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Frequently Asked Questions about Biochar

  1. What is biochar?
  2. What can biochar do?
  3. How is biochar produced?
  4. How do we know that biochar helps increase crop yields?
  5. How can biochar help farmers?
  6. How does biochar affect soil biology?
  7. How does biochar affect soil properties like pH and CEC?
  8. Can you add biochar to alkaline soils?
  9. How long does biochar last in the soil?
  10. How much CO2 can biochar potentially remove from the atmosphere?
  11. How does biochar work to reduce emissions of greenhouse gases other than CO2?
  12. Is biochar production sustainable?
  13. Does IBI advocate adding carbon derived from coal, old tires or municipal solid waste to soils?
  14. Could biochar impact climate through changes in soil albedo?
  15. Could black dust from biochar have an impact on climate?
  16. What are the costs and benefits of producing and using biochar?
  17. Can biochar be patented?
  18. Does the rise or fall of the biochar industry depend on comprehensive international agreements on climate change?
  19. What kind of biochar should you add to your soil, how much should you add, and where can you buy biochar?
  20. Can biochar be added to soils immediately after production?
  21. Is it true that most of the biochar added to soils is exported to rivers and oceans?

Biochar in Kiln by Kelpie Wilson

Biochar in the kiln. Photo courtesy of Kelpie Wilson

Bonechar in a carrier by Edward Someus

Biochar made from bones in a carrier for potting soils. Photo courtesy of Edward Someus.

Columbia Field Trials

Field trials in Colombia with different concentrations of biochar. Photo courtesy of Julie Major.

Biochar Pot Trials

Biochar pot trials in the greenhouse. Photo courtesy of Edward Someus.

1. What is biochar?

Biochar is a solid material obtained from the carbonization of biomass. Biochar may be added to soils with the intention to improve soil functions and to reduce emissions from biomass that would otherwise naturally degrade to greenhouse gases. Biochar also has appreciable carbon sequestration value. These properties are measurable and verifiable in a characterisation scheme, or in a carbon emission offset protocol. 

Biochar is the carbon (C) rich product when biomass, such as wood, manure or leaves, is heated with little or no available oxygen. In more technical terms, biochar is produced by thermal decomposition of organic material under limited supply of oxygen (O2), and at relatively low temperatures (<700°C). This process often mirrors the production of charcoal, which is perhaps the most ancient industrial technology developed by humankind. However, it distinguishes itself from charcoal and similar materials by the fact that biochar is produced with the intent to be applied to soil as a means to improve soil health, to filter and retain nutrients from percolating soil water, and to provide carbon storage.

Carbonization: The process of converting feedstock into biochar through reductive thermal processing. The process involves a combination of time, heat, and pressure exposure factors that can vary between processors, equipment, and feedstocks.

See Chapter 1: Biochar for Environmental Management – an Introduction, Biochar for Environmental Management: Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

2. What can biochar do?

Sustainable biochar is a powerfully simple tool that can 1) fight global warming; 2) produce a soil enhancer that holds carbon and makes soil more fertile; 3) reduce agricultural waste; and 4) produce clean, renewable energy. In some biochar systems all four objectives can be met, while in others a combination of two or more objectives will be obtained.

See Chapter 1: Biochar for Environmental Management – an Introduction, Biochar for Environmental Management: Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

3. How is biochar produced?

Carbonization is the process of converting a feedstock into biochar through reductive thermal processing. The process involves a combination of time, heat and pressure exposure factors that can vary between processors, equipment, and feedstocks. There are two main processes: pyrolysis or gasification. Energy products in the form of gas or oil are produced along with the biochar. These energy products may be recoverable for another use, or may simply be burned and released as heat. In addition, biochar can be made from a wide variety of biomass feedstocks. As a result, different biochar systems emerge on different scales. These systems may use production technologies that do or do not produce recoverable energy as well as biochar, and range from small household units to large bioenergy power plants.

See Chapter 8: Biochar Production Technology and Chapter 9: Biochar Systems, in Biochar for Environmental Management: Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

4. How do we know that biochar helps increase crop yields?

There is a large body of peer-reviewed literature quantifying and describing the crop yield benefits of biochar-amended soil. Field trials using biochar have been conducted in the tropics over the past several years. Most show positive results on yields when biochar was applied to field soils and nutrients were managed appropriately.

