time and place that the original terra preta soils were created. Archeology has pushed the time line back to 2500 years ago and to as recently as 500 years ago. It was clearly linked to an agricultural civilization with all the archaeological evidence lined up behind it. Although The apparent beginning coincided with the late European Bronze Age, I an unaware of any Archaeological evidence to suggest that we are dealing with a technology level that was any thing other than late stone age. That could still imply very limited access to some copper tools but nothing that would likely leak into the agricultural economy. Even the late European Bronze age I suspect had trouble using their only form of portable wealth to help their farmers.
So we can be fairly sure that our farmer worked with what tools could be made out of wood and stone. This is sufficient to girdle trees and to painfully do some wood cutting. So slash and burn becomes practical as does a limited wood processing industry. My best informant on this is the eighteenth century state of woodworking on the Pacific Northwest which then blossomed into the artistic explosion we know with the advent of steel axes. Cutting and splitting wood was possible but clearly not easy.
I then investigated traditional open air charcoal making which deforested much of the Eastern woodlands in the nineteenth century. Nothing like checking with the real experts who were relying on a thousand year old tradition. What is immediately evident, is that high yield charcoal making in open air is dependent on limiting air flow through the maximizing of packing ratio and the uniformity of that ratio. This is perhaps obvious but the fact that the packing ratio needs to be better than 75% is not obvious.
Packing ratio is a mathematical concept that measures the amount of open space to solid as a ratio. For example, a bucket of balls has a best packing ratio of 51%. This is not obvious.
Cutting hardwood to length and splitting out four inch blocks, which are then tight packed achieves both a 75% plus packing ratio but also good heat circulation. This is why the high yields were achieved.
To replicate the same packing ratio and heat transport with any biomass is a tall order. Most biomass is often almost unpackable, such as woodland waste or any branched crop. The simple jumbling together of waste ensures a lousy packing ratio and heat transport problems. In fact, it is fair to say that charcoaling woodland waste was also not very convenient without steel tools to cut the wood to length to get the needed packing ratio.
Once one realizes that the jungle is not a viable source for high volume ongoing biochar production, one must retreat to their crops. Recall that these fields are created first by the process of slash and burn which produces only a little charcoal which likely burns in the next cycle of slash and burn.
Again the packing ratio has a lot to say. Most of the burn happens just on the ground or above it. There is a lot less heat penetration of the soil than you would suppose. Recent comments on prairie grass misses this effect, since prairie grass has a packing ratio of possibly less than 20%, most of the heat is dumped into the atmosphere. I learned this lesson by attempting to roast a potato under a mound of better fuel than prairie grass. (the neighbors all came out to see the 'barn' burn:)).
If we want to produce biochar at all we have to grow the feedstock and then tightly pack it in order to get the necessary conditions in place. This limits us quickly to stalk plants that have a natural theoretical packing ratio of 77%.
Most grain crops seem to lend themselves to this except for their low volume on a per acre basis. Modern crops such as sugar cane would be possible if we did not use the cane. some other plants can be obviously used in this way. However, we very quickly are forced to consider corn simply because its non edible part consisting of the stalk represents a ten ton per acre source of biomass and a potential one to two ton source of char per acre.
This very high per acre yield is very necessary to the farmer because he has to see that he is visibly changing the seed bed and not expending a huge effort on haulage. Even today, this is the one crop producing enough bio mass to make terra preta practical.
The antique farmer had a waste product that he had to pull out of the ground and build into a waste stack to begin with so that he could raise the next crop. It was a likely ten ton stack since that was as far as he wished to haul this material. He then simply burned it as farmers do to this day. Even without proper packing some char was produced. It was not a big leap to optimize the packing and eventually to optimize the biochar production from this base.
I had reached these conclusions before I queried google scholar and ran down the pollen profile of the terra preta soils which immediately confirmed the predominance of corn pollen. Cassava also showed up which is also suitable for packing.
I will develop the rest of the story in my next post, but it can be found piecemeal in my earlier posts.