Milpa and Earthen Kilns

Kevin has done an excellent commentary on the Earthen Kiln Conjecture which I also posted over on the terra preta forum. It has made the whole rather lengthy but sometimes that can not be helped.

He integrates his knowledge of Milpa agriculture which is the three sisters transposed into the tropics. It seemed likely that this was so and it is nice to see it confirmed. I had heard of the Milpa system before but had not quite connected it to the Amazon and even Belize. It almost certainly is the precursor to terra preta.

The evidence suggests that terra preta production was ongoing, yet I am conscious that this was not necessary. Carbon sequestration at the one ton per acre per year rate would produce a base carbon content wildly beyond what was necessary.

Kevin’s remarks are in italics. Any additional remarks of mine will be bold.

Dear Robert
Robert Klein wrote:

I am reposting to my blog this article by David Bennet with Lehmann on Terra Preta in2005. This reconfirms the most critical information as well as describes the original scope of the Indian civilization itself. Again this lays out the limiting factors and fully supports my earthen kiln conjecture.
I like your Earth Kiln Conjecture, in that it sets out a possible explanation for the presence of charcoal in TP areas.

Firstly, the maize or corn exists in an environment that mitigated against its use for purely food production. There were alternatives far better suited to the non terra preta environment, starting immediately with manioc which is a rainforest friendly plant.
See:, that describes the Milpa System of primitive agriculture as is presently practised in parts of the Yucutan and Belize. Basically, a space is cleared in the jungle, and "The Three Sisters" (Maize, Beans, and Squash) were planted. This system works, and is simple but labor intensive, due to the need to clear new land every year. I would pose that Milpa was the original agriculture system, and that it evolved into the Terra Preta system, that did not require annual clearing of jungle for one crop and then 7 to 20 years of fallow. In summary, the addition of char to a Milpa Plot, would allow addition and retention of additional nutrients, to enhance growth.

I totally agree with this. Any beneficial improvement would be easily observed and copied in this system.

Secondly, the only viable source of meat protein to these peoples at this population density was through fish. Without confirmation, a pond with tilapia makes great sense. The waste from the daily meal could be readily folded into any growing seed hill. Human waste could simply have been buried in the field itself avoiding any storage.
Such a system would make sense in the context of smaller, dispersed villages. The important thing is that large villages and communities start from small villages and communities. Aquaculture was practiced in Chile or Peru, where fish were grown in the irrigation channels in the sides of mountainous terrain, where there was the grade for water conveyance. A key thing was that such "fish water" conveyed both phosphorous and potassium to the plants, in addition to the water. Tilapia are a very special fish, in that they can live on algae. Algae growth can be promoted by addition of manure to the irrigation water, and the Tilapia can grow under "green water conditions", as is employed in SE Asia Pond Culture. In larger communities, the field irrigation channels would provide a very convenient way for disposal of night soil.

I first came across Tilapia in conjunction with the Mayan ditch and bank system of producing rich gardens that was used throughout the Americas from Michigan to Chile.

> This is common practice to this day.
The making of the earthen kiln is no more difficult than uprooting the dehydrated corn stalks and properly stacking them to form an earthen walled kiln with a wall thickness of two to three root pads and an interior of tightly packed cornstalks.

Given that the Primitive Farmer went to all the work of clearing a hole in the Jungle, and given that he had one good year with a bountiful harvest, it would be a natural step to try and avoid the extra work required to clear more Jungle, to continue with the cropping/fallow cycle. It would be a lot easier to pull the maize stalks and stack them as you suggest, to dispose of them in an attempt at getting another year out of a particular Milpa Clearing. The Milpa System employs fire as a "clearing aid", and in the attached photo, charcoal is evident. Note however, that with the Milpa System, the exposed sticks and stalks would generally burn to completion. However, with Robert's Earthen Kiln Hypothesis, the "root ball walls" would tend to collapse onto the partially burned/charred corn stocks, smothering the fire, preventing it from going to completion, and thereby producing a much higher yield of residual char than would an open bonfire.