There is also evidence from thousands of years of traditional use of charcoal in soils. The most well-know example is the fertile Terra Preta soils in Brazil, but Japan also has a long tradition of using charcoal in soil, a tradition that is being revived and has been exported over the past 20 years to countries such as Costa Rica. The Brazilian and Japanese traditions together provide long-term evidence of positive biochar impact on soils. To read more about field trials and biochar, please see the IBI publications page.

While the larger questions concerning overall biochar benefits to soils and climate have been answered in the affirmative, significant questions remain, including the need for a better understanding of some of the details of biochar production and characterization. Work is ongoing to develop methods for matching different types of biochar to soils for the best results. IBI is working with private and public researchers around the world to develop protocols to answer these questions.

For more information see IBI’s research summary: Soil Improvement from Application of Biochar

5. How can biochar help farmers?

Biochar provides a unique opportunity to improve soil fertility for the long term using locally available materials. Used alone, or in combinations, compost, manure and/or agrochemicals are added at certain rates every year to soils, in order to realize benefits. Application rates of these can be reduced when nutrients are combined with biochar. Biochar remains in the soil, and single applications can provide benefits over many years. Farmers can also receive an energy yield when converting organic residues into biochar by capturing energy given off in the biochar production process. In both industrialized and developing countries, soil loss and degradation is occurring at unprecedented rates, with profound consequences for soil ecosystem properties. In many regions, loss in soil productivity occurs despite intensive use of agrochemicals, concurrent with adverse environmental impacts on soil and water resources. Biochar can play a major role in expanding options for sustainable soil management by improving upon existing best management practices, not only to improve soil productivity but also to decrease nutrient loss through leaching by percolating water.

See Chapter 1: Biochar for Environmental Management – an Introduction, in Biochar for Environmental Management: Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

6. How does biochar affect soil biology?

Decades of research in Japan and recent studies in the U.S. have shown that biochar stimulates the activity of a variety of agriculturally important soil microorganisms, and can greatly affect the microbiological properties of soils. The pores in biochar provide a suitable habitat for many microorganisms by protecting them from predation and drying while providing many of their diverse carbon (C), energy and mineral nutrient needs. With the interest in using biochar for promoting soil fertility, many scientific studies are being conducted to better understand how this affects the physical and chemical properties of soil and its suitability as a microbial habitat. Since soil organisms provide a myriad of ecosystem services, understanding how adding biochar to soil may affect soil ecology is critical for assuring that soil quality and the integrity of the soil subsystem are maintained.

See Chapter 6: Characteristics of Biochar: Biological Properties, in Biochar for Environmental Management: Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

7. How does biochar affect soil properties like pH and CEC?

Biochar reduces soil acidity which decreases liming needs, but in most cases does not actually add nutrients in any appreciable amount. Biochar made from manure and bones is the exception; it retains a significant amount of nutrients from its source. Because biochar attracts and holds soil nutrients, it potentially reduces fertilizer requirements. As a result, fertilization costs are minimized and fertilizer (organic or chemical) is retained in the soil for longer. In most agricultural situations worldwide, soil pH (a measure of acidity) is low (a pH below 7 means more acidic soil) and needs to be increased. Biochar retains nutrients in soil directly through the negative charge that develops on its surfaces, and this negative charge can buffer acidity in the soil, as does organic matter in general.

CEC stands for Cation Exchange Capacity, and is one of many factors involved in soil fertility. “Cations” are positively charged ions, in this case we refer specifically to plant nutrients such as calcium (Ca2+), potassium (K+), magnesium (Mg2+) and others. These simple forms are those in which plants take the nutrients up through their roots. Organic matter and some clays in soil hold on to these positively charged nutrients because they have negatively charged sites on their surfaces, and opposite charges attract. The soil can then “exchange” these nutrients with plant roots. If a soil has a low cation exchange capacity, it is not able to retain such nutrients well, and the nutrients are often washed out with water.

