The first time I encountered pyrolysis, it had nothing to do with soil.
Years ago, in my first job after studying packaging engineering, I worked at an environmental consultancy. One of the topics that crossed my desk was pyrolysis as a recycling process for plastic waste. That job did not last forever, but a former colleague once told me these topics would never let me go. He was right.
Long after I had moved on, a documentary caught my attention. It explored the idea of using charred plant material, produced through pyrolysis, as an electrode material for storing electrical energy. The same process I had first encountered in the context of plastic waste was now showing up in conversations about renewable energy and soil science.
Whether the potential was as large as the documentary suggested, I have no idea. But something about it stuck.
So I started reading. And the deeper I went, the more I realized that the real story of biochar was not about batteries or energy grids. It was about soil. About a practice that may be thousands of years old, one that modern science is only now beginning to understand. And about a material that deserves more attention than it typically gets in conversations about regenerative growing.
The Oldest Soil Amendment You've Never Heard Of
Somewhere in the Amazon Basin, there are patches of soil so dark and fertile they look like they belong to a different planet. Locals call them Terra Preta, black earth. Some of these soils have persisted for centuries, possibly much longer, and they remain remarkably productive without the inputs modern agriculture considers essential.
For researchers, the question was not simply what made these soils different, but how much of that difference was deliberately created. Pottery shards, fish bones, and charred plant material buried in the dark layers pointed toward an answer. These soils were not found. They were made.
Across the Amazon, Indigenous communities developed ways of enriching soils with organic waste, ash, and charcoal. Research with the Kuikuro people in the Upper Xingu region has shown that the deliberate creation of fertile dark earth, known locally as eegepe, is not simply a practice of the distant past. A 2023 study documented how Kuikuro communities continue to build carbon-rich soils through the managed accumulation of diverse organic materials.
That dark earth is what the rest of the world now calls Terra Preta. Charcoal is one of the ingredients that helped make these soils distinctive. Today, a related material produced deliberately for soil use is known as biochar.
Not Charcoal. Not Ash. Something Else Entirely.
This distinction matters. Ordinary barbecue briquettes are not a substitute for biochar. Their composition and possible additives make them unsuitable for garden beds. Scatter wood ash, and you get a short-lived pH boost. Biochar is different because of how it is made.
The process is called pyrolysis: heating organic material (wood, crop residues, garden waste) in an oxygen-starved environment. Without oxygen, the material does not combust into ash. Instead, it carbonizes into a porous, carbon-rich material. Think of it as a sponge with countless microscopic spaces.
Those pores are what make biochar useful in soil. Beyond its structure, biochar has another remarkable property: much of its carbon can be extraordinarily resistant to decomposition. A study by Schmidt et al. (2022) in the Biochar Journal found that biochar produced above 550°C and with a molar hydrogen-to-carbon ratio below 0.4 can retain roughly 75% of its carbon as polycyclic aromatic carbon, a molecular structure so stable it may resist decomposition for over 1,000 years. The remaining 25% is semi-persistent carbon with a projected lifespan of 50 to 100 years.
The authors argue that earlier research had consistently underestimated this durability. Most lab experiments ran for one to eight years and then extrapolated. The actual degradation curve, they suggest, is far slower than those projections implied.
Unlike much of the organic matter in compost, a substantial fraction of biochar carbon can remain in soil for centuries or longer. That difference in timescale is what makes biochar interesting as a carbon sink.
What Happens Underground
Fresh biochar in soil is like an empty apartment building: structurally sound but lifeless. The real transformation begins when soil microorganisms move in.
A 2026 meta-analysis by Zhang et al. in the journal Biochar analyzed 76 studies and 221 experimental comparisons and found that biochar increased soil organic carbon by an average of 52.4%. Part of that increase reflects the carbon added with the biochar itself, so it should not be confused with an equivalent increase in newly formed soil organic matter. The researchers also identified associations between different microbial communities and the size of that increase.
The patterns varied by bacterial group: comparisons associated with Proteobacteria showed the largest SOC increases (68.6%), while those linked to Acidobacteria (47.3%) and Chloroflexi (28.8%) were smaller. Dosage, climate, and soil properties also played a role.
Biochar can alter soil chemistry and structure, affecting pH, water retention, and nutrient availability. Whether those changes are beneficial depends on the soil and the biochar used. But some of its most interesting effects emerge through interactions with soil microorganisms. The charcoal provides the infrastructure. The microbes do much of the building.
This is why agronomists recommend "charging" biochar before use. Combining it with mature compost or other nutrient-rich materials before application can introduce nutrients and support microbial colonization. Some fresh biochars may temporarily reduce nutrient availability, depending on their properties and the soil.
Fire in the Garden
Here is where things get practical. Making biochar at home does not require industrial equipment.
The simplest method is the cone pit. Dig a cone-shaped hole, start a fire with dry branches, and keep adding layers of woody material. The key is that each new layer starves the layer beneath of oxygen while it chars. Once the material glows but does not flame, quench it with water. What remains is rough biochar.