It required only one smart farmer to think this up and try it out

Obviously, any other plant material, including wood can be built into the stack as available. The earthen wall nicely restricts air flow during the burn phase and lends itself to optimization by changing the thickness. It also minimizes the amount of human effort needed which is through the roof if you are attempting to cover a pile of stubble or branches. This gives you a kiln with vertical earthen walls and a possibly domed top that can be easily covered with earth. Again, field trials will optimize this protocol very easily. The kiln could be squared of or perhaps even circular though unlikely. The only tool to this point is a strong back or two.

Nowadays, the Milpa Farmers have the benefit of steel machetes, and would be able to easily cut the stalks from the root ball. Without a machette, it would be much easier to pull up the entire stalk, and stack the stalk and root ball in the manner suggested by Robert. Certainly, the incremental effort to pull and stack the stalks would be less than the effort to move to another site and clear more jungle.

> We have gathered several tons of corn stover over perhaps an acre of land with only a little more effort than that required to clear the field and burn the waste. Now we must fire the kiln. The easy way is to take a clay lined old basket and fill it up with coals from a wood fire. Carry this ember charge to the center of the kiln top and tip the charge onto the exposed center and place the basket as a cap to the newly forming chimney.

> More clay may be necessary to widen the chimney cap. Throw more earth on top of
this to prevent breakout of the fire. Keep growing earth on any breakout points that start.
The chimney will serve to burn all the volatiles produced as the hot zone expands to fill the collapsing kiln until they are exhausted.

If the Farmers were simply trying to get rid of vegetative waste, to avoid opening up new Jungle, then they may not have been very interested in plugging up any air leakage points. Less labor would be involved is simply "stack and burn", rather than tending the earthen kiln. They had no need to burn the volatiles to completion. Indeed, the smoke would probably be beneficial, through dispersing mosquitoes and insects.

> There upon the hot zone will cool off leaving a blend of biochar, ash and earth and
ome root ends for the next kiln. And yes, we should have a lot of fired clay.

This is very interesting. Loose earth from the root balls would not be compacted sufficiently to yield the pottery shards we now associate with Terra Preta. However, the process could very well have produced "microshards" of "pottery". Actually, this "fired soil" would not be "microshards", in that the term "shard" usually refers to "broken pieces of pottery", and it would not be "pottery", in that the term usually refers to "a formed clay shape that was fired to enhance its properties." It would be expected that this would be a "low temperature firing", and it is thus not likely that the "fired root ball pottery particles" would be able to endure the ravages of 500 to 4,000 years of tropical weathering.

I would actually be surprised to see any firing taking place in the soils themselves. However, the thin clay plate sitting on top of the chimney preventing a full burn out would get hot enough to fire. Of course, they may simply have fired sun dried plates elsewhere, but so far I have seen no evidence of such kilns.

> The biochar itself will be a range of nonvolatile combustion products that will range from even dried vegetation to activated charcoal following a nice bell curve. The material can be then gathered in baskets and redistributed into the field onto the seed hills again reducing wastage and effort. I realized originally that the only ancient plant that could accommodate a high enough volume of terra preta production was good old maize. It just seemed an unlikely option for tropical rainforests. That is when I started looking for references to the pollen record. The article by David Bennett and Lehmann is one of those references that then emerged. I would like to get a full spectrum of the pollen profile since it seems very likely that while the fence rows held the food trees, it seems more likely that they also used a variation of the three sisters using some form of convenient legume. Squashes also, of course, but not nearly as important. The key point of all this is that a family can convert a field into terra preta in one short season, allowing them to repeat the process thereafter as necessary until the field is completely transformed to depth. Today, we can do the same thing using shovels and a garbage can lid.

Terra preta: unearthing an agricultural goldmine Nov 14, 2005 10:36 AM, By David BennettIn, there is reference to "insufficient period of fallow". It would indeed be advantageous to be able to extend the productive period of a Milpa, to avoid the need to clear more jungle. Weeds are a problem in a fertile soil. What they needed for sustained cropping of a given milpa area would be

1: A mulch system, that focused growth where they wanted it, and
2: Plant nutrients.

