Biochar: cutting-edge technique with ancient roots

photo l. fowler

Dating back thousands of years to a civilization in the Amazon Basin, soil scientists have analyzed regions of dark rich soil found within historically inhabited areas and found high concentrations of charcoal and organic matter. The roots of what is called biochar today, is found in the disposal methods used by ancient Amazonian inhabitants • where wastes were “baked” under a layer of soil producing a soil amendment. This method, know as pyrolysis, a process in which organic waste (wood chips, byproducts of agriculture) is burned with little or no oxygen, producing oil and a synthetic gas called syngas, and a solid residue that resembles charcoal or terra preta, literally “black earth” in Portuguese. This amendment, along with other organic and household wastes, over the centuries modified the soil horizon. This buried terra preta covers up to 10% of the Amazon basin today and retains a remarkable amount of carbon. So recent interest in biochar production and mixing soil is debated as an option for atmospheric carbon sequestration and providing synchronous benefits for bio-energy co-production and soil enhancement.

“It has an appealing symmetry to it: If we got ourselves into this climate mess by digging up and burning coal, maybe we can fix it by creating some more coal and putting it back into the ground.”

Dave Levitan, Journalist

It is the hypothesis that humans deliberately developed a process of “slash-and-char” to create fertile soil. The process, using the plant material or crop remains, was cut and ignited, then left to smolder, which eventually produced char. The process isolated most of the carbon in the vegetation, creating a particularly hospitable amendment, which in turn benefited micro-organisms that transformed the degraded soil to extremely rich and stable humus.

Biochar takes its place in the geo-engineering ideas that are often scorned as a planet saving climate change mitigators, like solar radiation management and ocean fertilization, but, biochar has extra added value that these other endeavors cannot claim. Biochar enriches soil and improves structure, can aid in soil compaction restoration, nutrient and water use efficiency, balance pH/acidity, can improve crop production, and other environmental benefits. In Hans Schmidts article, 55 uses of Biochar – Ithaka Journal, uses for biochar include, treatment of drinking water, treatment of waste-water, insulation, air decontamination and the decontamination of earth fondations, humidity regulation, and biogas production to name just a few.

Hyping any theory for solutions to global warming will evoke responses to the contrary as tin his article by George Monbiot.

Woodchips with everything. It’s the Atkins plan of the low-carbon world…The latest miracle mass fuel cure, biochar, does not stand up”  

George Monbiot, Environmental Journalist

His biting criticism of the “Green Miracle”…turning the plant’s surface into charcoal” is purported as the work of naïve “Magical thinkers”, however, his article offers no empirical evidence or scientific research to support the commentary. Claims made that biochar improves plant growth are refuted by A. Ernsting and Rachel Smolker of Biofuelwatch  – stating that burying carbon bears little relation to the farming techniques of ancient Amazon and in some cases suppresses plant growth.

This mixture of hype and science clouds the understanding of the mechanistic model of biochar as a soil amendment. Many, myself included, have used biochar for several years with discernible positive effect, but lack the solid concepts and theories as to how the effects were accomplished because biochar is a diverse chemically complex material and its actions in soil are difficult to untangle mechanistically. My continued use of biochar as soil amendment is based on the results and the acknowledgement that some of the most resilient soils in the world contain a significant amount of natural biochar. Nature produces megatons of biochar in the process of naturally occurring wildfires, Prairie fires also generate a large amount of biochar as tall grasses quickly burn hot, close to the ground where roots start, air is excluded so the base of the grasses will pyrolyze and not burn completely. (Old, 1965).

“Recently, scientists (Mao et al, 2012) have looked more closely at the Mollisols, and found that they contain charcoal that is “structurally comparable to char in the Terra Preta soils and much more abundant than previously thought.”

Kelpie Wilson, Writer/Mecanical Engineer

Modern biochar manufacturing uses precise controls to maximize charcoal yield and quality while minimizing air pollution from combustion. Many biochar systems use green landscape waste, agricultural or forestry residues, or salvaged wood, effectively recycling material that might otherwise go to waste.

My application of biochar in the landscape, for the most part, is to rebuild soil structure due to compaction in tree sites. Since most above ground issues in trees and woody perennials are caused by below ground issues, improving soil, especially to overcome compaction, grabbed my attention when researching techniques for combating compaction during construction.