See Chapter 14: Biochar effects on soil nutrient transformations; Chapter 15: Biochar effects on nutrient leaching; and Chapter 16: Biochar and Sorption of Organic Compounds, in Biochar for Environmental Management: Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

8. Can you add biochar to alkaline soils?

Most biochar trials have been done on acidic soils, where biochars with a high pH (e.g. 6 – 10) were used. One study that compared the effect of adding biochar to an acidic and an alkaline soil found greater benefits on crop growth in the acidic soil, while benefits on the alkaline soil were minor. In another study, adding biochar to soil caused increases in pH which had a detrimental effect on yields, because of micronutrient deficiencies which occur at high pH (>6). Care must be taken when adding any material with a liming capacity to alkaline soils; however, it is possible to produce biochar that has little or no liming capacity that is suitable for alkaline soils.

See Chapter 5 Biochar: Nutrient Properties and Their Enhancement, in Biochar for Environmental Management: Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

9. How long does biochar last in the soil?

The stability of biochar is of fundamental importance in determining the environmental benefits of biochar. There are two reasons why stability is important: first, stability determines how long carbon (C) applied to soil as biochar will remain sequestered in soil and contribute to the mitigation of climate change; and secondly, stability will determine how long biochar can provide benefits to soil and water quality. Biochar is not a single material, and its characteristics vary depending upon what it is made from and how it is made. Most biochars have a small labile (easily decomposed) fraction in addition to a much larger stable fraction. Scientists have shown that the mean residence time of this stable fraction is estimated to range from several hundred to a few thousand years.

For more information see IBI's work on creating a Biochar Protocol and IBI’s research summary: Biochar Recalcitrance in Soil

See Chapter 11: Stability of Biochar in Soil, in Biochar for Environmental Management: Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

10. How much CO2 can biochar potentially remove from the atmosphere?

The IBI promotes the use of waste biomass for the production of biochar. Large amounts of agricultural residues, municipal green waste and forestry biomass are currently burned or left to decompose and release CO2 and methane back into the atmosphere.

IBI has produced a preliminary analysis of several different carbon offset scenarios titled, “How Much Carbon Can Biochar Systems Offset--and When?” based on using only biomass from waste streams. Even a conservative scenario, using only 27% of the world’s crop and forestry wastes for biochar, could sequester 0.25 gigatons (Gt) of carbon a year by 2030 with biochar alone. If the energy co-product of biochar production is used to offset fossil fuel use, then the carbon offset potential of biochar more than doubles to 0.6 Gt of carbon a year by 2030.

A more optimistic scenario shows that by the year 2050, approximately 2.2 Gt of carbon could be stored or offset on an annual basis. The assumptions used that produced the high end figure are as follows:

  • Assumes 80% of all crop and forestry residues (assuming current agricultural and forestry production levels) are available to be converted to biochar and energy.
  • Assumes energy produced in pyrolysis was used to replace energy that would have come from coal.
  • Assumes significant decreases in N2O emissions resulting from biochar use.
  • Assumes an increase in Net Primary Production accrues from use of biochar in soils.

11. How does biochar work to reduce emissions of greenhouse gases other than CO2?

Recent studies have indicated that incorporating biochar into soil reduces nitrous oxide (N2O) emissions and increases methane (CH4) uptake from soil. Methane is over 20 times more effective in trapping heat in the atmosphere than CO2, while nitrous oxide has a global warming potential that is 310 times greater than CO2. Although the mechanisms for these reductions are not fully understood, it is likely that a combination of biotic and abiotic factors are involved, and these factors will vary according to soil type, land use, climate and the characteristics of the biochar. An improved understanding of the role of biochar in reducing non-CO2 greenhouse gas (GHG) emissions will promote its incorporation into climate change mitigation strategies, and ultimately, its commercial availability and application.