Now, people don't live in the fields where they are attempting to grow their food crops. They would live adjacent to their fields. They would, of course, be producing Nightsoil, and naturally, they would need to dispose of it. Fresh manure and night soil could

The seed hills occupy twenty five percent of the available space. The night soil can be covered with soil and placed in a new location each time. I saw this recently described in India. (it is only a problem in cold climates were breakdown is postponed.

damage the crops. For the simple reason of smell, it would make sense to have adjacent fields working on "short fallow system"... crop one field area, while applying the humanure to an adjacent area. This would allow pathogens the time to degrade to a safe level. An additional "health protection benefit" of the "Three Sisters" is that they are all "above ground crops."

They had pots made of "pottery", and these pots over time would break. It would not take long for a Farmer to discover that pottery shards make an excellent mulch, in that plants do not grow up through pottery pieces. A further benefit of such pottery mulch is that it is fireproof. It would be a relatively easy thing to simply "burn the weeds". A further benefit using such a "fireproof mulch" is that there would tend to be moisture retention below the shards, and this moisture would tend to prevent loss of organic material from the soil. Fire burning of the weed tops with pottery shards as a "fireproof mulch" would result in an increase of organic material in the soil, from the weed root system.

There is way too much ‘pottery’ to be explained by household breakage. And a clay plate or clay lined basket was clearly necessary to carry an ember charge and cap the resultant chimney. It would shatter in the heat.

> > Many soil scientists insist an ancient Amerindian agrarian society will soon
Ø make a huge impact on the modern world. They say once the intricacies and
Ø > formulation of the society’s “terra preta” (dark earth) is unlocked, the
Ø > benefits will help stop environmental degradation and bring fertility to
Ø > depleted soils. Developing and developed nations will benefit.

Ø > Milpa and Terra Preta were NOT "systems designed to prevent environmental degradation... they were systems designed to provide a supply of wholesome food on a regular and dependable basis. Certainly, obvious signs of "environmental degradation" would be dealt with, and the one concern I could see that they would have is loss of soil through erosion. Flat pottery shards would absorb the energy of falling rain, and reduce soil erosion problems.

> Orellana
>>> The story goes that in 1542, while exploring the Amazon Basin near Ecuador
> in search of El Dorado, Spanish conquistador Francisco de Orellana began
> checking the area around one of the Amazon’s largest rivers, the Rio Negro.
> While he never found the legendary City of Gold, upon his return to Spain,
> Orellana reported the jungle area held an ancient civilization — a farming
> people, many villages and even massive, walled cities.
>>> Later explorers and missionaries were unable to confirm Orellana’s reports.
> They said the cities weren’t there and only hunter-gatherer tribes roamed the
> jungles. Orellana’s claims were dismissed as myth.
>>> Scientists who later considered Orellana’s claims agreed with the negative
> assessments. The key problem, they said, was large societies need much food,
> something Amazonia’s poor soils are simply incapable of producing. And without
agriculture, large groups of people are unable to escape a nomadic existence,
> much less build cities.

> Milpa could very well progress to Terra Preta, and with the sanitation requirements for larger communities, there could very well have been a food system that evolved to support it. A classic symbiotic relationship.

> Dark earth
>>> More recently, though, Orellana’s supposed myths have evolved into distinct
> possibilities. The key part of the puzzle has to do with terra preta.
> It turns out that vast patches of the mysterious, richly fertile, man-made
> soil can be found throughout Amazonia. Through plot work, researchers claim
> terra preta can increase yields 350 percent over adjacent, nutrient-leached
> soils.

> There is absolutely no mystery or miraculous occurence here... plants grow well in nutrient rich soil and they grow poorly in nutrient poor soil. With all the burning and vegetation, it would be natural for some of the Milpa Farmers to have noticed that black soil seemed to last longer before yields fell off to the point that a fallow period was necessary. These Farmers were primitive, but they weren't stupid.