A step up is the Kon-Tiki kiln, essentially a cone-shaped metal vessel that uses the same flame-curtain principle but with more control over temperature and airflow. Studies of flame-curtain kilns have reported high temperatures in the active pyrolysis zone and relatively low methane emissions when dry biomass is used and the kiln is operated correctly. Other emissions, including carbon monoxide and particulates, still need consideration.
The third option is a TLUD stove (Top-Lit Updraft), which burns gases released during pyrolysis as fuel, making the process cleaner and more efficient.
All three methods can work at garden scale using materials most gardeners already have: pruning waste, fallen branches, untreated wood scraps. A few things worth noting: use only clean, untreated, dry biomass. Work outdoors, well away from buildings, and check whether open burning is permitted in your area. Keep water or a fire extinguisher nearby. Make sure the biochar is fully quenched and cooled before storing it. Smoke, particulates, and carbon monoxide are real concerns, and production quality matters. This is worth taking seriously. Home-produced biochar has not been tested for contaminants. Feedstock choice and process control directly affect whether the result is suitable for soil use.
From Backyard to Raised Bed
You do not need a large property to use biochar. Charged biochar mixed into a raised bed or container can improve water retention and nutrient availability, though results depend on the biochar and the soil it goes into. In a meta-analysis of field trials, adding biochar alongside inorganic fertilizer increased crop yields by an average of 15%. Results varied with soil type, crop, and management practices.
For those making it at home, the process scales naturally. Even small-scale production can yield useful amounts of biochar, although the quantity and quality depend on the feedstock and how carefully the process is managed. A Kon-Tiki kiln serves a market garden. Biochar stores well if kept dry and free of contamination, so it can be made in autumn and applied in spring when beds are prepared.
The connection to regenerative practice is direct. Well-made biochar in soil is carbon that may not return to the atmosphere for centuries. Every charged piece of biochar is a potential micro-habitat for the organisms that build soil fertility. Unlike most amendments that break down and need replacing, biochar's structure persists, and the microbial communities it supports can develop over time.
Beyond Soil: The Material That Keeps Surprising
Here is where the story circles back to that documentary.
It turns out the same porous carbon structure that makes biochar effective in soil also makes it interesting to materials scientists. Researchers are actively studying biochar-derived carbon as an electrode material for supercapacitors, a type of energy storage device that charges and discharges faster than conventional batteries.
A 2025 review by Mishra and colleagues examined the growing body of research into modified biochar-based materials for supercapacitors. The results are promising, though the materials involved are not ordinary garden biochar. They are specially activated carbon, engineered under controlled conditions to maximize surface area and electrical properties. But the underlying idea is remarkable. A material derived from plant waste, in a specially processed form, could one day store renewable energy.
For anyone already making biochar for their garden, that is a strange and satisfying thought. You are working with a material whose full potential science has not finished mapping.
A Quiet Act of Building
There is something satisfying about turning garden waste into something that will outlast the garden itself. Biochar is not a shortcut, not a product that promises instant results. It is a long game, played in the same soil, season after season.
The fire transforms. The soil receives. And something that would have rotted in a few years becomes part of the ground for centuries to come.
Field Notes explores the intersection of soil, science, and growing — through curiosity, careful research, and a willingness to question familiar assumptions.
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Sources
- Zhang, G., Deng, L., Liao, Y., Wu, J., Zhao, X., & Shangguan, Z. (2026). Microbial regulation mechanisms of soil organic carbon sequestration by biochar application. Biochar, 8(1), 57. DOI: 10.1007/s42773-026-00575-2
- Schmidt, H.P., Abiven, S., Hagemann, N., & Meyer zu Drewer, J. (2022). Permanence of soil applied biochar. Biochar Journal. (Polycyclic aromatic carbon persistence >1,000 years; semi-persistent fraction 50–100 years)
- Ye, L., Camps-Arbestain, M., Shen, Q., Lehmann, J., Singh, B., & Sabir, M. (2020). Biochar effects on crop yields with and without fertilizer: A meta-analysis of field studies using separate controls. Soil Use and Management, 36, 2–18. https://doi.org/10.1111/sum.12546
- Schmidt, M.J., Goldberg, S.L., Heckenberger, M., et al. (2023). Intentional creation of carbon-rich dark earth soils in the Amazon. Science Advances, 9(38), eadh8499. https://doi.org/10.1126/sciadv.adh8499
- Cornelissen, G., Pandit, N.R., Taylor, P., Pandit, B.H., Sparrevik, M., & Schmidt, H.P. (2016). Emissions and char quality of flame-curtain "Kon Tiki" kilns for farmer-scale charcoal/biochar production. PLOS ONE, 11(5), e0154617. https://doi.org/10.1371/journal.pone.0154617
- Norwegian Geotechnical Institute (2024). Flame curtain kilns revolutionise biochar production. (Methane emission reduction with dry biomass)
- Mishra, R.K., Kumar, D.J.P., Chinnam, S. et al. (2025). Recent progress in post-modified biochar-based material for supercapacitor applications: a review. Materials for Renewable and Sustainable Energy, 14, 63. DOI: 10.1007/s40243-025-00339-1
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