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.