I completely agree that an association would be made between high carbon content and sustained soil fertility. I made the exact same observation as a child on our farm from what must have been a patch used to burn out a huge amount of wood during the original clearing of the land. It had the thickest and best grass on the farm.

> Many well-respected researchers now say terra preta, most of it still hidden
> under jungle canopy, could have sustained large, agronomic societies throughout Brazil
and neighboring countries.

> The "well respected researchers" don't deserve much respect, if all they can say about Terra Preta is that "... it could have sustained large agronomic societies...". They would deserve much more respect if they provided more insight into Terra Preta. :-) The above statement may have some profound content..."... terra preta, most of it hidden under Jungle canopy..." Is it perhaps possible that terra preta is simply the natural jungle soil?
> Amazing properties
>>> The properties of terra preta are amazing. Even thousands of years after> creation, the soil remains fertile without need for any added fertilizer.

This is a stretch. A very big stretch. It goes against all known "Agricultural Paradigms". Mother Nature is very strict with her rule "You never get something for nothing." The above statement would only be true if a fertile, nutrient laden soil was not used for growing, or if nothing was removed from the site as crops, or through leaching, or as food for soil organisms, or as an oxidation product..

Not so fast – the carbon grabs nutrients and holds them. Only a little is actually used each season. Without terra preta the remaining nutrients are washed away. With terra preta any fresh waste nutrients are recaptured and also made available. Thus it is no surprise that reports of sixty years without fertilization are heard. Our own system is incredibly wasteful, distorting our expectations.

> For those living in Amazonia, terra preta is increasingly sought out as a
> commodity. Truckloads of the dark earth are often carted off and sold like
> potting soil.
> Certainly, there are people who make their living all over the world bringing in topsoil, compost, and manure to areas where the soil is deficient in organic matter and nutrients.

> Chock-full of charcoal, the soil is often several meters deep. It holds
> nutrients extremely well and seems to contain a microbial mix especially suited
> to agriculture.
> Certainly, this would work. Note, however, that black soil found in a wet depression could very well have been formed naturally, without the presence of man-made charcoal. The soils are referred to a "Black Carbon" soils, and "black carbon" can occur naturally through decomposition of organic matter in anaerobic conditions.
> Thus far, despite great effort, scientists have been unable to duplicate
> production of the soil. If researchers can ever uncover the Amerindians’ terra
> preta cocktail recipe, it will help stop the environmentally devastating
> practice of slash-and-burn agriculture in the Amazon jungle. Terra preta’s
> benefits will also be exported across the globe.

> The above passage reads well, but it doesn't say much about the caliber of scientific effort being directed at figuring out how to "reverse engineer" Terra Preta!! :-)

> However, even without unlocking all of the soil’s secrets, things learned in
> the study of it are already being brought to row-crop fields.
> Among researchers studying terra preta is Johannes Lehmann, a soil
fertility management expert and soil biogeochemistry professor at Cornell
university. Lehmann, who recently spoke with Delta Farm Press, says things learned
from terra preta will help farmers with agricultural run-off, sustained fertility and input
costs. Among his comments:
>>> On how Lehmann came to terra preta research…
>>> “I spent three years living and working in degraded Amazonia field sites.
> Inevitably, if you work in the central Amazon, you come across terra preta.
> “The visual impact of these soils is amazing. Usually, the soils there are
> yellow-whitish colored with very little humus. But the terra preta is often 1
> or 2 meters deep with rich, dark color. It’s unmistakable. We know terra preta
> are preferentially cropped.”
>>> On the various properties of terra preta and its modes of action…
>>> “There are a few factors that contribute to this fertility — sustainable fertility.
> Remember, these are soils that were created 1,000 to 5,000 years ago and were
> abandoned hundreds or thousands of years ago. Yet, over all those hundreds of
> years, the soils retain their high fertility in an environment with high
> decomposition, humidity and temperatures. In this environment, according to
> text books, this soil shouldn’t exist.
>>> “That alone is fascinating for us.