See Chapter 13: Biochar and emissions of non-CO2 greenhouse gases from soil, in Biochar for Environmental Management: Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

12. Is biochar production sustainable?

Of all the key factors that will support the fastest commercialization of the biochar industry, feedstock supply and sustainable yield issues are by far the most important, from both a broad sustainability perspective and from the financial and commercial points of view. This will require the sources of biomass selected for biochar production to be appropriate and be able to withstand a comprehensive life cycle analysis. Biochar can and should be made from waste materials. These include crop residues (both field residues and processing residues such as nut shells, fruit pits, etc), as well as yard, food and forestry wastes, and animal manures. Large amounts of agricultural, municipal and forestry biomass are currently burned or left to decompose and release CO2 and methane back into the atmosphere. Making biochar from these materials will entail no competition for land with any other land use option.

Biochar can be a tool for improving soils and sequestering carbon in soil. However, this technology as any other must be implemented in a way that respects the land rights of indigenous people and supports the health of natural ecosystems. The goal of biochar technology as IBI envisions it is to improve soil fertility and sequester carbon, taking into consideration the full life cycle analysis of the technology. Properly implemented, biochar production and use should serve the interests of local people and protect biodiversity.

See Chapter 20: Socio-economic Assessment and Implementation of Small Scale Biochar Projects; and Chapter 21: Taking Biochar to Market: Some Essential Concepts for Commercial Success, , in Biochar for Environmental Management: Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

13. Does IBI advocate adding carbon derived from coal, old tires or municipal solid waste to soils?

No. Coal is not a renewable resource. Biochar refers specifically to materials made from present-day biomass, not fossil carbon. Tires and other potentially toxic waste materials are not appropriate as sources of biochar for soil improvement.

14. Could biochar impact climate through changes in soil albedo?

After centuries of agriculture, soils globally have become depleted of carbon, compared to pre-agricultural conditions. Agricultural development goals include restoring carbon to carbon-depleted soils. Unavoidably, adding carbon to soils darkens them, changing their albedo (a measure of sunlight reflectance). Fortunately, darker, carbon-rich soils are more fertile and will be more easily re-vegetated. Vegetation has a lighter albedo, so the albedo problem is very temporary in nature and is not a significant issue.

15. Could black dust from biochar have an impact on climate?

Small particles of black carbon are produced from the incomplete combustion of fossil and biomass fuels. When deposited on snow and ice, they are able to absorb heat and energy. The smallest black carbon particles associated with biochar production and application are much larger, in the millimeter range, than the particles associated with global warming, in the nanometer range. Thus application of biochar would result in little opportunity for long-range transport and deposition into the sensitive Arctic and mountain regions.

Dust is a certainly a concern with biochar application, but best practices require that biochar applications be done during periods of low wind to prevent the blowing of fines. Agricultural techniques already exist to apply powdered fertilizers and other amendments. Several techniques are available to help keep wind losses to a minimum: biochar can be pelleted, prilled, mixed into a slurry with water or other liquids, mixed with manure and/or compost, or banded in rows. The optimization of biochar application to soil is important, and the farm technology and methods are available to do the job.

16. What are the costs and benefits of producing and using biochar?

The benefits that potentially flow from biochar production and use include waste reduction, energy co-production, improved soil fertility and structure, and climate change mitigation. Not all of these benefits are accounted for under current economic systems, but under the carbon constrained economies of the future, the climate mitigation benefit is likely to be accounted for as an economic benefit. Biochar benefits are partly offset by the costs of production, mainly hauling and processing feedstocks. Profitability of biochar systems will be especially sensitive to prices for energy and for greenhouse gas reductions and offsets.

See Chapter 19: Economics of Biochar Production, Utilisation and Emissions, in Biochar for Environmental Management: Science and Technology, edited by Johannes Lehmann and Stephen Joseph, Earthscan 2009.

17. Can biochar be patented?

While some biochar producers may be able to patent a specific biochar production process or method, there exist a number of open-source, low-cost, clean technologies that can make biochar at the home or village level, and more are being developed.

