Biochar is a solid material obtained from the carbonization thermochemical conversion of biomass in an oxygen-limited environments. In more technical terms, biochar is produced by thermal decomposition of organic material (biomass such as wood, manure or leaves) under limited supply of oxygen (O₂), and at relatively low temperatures (<700°C). This process mirrors the production of charcoal, which is perhaps the most ancient industrial technology developed by humankind. Biochar can be distinguished from charcoal—used mainly as a fuel—in that a primary application is use as a soil amendment with the intention to improve soil functions and to reduce emissions from biomass that would otherwise naturally degrade to greenhouse gases.

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.

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.

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-known 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.

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.

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.

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.

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.

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.

Biochar is a spectrum of materials, and its characteristics vary depending upon what it is made from and how it is made. One unifying characteristic of biochars, however, is that it mineralizes in soils much more slowly than its uncharred precursor material (feedstock). Most biochars do have a small labile (easily decomposed) fraction of carbon but there is typically a much larger recalcitrant (stable) fraction. Scientists have shown that the mean residence time (the estimated amount of time that biochar carbon will persist in soils) of this recalcitrant fraction ranges from decades to millennia.

The persistence of biochar when incorporated into soils is of fundamental importance in determining the environmental benefits of biochar for two reasons: first, it determines how long carbon in biochar will remain sequestered in soil and contribute to the mitigation of climate change; and second, it determines how long biochar can provide benefits to soil and water quality.

The carbon lattice structure made up of fused polyaromatic carbon rings is hypothesized to be the key property that confers a resistance to mineralization (conversion from organic carbon to carbon dioxide via respiration) by soil microbes that utilize organic matter i.e., hydrocarbons, as food (Lehmann et al, 2015). The energy required by microbes to access the carbon in biochar appears to be greater than that acquired when it is released. In contrast, carbon compounds in the original biomass (feedstock) are a net positive energy sources and are more readily mineralized by soil microbes.

The fused carbon ring structure of biochar can be measured in the laboratory using a range of established techniques, some low cost and relatively easy to conduct, others more sophisticated and requiring high-tech equipment that analyzes nano-structural properties. In combination with empirical (measurement-based) modelling exercises which show how biochar carbon mineralizes over time using field and laboratory incubation trials for validation, the degree of carbon aromaticity can be used to predict how much biochar would remain in soils over discrete time periods, for example 100 years or 1,000 years. Persistence is then quantified as mean residence time (MRT)—the average time that biochar is present in the soil.

Large amounts of forestry and agricultural residues and other biomass are currently burned or left to decompose thereby releasing carbon dioxide (CO₂) and/or methane (CH4)—two main greenhouse gases (GHGs)—into the atmosphere. Under biochar conversion scenarios, easily mineralized carbon compounds in biomass are converted into fused carbon ring structures in biochar and placed in soils where they persist for hundreds or thousands of years. When deployed on a global scale through the conversion of gigatonnes of biomass into biochar, studies have shown that biochar has the potential to mitigate global climate change by drawing down atmospheric GHG concentrations (Woolf et al, 2010).

According to one prominent study (Woolf et al, 2010), sustainable biochar implementation could offset a maximum of 12% of anthropogenic GHG emissions on an annual basis. Over the course of 100 years, this amounts to a total of roughly 130 petagrams (106 metric tons) of CO₂-equivalents. The study assessed the maximum sustainable technical potential utilizing globally available biomass from agriculture and forestry. The study assumed no land clearance or conversion from food to biomass-crops (though some dedicated biomass-crop production on degraded, abandoned agricultural soils was included), no utilization of industrially treated waste biomass, and biomass extraction rates that would not result in soil erosion.

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.

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.

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.

Biochar offers direct, present day benefits to farmers of all sizes in the form of greater crop productivity as well as numerous other quantifiable environmental benefits, among them climate change mitigation. While efforts are underway to develop mechanisms to quantify and monetize the climate benefits of biochar—chiefly in the form of carbon offset methodologies—these would only add to the existing financial incentives for farmers and other stakeholders to adopt biochar.

Biochar production and use comprises a complex system and its sustainability must be parsed out into various components. 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. Large amounts of agricultural, municipal and forestry biomass are currently burned or left to decompose and release CO₂ and methane back into the atmosphere. 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. 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.

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.

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.

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.

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.

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.

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.

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.

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.