Wikipedia tells us Terra Preta is said to have a minimum proportion of 2.0-2.5% organic matter at 50cm depth as the photo below of field-based Terra Preta indicates.
Since most naturally occurring fertile soil becomes anaerobic at about this depth 60cm (2 feet), I wanted to try and emulate Terra Preta levels down to that depth.
It’s important to note that as opposed to field sites, Terra Preta amongst village areas appears much deeper and laden with clay pottery shards that may have originally been buried humanure waste vessels.
My aim is then 60cm (2 feet) of at least 2.5% biochar.
I have created my back of the envelope calculations based on a soil that has an existing 0.5% in the top 10cm, and I’m using the cheapest charcoal available locally which happens to be this lump charcoal.
I’ve also calculated the lump charcoal bulk density and estimated it’s skeletal density and plugged that information into my biochar spreadsheet.
What I calculated is that to achieve an average of 2.5% organic carbon in that 60cm profile requires approximately three 29 litre boxes of this 10kg lump charcoal in raw form per square meter, or 9.5kg when micronised.
Notice the 8% carbon target in the top 10cm in my spreadsheet. Each subsequent 10cm layer is a linear halving of this that when all added up results in 15% total organic carbon distributed through the soil profile. When averaged over the 6 layers this results in the magical 2.5% that Terra Preta is said to have at 50cm.
As I’ve previously written, that 8% is similar to the amounts in biochar field trials that achieve highest yields. While the 2.5-3.0% appears to be the tipping point where plants are sequestering the most amount of carbon into the soil.
There’s your reason Terra Preta is so productive.
Add 15% or 9.5kg of the above micronised charcoal per square meter into the top soil, and you’re gold. Less if your charcoal has a higher bulk density. Over time and under continuous cropping without fallow land the plants roots and microbes in the subsoil will build the soil down two foot, and probably at an accelerated rate in the tropics and high rainfall zones. At over 8% the ecosystem begins to dissolve that carbon and leach it deeper into the soil profile building the soil from top down.
This may be how Terra Preta is said to grow back after mining the top soil and leaving 20cm of it to regrow at pace in the tropics, especially if the micronised charcoal is well mixed with the soil profile.
Mixing that 15% evenly into the two feet for an average of 2.5% carbon, the point where plants then sequester the most soil carbon and build soil, may be another, potentially slower option. The plants will have to build it up to 8% in the top soil for most productivity, at which point dissolved organic carbon is increased and begins to leach through the profile finding an equilibrium.
At a bare minimum to get an existing 0.5% SOC soil to a 2.5% in the topsoil and a 4% total SOC, then about 2.5kg micronised charcoal is needed, or 8.3kg of the above lump charcoal. 2.5kg when micronised.
This is all speculation on my part from my learnings.
And please note: I am yet to implement decreasing air and water volume as the depth increases into the model so take what I’ve written with a grain of biochar.
Native forest soils also tend to decline in a more non-linear manner compared to the linear grasslands when it comes the to soil carbon soil profile used in my model, so the choice of crops may affect the building of soil and the maximising of photosynthesis for a given area.
And remember that because the charcoal is pyrolyzed it is longer lasting in the soil, whereas carbon sequestered by plants tends to oxidise, why we may not see this natural process occurring in all high carbon soils.
End Over Out.
The mysteries of Terra Preta aka “black soil.” How was it formed?
Thought I’d have a stab at my own recipe. Untested as yet.
I actually came up with a couple of recipes depending on whether amendments are made in a village or field.
Firstly, in village areas I think most of the material is cooking & construction material burnt and then thrown into a hole, garbage or compost pile. I haven’t seen any baked soil layer in photos that would be reminiscent of growing on top of a deep fire pit. But I haven’t looked hard.
Materials including wood ash, charcoal, bones & egg shells were likely cooked with fire. And although it takes 842C to create calcium oxide, cooking can help expedite the breakdown of the bones that then form an excellent soil amendment.
Any cooking liquid waste like from tea, soup, broth can go on the pile too. Humanure and urine? Up to you.
This mess was probably then left to compost with other wastes like animal carcasses, plant materials, and all their clay pottery failures!
Basically the bulk of waste material the villagers would have created in daily life.
How much if any processing like crushing or grinding of the material was done is a good question, as is the kind of domesticated animals they may have kept too. Images of tribes suggest pig, fowl, and possibly bovine. Perhaps they ran the animals over the burnt wastes to crush and inoculate it with manures for them, or collected the manures for the waste pile?
However they did it, they likely had a variety of ground charcoal and rockdust for decorating themselves and their environment, so I’ve included those.
My Village compost recipe:
15% Wood ash,
5% Pyrolised Calcium (Bone, egg shell, etc),
50% Assorted fresh plant and animal material waste for microbes,
10% Ground Clay+Rockdust, Optional: River silt.
+Cooking & water waste.
To make it easier to remember my recipe I decided to simplify the proportions by starting with any amount of plant/animal matter, then adding half as much in charcoal, and keep halving with wood ash, then clay+rockdust and finally pyrolized calcium. I remember it as adding from dark to light coloured material.
I’m calling it Half Terra Preta because I was also thinking about the other half in the fields, and how they may have amended those extensive raised beds shown from air in the documentary.
I’m thinking something like large in-situ hugelkultur biochar soil mounds. Where they would slash land, let the greens rot down into the topsoil or possibly let animals on to forage and fertilize it in, then collect the larger brown materials in long and wide hugelkultur-like mounds.
Hold a feast along it, cook up animals, burn the material and bones, maybe cook in big clay vessels and have some firewalking too. 🙂
Then cover using subsoil creating those irrigation trenches on either side, while also saving the topsoil for later.
With the coals still hot it will bake the subsoil placed on top of the charcoal and keep the fire in check. Kinda like this:
The heat would likely kill those soil microbes however. Heat also changes the soil structure in a way that reduces clay dispersion from rain, important in the tropics! See:
So I thought it would then be a good idea to wait for the coals to stop burning and cooking the soil, at this point you could crush and mix the two before mounding the living topsoil back on, or just mix the lot.
You’d end up with a charcoal+bone base, baked subsoil middle, then fertilized topsoil, all ready to mix before planting out.
I call this half the Half-baked Terra Preta, and it could be the source of the village charcoal used for cooking too.
How did they make it? David gives two interpretations of his own:
Micronized biochar 20%,
Optional mentions: Bone meal, Eggshells, Milk, Pottery shards, Clay fines, Cow urine
40% Cow Manure,
60% Aged wood shavings,
2 cups Micronized Biochar,
1 Hand Wood ash,
Microbes (EM, soil, compost tea),
Sugar (jaggery, molasses),
Optional mentions: Fish fertilizer, Soy protein, Bran