18. Does the rise or fall of the biochar industry depend on comprehensive international agreements on climate change?

Biochar offers direct, present day benefits to farmers of all sizes in the form of greater crop productivity, and efforts are underway to promote widespread testing of biochar in many different types of soils. Financial instruments that would lead to direct benefits for farmers in the form of carbon trading, for example, would certainly provide more incentives for biochar technology adoption. This has been and is being promoted by IBI at a number of UN events through the UN Framework Convention on Climate Change (UNFCCC)

19. What kind of biochar should you add to your soil, how much should you add, and where can you buy biochar?

It is important to note that not all biochar is the same. Biochar is made by pyrolysing biomass—pyrolysis bakes the biomass in the absence of oxygen, driving off volatile gases and leaving behind charcoal. The key chemical and physical properties of biochar are greatly affected by the type of feedstock being heated and the conditions of the pyrolysis process. For example, biochar made from manure will have a higher nutrient content than biochar made from wood cuttings. However, the biochar from the wood cuttings may have a greater degree of persistence over time. The two different biochars will look similar but will behave quite differently. IBI has recently produced IBI Biochar Standards to provide more clarity on the characteristics of biochar.

Some biochar materials, for example those made from manures and bones, are mainly composed of ashes (so-called “high mineral ash biochars”), and thus can supply considerable amounts of nutrients to crops. Keep in mind that this fertilizer effect will likely be immediate and short-lived, just as is the case with synthetic fertilizers. Conversely, the carbon content of high mineral ash biochars is low (e.g. < 10%), and thus longer-term nutrient retention functions will be less for a given amount of material.

Given the variability in biochar materials and soils, users of biochar should consider testing several rates of biochar application on a small scale before setting out to apply it on large areas. Experiments have found that rates between 5 – 50 t/ha (0.5 – 5 kg/m2) have often been used successfully. 

The biochar market is still in its infancy, but there are sellers of the product. You may be able to find a biochar seller in your area by searching the IBI Business Member Directory.

20. Can I use biochar immediately after producing it?

Biochar straight out of the pyrolysis unit might take some time to reach its full potential in soil, because it needs it's surfaces to "open up", or "weather". This happens naturally in soil, but the process can be sped up by mixing biochar with compost, for example. Nutrient retention with biochar is thought to improve with time, along with crop benefits. Mixing biochar with compost is a great idea, since apart from the ash (and there might only be small amounts of it in biochar), biochar is not a fertilizer in itself so the compost can provide nutrients which the biochar can help retain.

21. Is it true that most of the biochar added to soils is exported to rivers and oceans?

A study published in the April 19, 2013 issue of Science magazine titled “Global Charcoal Mobilization from Soils via Dissolution and Riverine Transport to the Oceans” examined the proportion of benzene polycarboxylic acids (BPCA) as a proxy for black (pyrogenic) carbon (BC) in dissolved organic carbon (DOC) of a number of rivers. While the paper makes an important contribution to the global knowledge base on DOC fluxes in the environment, IBI believes there are a number of clarifications needed to reduce the propagation of erroneous conclusions about biochar behavior in soil.

We concur with the finding that the export of BC to terrestrial ecosystems via rivers is significant. This should not be interpreted, however, as being greater than the export of uncharred material. In fact, the export of BC is only 10% of the total export of organic carbon, which is on the same order of magnitude or even smaller than the proportions that the authors cite for BC contents in soils of 5 - 40%. Therefore, BC in soil is not preferentially exported from watersheds.

Based on citations the authors conclude that production rates of BC exceed decomposition rates and thus “a relatively labile BC pool must exist, allowing for considerable losses from soils.” However, the studies cited acknowledge high uncertainty in the rates of BC production, and, in the case of BC degradation, do not support inferences about BC degradation via microbial metabolization—rather just total losses from soil, be it via erosion, leaching or mineralization. Based on uncertainty in both production and decomposition of BC, we believe that further research is warranted to understand BC fluxes in the environment.

Finally, the article concludes by implying that use of biochar may reduce DOC bioavailability with cascading effects on microbial and aquatic food webs. This, however, would only be correct if all biochar were made from biomass where the baseline scenario is accumulation in soil. In fact, most biochar proponents—including IBI—advocate for use of biomass feedstocks that are currently burnt, land filled or disposed of in ways other than returning them to soils. Furthermore, even aggressive scenarios of biochar addition would still only be a fraction of total annual biomass residues that are already returned to soils and the impact on DOC bioavailability would thus be small.