How Biochar Works

Soil has three components – chemical/nutritional, biological, and physical. The chemical component consist of macro- and micro nutrients in the soil as well as pH level and the redox potential. Biological is the quantity and function of anything living in the soil including organic matter, which is dead and decomposing remains and the waste products of once living things. The Physical component relates to the texture and structure of the soil, which is content of sand, silt, and clay and how these particles are aggregated and layered. The combination of texture and structure determines properties of soil strength, level of aeration, capacity to hold water, and rate of percolation/infiltration.

I use a holistic approach to managing trees and shrubs. Balancing all three components with direct application of fertilizers to address the chemical /nutrient component, feeding the food web and addressing the biological component with application of composts, and employing a number of mechanical tools to address the physical component including: tillage with hand tools and excavation equipment, augers, and air tillage.

These methods can be labor intensive and expensive so adding biochar to my management program has reduced other inputs and the benefits of use has enhanced nutrient use efficiency and availability and microbial habitat and respiration, but the most significant improvement has been in physical soil properties. Use of biochar has enhanced long-term porosity and has assisted in stabilizing macropores and soil aggregates against further compaction.

Benefits of Biochar

The structure of biochar, which is sheets of carbon stacked together, is what resists compaction and compares to other soil organic matter with a high surface-area-to-volume ratio and nutrient exchange capacity. the woody materials retain their internal vessel structure in the process of pyrolysis, creating a network of macro and micro pores which air, water, and soil microbe can move through.

The property of soil air-filled porosity of biochar counteracts the effects of soil compaction by restoring downward infiltration of rainwater, the movement of air to deeper layers of soil, and the ability of roots to penetrate the soil and develop more robust root systems. Micropores in biochar hold water at field capacity and can buffer both wet and dry conditions by providing both drainage and water-holding capacity.

“If you could continually turn a lot of organic material into biochar, you could, over time, reverse the history of the last two hundred years.”

Prof. Bill McKibben, Middlebury College, Founder of

Nutrient efficiency is achieved when nutrients bond to biochar until they can be used by plants and microbes resulting in reduced fertilizer use with same results and reductions in leaching and runoff of nutrient pollutants which harm waterways.

Another benefit of biochar is long-term carbon sequestration. Biochar is largely stable in the soil for decades resulting in a net increase in soil carbon and a decrease in atmospheric carbon.

“Biochar can be used to address some of the most urgent environmental problems of our time—soil degradation, food insecurity, water pollution from agrichemicals, and climate change.”

Dr. Johannes Lehmann, Cornell University, Chairman of The International Biochar Initiative Board of Directors

Limitations of Biochar

Since biochar predominantly influences the physical components directly, it can take several years to see the benefits of an application and roots taking advantage of improved conditions. Biochar is not necessarily an “immediate fix” but a long term solution. I find that biochar integrates with the soil faster when it is applied with mulch.

To use biochar as a cost effective way to remove CO2 from the atmosphere and to make a significant difference, would require an impressive scale. The practicality of this tool for use in combating climate change is still in question, but use as a soil amendment  • to restructure compacted soils, assist in water retention, decrease nutrient run-off, and improve soil fertility is not.

Further Reading

Literature Cited

Beeson Jr RC, Keller KG. 2001. Yard wast compost as a landscape soil amendment for azaleas. Journal of Environmental Horticulture. 19(4):222-25

Gilman EF. 2004, Effects of amendments, soil additives and irrigation on tree survival and growth. Journal of Arboriculture. 30(5):301-310

International Biochar initiative

Refilling the Carbon Sink: Biochar’s Potential and Pitfalls, David Levitan, December 9, 2010

Woodchips with everything. It’s the Atkins plan of the low-carbon world, George Monbiot, The Guardian

Wilson K:
How biochar works in soil,
the Biochar Journal 2014, Arbaz, Switzerland.
ISSN 2297-1114
Version of 31 th October 2014

Schmidt HP
55 Uses of Biochar
Ithaka Journal 1/2012: 286–289 (2012)
Editor: Delinat-Institute for Ecology and Climatefarming, CH-1974 Arbaz,
ISSN 1663-052

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