Biochar, The Soil Capacitor

Work With Nature

David talks about his biochar experiments and that got me thinking…

Recently I watched a great talk about the negative priming effects of pyrogenic carbon on soil organic carbon that you may find interesting:

Extrapolating from Silene’s results, when biochar concentration is high enough (~3%) there should be a halving of soil organic carbon (SOC) priming, and this should cause a doubling of SOC sequestration and effectively grow high carbon content Terra Preta soils faster. This correlates well with other research I’ve seen by David Johnson.

What the biochar is doing is interesting. I’ve hypothesised that microbes change metabolic strategy in the presence of enough carbon and in particular high electron transfer biochar, as recently biochar has been shown to increase electron transfer within soils.

So in addition to nutrient sorption, biochar may be acting as a sort of microbe electricity grid, and moving their metabolism from one of oxidation to reduction as they get their energy from the grid, thereby facilitating more SOC sequestration.

If this is the case, to facilitate this we may want high electron transfer biochars that have large surface areas that are effectively many aggregate soil capacitors, which made me think of Robert Murray-Smith’s recent videos in which he creates his own graphene inks for batteries and capacitors, and has been recently been talking about his strange capacitors.

I know from other research that the most productive soils long-term are those that are most connected ecologically, not fungal dominated, though that helps up to a point, and creating these connected soils is important if we want productive systems. This electron transfer effect that biochar has may be one small part of the puzzle along with plant roots, mycorrhizal fungi and other interconnected ecosystems we’ve yet to discover.

Also, if I calculated correctly, in Silene’s video, 450C carbon-13 tagged biochar soil appears to respire at a rate about 13x slower than SOC, so it’s not going to stay around forever.

Biochar to Terra Preta aka “Black Soil”

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.

terra preta 60cm.jpg

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.