This resolves an issue that was bothering me from the beginning and I think is now totally clarified.
It was obvious that a full blown Terra preta soil developed over decades, if not centuries and entailed a lot of ongoing effort every year. Yet it was also obvious that even one year’s effort gave you a productive soil to work with. Now we have a clear resolution of this problem.
Terra preta was solely associated with the settlement site itself where we could expect each family to sustain a hectare of land. The use of an ad hoc earthen kiln would convert a season’s supply of plant waste and cultural waste into a mound of soil and biochar. The clay shards were likely there only because of breakage. Human waste and the like were likely buried in the general waste tip. Every season, this tip was packed down and covered first with a layer of palm fronds to keep the final layer of soil from initially smothering the burn. This was then fired to produce a load of soil and char. This enhanced soil was then carried to the seed hills.
When we come to Terra Mulata we have a slightly different story. We are now dealing with exterior fields that were producing large crops likely to support the state itself (taxes and trade inventory). Corn is a great way to pay your taxes and a great way to store a surplus for much later expenditure and is thus a great example of what is possible.
In this case, they only put in enough char to preserve the soil fertility. There are also no cultural artifacts such as clay shards eliminating the hypothesis that the shards had anything to do with the manufacture of Terra Preta.
The abstract shows that the agricultural culture was very well managed with the existence of large field monocultures not unlike today. You must appreciate that this existed when the only source of energy was man himself. The rise of large field farming in Europe at least had oxen and horses to support it. The diversity of crops is also a surprise. Where were the markets?
We are slowly overcoming the centuries of scholarly dismissal of this culture’s wonderful achievements and are now learning from their achievement.
An earthen corn kiln likely works just fine without the special use of wet clay. And it is very clear that all our tropical soils can be converted to Terra Mulata within a much shorter time frame than expected and does not really need to done too often after initially established.
Going through the numbers again for the corn kiln in particular we have the following;
A given acre of land is cleared by fire. The three sisters are planted in seed hills occupying about twenty five percent of the land. The three sisters consists of corn, legumes for beans and nitrogen fixing and the odd squash to provide soil cover, lowering weed problems and partially protecting the soil from rain erosion.
At the end of season, the dehydrated corn stalks are pulled and used to produce an earthen kiln. There is about ten tons of stover per acre. This will reduce to a little less than two tons of char and admixed soils. Once done, the soil char mix is carried in baskets back to the seed hills.
Eventually the build up of charcoal will naturally migrate into the surrounding soils through any tillage done for other crop types.
Terra Preta and Terra Mulata in Tapajônia: A Subsidy from Culture
Joseph M. McCann
Abstract. The large, multi-ethnic chieftaincy centered on Santarém, Brazil disappeared rapidly in the face of early European slave raids and subsequent missionization, but its physical legacy persists. Ornate ceramics, bamboo forests, relic crops, roads, wells, and manmade waterways in association with patches of anthropogenic dark earth corroborate 17th century chroniclers’ depictions of settled farmers. The evidence suggests the people of Tapajônia lived in permanent settlements and practiced intensive agriculture; they extended the ranges of useful plants, established plantations of fruit-bearing trees, and enhanced soil fertility through inputs of ash, organic mulch and household wastes, creating two kinds of persistently fertile dark earths, the classic terra preta formed under settlements and the somewhat lighter, less chemically enriched terra mulata of agricultural contexts.
Patches of terra preta and terra mulata range from less than 1 ha to more than 100 ha, and are abundantly distributed throughout the region. The spatial distribution and landscape orientation of dark earth patches do not appear to significantly favor varzea or other riverine locations including bluffs, and the largest expanses are located in interior settings. This pattern does not fit the expectations of existing models of Amazonian settlement and subsistence. Rather, it suggests a strategy for optimizing access to a wide range of important resources within a complex landscape mosaic. In addition to inherent ecological factors, the choice of settlement and field locations may have been influenced by access to trade and kinship networks, vulnerability to attack, seasonal transhumance strategies, and cumulative anthropogenic modifications of the landscape.
These modifications, including the enhancement of soil fertility and concentration of useful species and crop germplasm, continue to benefit today’s caboclos, in contrast to most recent development in Amazonia (e.g. mining, ranching, logging). As such, they are an important subsidy from an "extinct" culture. However, access to dark earth is limited by population growth and changing land tenure systems, and the techniques (such as applying mulch and ash, and inoculation with microbiota) which led to their creation are no longer practiced by today’s shifting cultivators. Furthermore, the cultivation of dark earths destroys archaeological artifacts and stratigraphic context that could shed much light on these practices. Clearly, further study of the processes of creation and persistence of Amazonian dark earths are warranted, so that they may serve as models for the development of high-yield, land intensive, yet sustainable land management strategies in the tropics.
J. McCann, Department of Social Sciences, New School University, New York, NY 10011. Email: email@example.com; firstname.lastname@example.org