What is Biochar?
Biochar is charcoal applied to agriculture. Charcoal can end up in the soil by many different means; for example, charred material from forest fires ends up being incorporated into the organic carbon of the soil of the forest over time.
If you rake the remaining coals from a campfire into the soil, you’ve essentially added charcoal to the soil. However, the modern discovery of massive plots of land in the Amazon which were systematically enriched by the addition of charcoal in ancient times. These soils are known as “terra preta”, or “dark earth”, and appear to have persistent fertility compared to the surrounding soils in spite of hundreds if not thousands of years without deliberate cultivation. The frequent rains in the Amazon tend to leach away water-soluble nutrients, but terra preta holds on to these nutrients.
In pre-columbian times the soil of the Amazonian forest was not very fertile and so the local people put charcoal, as well as plant and animal waste in it to nourish it. The soil responded beautifully and still today the “Terra Preta” of the Amazonian forest is considered precious and particularly good.
What is soil organic carbon?
Soil organic carbon includes any carbon in the soil that is organic (as in “from an organism”) or biological in nature rather than mineral in nature. For example, limestone and various carbonate bearing minerals contain carbon, but the way carbon behaves in these minerals is not the way carbon behaves in compost or decaying biological matter.
Examples of soil organic carbon include manure, worm castings, biochar, and the carbon component of compost and in-situ decay products of plant and animal matter in the soil, such as cover-crops which have been crimped or rolled down or ploughed under. Soil organic carbon from decay is known as humus; biochar is pyrogenic, and is not considered humus.
Even though organic carbon is not consumed as a nutrient for the plant, in soil, it offers chemical, biological, and physical benefits which support the soil biome and the plant’s access to water and nutrients.
One curious effect is the ability of soil organic carbon to improve drainage in waterlogged soils while also improving water retention in fast draining sandy soils. This comes from the combination of high surface area and high porosity. Ignoring chemical effects for a second, simply increasing surface area will improve water retention because all of that surface provides more wettable area; increasing porosity generally improves drainage because water can percolate through the material.
Benefits of increases SOC (Soil Organic Carbon) are shown at the left. Among these benefits, Cation Exchange Capacity is probably the primary benefit of increasing soil organic carbon.
Charcoal doesn’t get consumed by the plant, and it isn’t a nutrient or a fertilizer. So what exactly is it doing to the soil that makes it so beneficial? It holds in place nutrients and water, as well as increases the rate of electron exchange among microorganisms.
Here below are some details and recent findings:
Cation Exchange — a key mechanism of soil fertility
- Biomass reverts to CO2 as it decomposes. Nearly all of the mass of compost disappears within a couple years.
- In contrast, the carbon in biochar is generally stable, and resists decomposition. The fraction that does decompose does so very slowly.
- Depending on the feedstock and pyrolysis process, the recalcitrant carbon in biochar may persist in soil for decades to millennia.
The advantage that the addition of biochar to soil offers for carbon drawdown is that the carbon doesn’t return quickly to the atmosphere but instead stays in the ground.
Biochar’s Advantage — persistence in the soil
Many plant nutrients are cations (e.g.: NH3+, Ca2+, Mg2+, K+) which are water-soluble and often leach out of soil. Cation exchange (CE) sites are usually negatively charged, such as weak organic acid conjugate bases, which loosely hold on to these cations, reducing nutrient loss.
To absorb nutrients, root hairs exude CO2, which turns into carbonic acid (H2CO3) in water. The hydrogen ion released by carbonic acid displaces and releases the nutrient ion from the CE site, and the root hairs then absorb the nutrient.
With this in mind – by adding biochar to compost and compost to soil the nutrient become much more available to the plant.
The importance of Biochar for Carbon Drawdown
Considering the climate crisis and how much carbon dioxide we’ve put into the air, carbon sequestration is going to be a necessary part of addressing climate change in the long run. Reducing emissions is not enough; we must also remove carbon from the air and take it out of the carbon cycle. The climate change we see today is due to us taking carbon that was long removed from the carbon cycle, and putting it into the air by burning coal and other fossil fuels. To fix this, we must take the carbon out. The challenge is daunting: billions of tons of carbon have been put into the air from humanity’s use of fossil fuels. Our options are limited.
The widespread use of biochar produced from agricultural and forestry waste is one of the most promising ways to accomplish this. There is a two-fold need. Agricultural land worldwide has been depleted of soil organic carbon by chemically intensive agricultural practices that let farmers reap diminishing returns using fertilizer rather than investing in the quality of the soil for the long term; biochar can address this problem. At the same time, the quantity of soil organic carbon this represents is massive— billions of tons— enough carbon holding capacity to sequester the carbon we’ve put into the air by burning fossil fuels.
See http://soilnotoilcoalition.org/ for more information on the phenomenal carbon-holding capacity of soil. Soil not only holds carbon, it does so productively, increasing the plant life and its fertility while doing so.
- Putting biochar in the soil effectively takes its carbon content out of the carbon cycle for the foreseeable future.
- Unlike sequestering CO2, which takes two oxygens for every carbon sequestered, biochar sequesters carbon itself. And it does so while producing energy if the char comes from gasification byproducts.
- Putting biochar in soil is essentially “reverse coal mining”— taking carbon dioxide from the air, making solid carbon, and burying it in the ground.