> Amazonian Jungles have been in existence for much longer than the presence of Man in Amazonia. They are a natural phenomenon. They work as a result of the layer of humus on the surface of the jungle floor that captures available nutrients and releases them to jungle vegetation. An abandoned Terra Preta plot could be expected to remain fertile for a very long time, PROVIDING THAT no crops were removed from the site.

Except this is in contrast with the rapid fertility loss of all tropical soils because of the rapid movement of rainwater deep into the soils

>> “Among the most important properties are high nutrient concentrations
> (especially for calcium and phosphorus). Most likely, this is linked to a
> unique utilization of agricultural and fishery waste products.

> Certainly, one would expect higher levels of soil nutrition in the vicinity of human habitation, where they had a nightsoil and food waste resource that was at the same time, a disposal problem and a tremendous agricultural resource.

>> “We believe that fish residues are an important portion of the high
> phosphorus concentrations. Phosphorus is really the number one limiting
> nutrient in the central Amazon.

> Near River/Lake systems, natural fish could provide a good source of protein, and fish bones for fertilizer. Pond Culture may have been employed further away from rivers and lakes. Human and animal manures resulting from "new phosphorous" being brought into the area as a result of the people "importing" foods from outside the community would also result in an "above average phos level.

Also terra preta does not let the unused phosphorus to escape.
>> “Another interesting aspect of terra preta’s high fertility is the char
> (charcoal) content of the soil. This was deliberately put into the soil by the
> Indians and doesn’t only create a higher organic matter — and therefore higher
> fertility through better nutrient-retention capacity — but this special type of
> carbon is more efficient in creating these properties.
>>> “You can have the same amount of carbon in terra preta and adjacent soils
> and the infertile soil won’t change. Terra preta’s abilities don’t just rely on
> more carbon, but the fact that its char and humus is somehow more efficient in
> creating beneficial properties. That’s the truly unique aspect.”

> This is very interesting. He might be differentiating between "Black Carbon Soils" that contain "pyrocarbon" and those that only contain "natural black carbon." It is also possible that on the "poor" Black Carbon Soil plots, the Cation Exchange Sites on the charcoal and natural black carbon may be occupied by cations that were not beneficial to the plants, and thus unable to hold the nutrients that were the "bottleneck to growth."

> Having lived in the Amazon and studied it, how much terra preta does
> Lehmann believe there is?
>>> “There are no precise numbers of how much terra preta there is (in
> Amazonia). No one has done any large-scale investigation of that. It’s very
> difficult to find out in the Amazon’s jungle environment. Suitable
> remote-sensing techniques haven’t yet been used.
>>> “So (the 10 percent) estimates sometimes cited are crude extrapolations from
> the few areas we’re familiar with. But we know that in familiar areas there are
> huge patches of terra preta. These are hundreds of hectares large. When there
> have been maps produced of areas containing terra preta — say an area around a
> stream — patches are everywhere.
>>> “It is also true that terra preta is widespread. Almost anywhere in the
> central Amazon, you can step out of the car and ask a local ‘Is there any terra
> preta around?’ and they’ll show you. It’s everywhere.”

Effort should be made to determine if these Black Carbon Soils were the initial result of natural black carbon formation, and if the Anthropogenic contribution of charcoal to Black Carbon Soils was an incidental result of working a natural black carbon soil.

>> What were the Indians growing? Tree crops? Row crops?
>>> “There has been some pollen analysis. It suggests manioc and maize were
> being grown 2,000 to 3,000 years ago. In the pollen bank, these crops didn’t
> pop up sporadically but in large numbers.
>>> “But all kinds of crops were grown by the Indians. Palm trees,
> under-story fruit trees, Brazil nut trees — all were very important.”
>>> On the differences between slash-and-burn and slash-and-char agriculture…
>>> “We have very good indications that the Amerindian populations couldn’t have
> practiced slash-and-burn and created these soils.

