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.