The carbon removed from the atmosphere by biochar is not merely the carbon in the biochar, though this portion is the most long-lasting fraction. The whole point of biochar is to support the soil biome and the plant life that depends on it. Not counting the plants themselves, the carbon that ultimately gets captured, though not technically sequestered, includes an ongoing load of plant exudates, which the soil fungi use to grow their own structures made of carbon. The result is that the biochar results in much more carbon captured than the carbon content of the char itself.
Soil carbon and the carbon built up around it have the potential to take out 6 gigatons of carbon from the atmosphere, more than half of the estimated 10 gigatons of carbon we need to remove to correct climate change in the long term. See this article by Erich Knight: The Civilization of Soils; Hall Marks of The Unintended & Intended Anthropocene about the carbon multiplier effect and the importance of carbon in soil health
The Carbon Multiplier Effect
In summary, each unit of black carbon (humus and biochar)
- Holds 6 units of water
- Supports 10 units of green carbon (plant exudates) in soil
- Which grow 10 units of white carbon (fungal mycelium)
- This represents a massive amount of carbon capture triggered by the addition of black carbon!
Charcoal is made by burning wood with insufficient oxygen; the heat is only enough to induce pyrolysis. In the old fashioned charcoal mound, wood is piled around a central pole until it forms a mound. The mound is then covered in twigs and leaves, and this cover is pasted over with a thick shell of mud. Small openings around the bottom of this mud shell enable air to leak in.
An opening at the top of the mound is used to light the feedstock; the fire descends into the mound, but with oxygen access restricted, it chars the feedstock rather than entirely consuming it. When flame is visible through the holes at the bottom, the material has charred all the way to the bottom, and the holes are plugged to stop the process. The mound is then left for several days to cool off, since the mud shell is a good insulator; breaking it open too early can cause the charcoal inside to ignite upon accessing oxygen. This method is still practiced in traditional societies all over the world.
With this method much of the combustible volatiles from the wood escaped out the top of the mound. This results in a great deal of air pollution and is inefficient, since the energy content of the escaping smoke does not contribute to making char.
Modern char production processes force the smoke to come out near the bottom of the vessel containing the feedstock, where it ignites, contributing heat to the production of char. This is much more efficient, and greatly increases the yield of char, since less raw material is consumed for the production of char. It is also a lot less polluting since the smoke is consumed rather than being released into the atmosphere.
However, as you can see in the picture on the right, even modern retorts waste a tremendous amount of energy; the heat from those two flames basically dissipates without contributing any useful work.
Our char is pyrolyzed at low temp pyrolysis temperatures (300˚-500˚C), then passes through a brief exposure to high temperatures in the combustion zone . Then, it resides in the reduction zone (900˚-600˚C) until purged by the char removal system.
5% of the feedstock becomes char; this process is optimized for gas production, which consumes char.
The char produced is ideal for co-composting.
The benefits of high temperature Biochar
As biomass is heated, it hits a temperature range where it loses a large portion of its mass as smoke. What remains is charcoal, which roughly corresponds to the fixed carbon fraction of the biomass. From 400-600˚, the remaining carbon continues to diminish, as higher temperatures volatilize a diminishing fraction of the remaining carbon. Higher temperature processes result in increased recalcitrant carbon.
This famous graph, produced in 2007 by Johannes Lehmann, the pre-eminent biochar researcher, suggests that the optimal temperature range for manufacturing biochar is between 450 and 550. It was thought that at higher temperatures, char is liable to repel worms, but we now know better. This is not to say that this temperature range is bad, only that there are more factors that need to be considered. With so many variables influencing the quality of biochar, there is not one optimum char for all qualities. For example, we now know what the temperature of processing influences the conductivity of biochar.
Higher temperature char is more conductive, and this conductivity confers significant benefits, facilitating many biological processes carried out by soil microbes. Higher temperature char also has a larger recalcitrant fraction, and this recalcitrant fraction persists in the soil for longer.
Variability in biochar quality: production+feedstock
High-Temperature Char Characteristics
Our Biochar is a high temperature process char with extremely high porosity and fairly high conductivity. However, it is also fairly alkaline, due to the soluble ash content left from the gasification process. Co-composting the char should neutralize the alkalinity to a great extent.
Co-composting should also increase the CEC and water holding capacity even further; the cation exchange organic acid groups formed as compost materials ferment end up coating the massive surface area. These not only facilitate cation exchange, they also increase water retention.
- 80% fixed carbon, 32.5% recalcitrant carbon
- Extremely high surface area: 84.4% void space
- Electrical Conductivity: 28.6 mmhos/cm
- Alkaline: pH 10.2.
- Water holding capacity: 46ml per 100 grams of dry char
- CEC: 68.8 meq/100g dry. (Good soil has about 20 meq/100g)
Remember to Charge Your Biochar
- Fresh biochar acts like an active carbon filter; if added directly to soil it absorbs nutrients from the soil until it is loaded, suppressing yields for several seasons.
- Unless properly wetted, biochar may irritate worms and other large invertebrates, and drive them away from the area of application. Once wetted, biochar is more hydrophilic than hydrophobic.
- Fresh biochar may resist wetting due to gases adsorbed on its surface. Time and temperature help with wetting biochar.