> This statement should be clarified. There is indeed very good evidence that the Mayans in the Yucatan have indeed been practising "slash and burn" agriculture on a sustainable basis for thousands of years. Milpa is "slash and burn" on a patchwork basis.

>> “It’s also highly unlikely that a population relying on stone axes would
> have practiced slash-and-burn anyway. The normal soils are so poor that with a
> single slash-and-burn event, you can only crop without fertilizer for two years
> at most. Then the soil has to be left fallow again.

> Yes, that is what the Mayans found also. This is where the addition of Humanure could have led to sustained "single site tropical agriculture." Additional nutrients would give immediate feedback to the Farmer, and would encourage him to do it again next season"

>> “Primary forest trees have a diameter of 2 or 3 meters. If all you had was a
> stone ax in your hand, you’d find a different way to deal with agriculture than
> felling these huge trees every two years.

> Huge trees take a long time to grow, especially in nutrient poor soils. The cycle time of cropping a Milpa Site is about 7 to 20 years; replacement trees would be nowhere as large as 2 to 3 meters in diameter. Note that such large trees can be easily taken down by primitive technology.... simply chop or burn the anchor roots and wait for the first good windstorm. When the tree fell, it could be disposed of by burning. These tree stems and branches could have been a significant source of charcoal for the site.

The easiest way was to simply girdle all the trees and come back in a couple of years. The voracious climate would be quickly reducing the remaining material.

>> “The difference between (the two systems) is the slash-and-char wouldn’t
> burn in an open fire. Charcoal would be produced under partial exclusion of
> oxygen. We envision that happening by natives covering up piled up logs with
> dirt and straw. These charcoal-making systems are still being used around the
> world.”

In the photo referenced above, there is clear evidence of charcoal having been produced, and there is no evidence of effort been expended to prevent total burning of the wood.

>> How close are researchers to duplicating terra preta?
>>> “We’re working intensively. We don’t need to take any terra preta anywhere.
> What we want to do is become knowledgeable about how terra preta was created
> and then create it elsewhere with local resources.
>>> “Research on this is ongoing in Columbia, in Kenya. I have research
> colleagues in Japan and Indonesia also working on this. At the moment, there is
> a lot of excitement but there’s a lot of work to do.”
> It would indeed be interesting to know the avenues being pursued by the various researchers.

>> How terra preta could help industrialized countries…
>>> “We envision systems based on some of the principles of terra preta. And
> this isn’t just for tropical agriculture. This could be very important for U.S.
> agriculture.
>>> “Terra Preta could mean a reduction in environmental pollution. What works
> as a retaining mechanism in Amazonia could work in the United States where
> there are concerns of phosphates and nitrates entering groundwater and streams.
> We have only begun to realize the potential of how this could reduce pollution
> in industrialized countries.

> "Pollution in industrial Countries" was not a concern of the Amazonians. Having a fertile soil and a secure food supply was a concern. Segregating Municipal Sewage from toxic Industrial Waste should allow safer and more widespread application of Municipal Sewage into agricultural systems, reducing such sewage pollution

>> “Luckily the principles of creating bio-char soils will be very similar no
> matter what area of the world you’re in. Results obtained in Brazil will be
> pertinent for the United States.
> One should be careful here. There are many very fertile "Black Soils" throughout the world that have "Natural Black Carbon", and where there is no "bio-char" that was made by a pyro process.

>> “In terms of widespread adoption, it’s still some way away. There are still
> knowledge gaps. For instance, we know there are important differences in the
> effects of bio-char on soil fertility depending on what material you use and
> what temperature and under what conditions the char is produced. That’s
> something we should be able to resolve within a year or two. Once that’s done,
> we can take the systems to Extension Services around the world and make larger
> scale, on-farm research plots.

Fertilizer additions seem to be an important part of the research work. Little is said about the importance of fertilizers and nutrients, the emphasis is primarily on the "bio-char", with little apparent recognition of the importance of "natural black carbon" in the soils.

Best wishes


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