Introduction to Biochar in Agriculture

David Yarrow spreads biochar from white cart onto a garden bed he is preparing in Colrain, Massachusetts. The biochar was produced the day before in a homemade burner.

David Yarrow spreads biochar from white cart onto a garden bed he is preparing in
Colrain, Massachusetts. The biochar was produced the day before in a homemade burner.

Most readers of this journal have heard of biochar. It is hard not to, if you are an informed citizen concerned about contemporary issues. Biochar use has been heralded as a breakthrough development in soil health, crop production, carbon sequestration, environmental cleanup and pollution control, among other positive social purposes. Sales of the product have been tripling every year since 2008 and powerful players on the world stage, including China, have been studying its potential for solving varied growing problems.

We thought it might be time to take a closer look at biochar, especially its applicability to agricultural soils and enabling them to provide more and healthier food.

History of Char

Charring is produced by pyrolysis, which is heating biological material – agricultural residues, wood chips, virtually any organic matter – in a low-oxygen environment. The lack of adequate oxygen prevents normal burning, but enables volatile or semi-volatile materials to off-gas. Depending on the material and the temperature, other materials may be driven off and chemical changes will occur, potentially leaving just a porous carbon structure.

Black carbon is the generic term given to all solid chemical-thermal conversion products of carbonaceous materials, including charred residues. Biochar has been recently added as one material in the black carbon spectrum.

Char has an ancient history. The use of charcoal in cave paintings has been dated to 30,000 BC. Its use as a fuel began over 7000 years ago in the smelting of copper and more recently in smelting iron and producing glass. The Egyptians were pyrolyzing biomass 5000 years ago to form pyroligneous acid (composed of wood vinegar, tars and smoke condensates) for embalming.

Wood pyrolysis has been used for extracting valuable gases and oils for commercial purposes for hundreds of years. At its peak a standard distillation apparatus or retort could process 10 cords of wood within a 24 hour period. For these “industrial” purposes the gases and oils were the product, and char a secondary byproduct. In the 1800s, however, coal burning began replacing charcoal as an energy source and by the 1920s petroleum refining replaced biomass pyrolysis as the leading source of chemicals, distillates and volatiles.

One of the earliest examples of the agricultural uses of char is terra preta, the rich, fertile dark soils of the Amazon Basin which the indigenous people created by adding the remains of fire pits and middens full of bones and trash to the normally light tropical soils. Other similar char uses in agriculture date back to the early 1600s in Japan and potentially earlier in China. These, along with the natural deposition of char from forest fires, prairie fires, volcanoes, etc. have resulted in the widespread presence of it in soil organic matter.

Historically, char has been a waste product because the primary purpose of making it had been on optimizing the liquid and gas products of biomass for energy conversion and other uses, not on biochar for carbon sequestration. Thus despite the long research history of pyrolysis, more study is needed to optimize yields of biochar itself, and standardize its properties.

Biochar in agriculture

A number of studies have taken place concerning the effectiveness of biochar in promoting yields. According to those studies the positive impacts of biochar seem to outweigh the negative ones. A 2011 meta-analysis found an overall average yield increase of 10%, rising to 14% in acidic soils, from use of biochar. Approximately 50% of the compiled studies observed short-term positive yield or growth impacts from use of the material, 30% reported no significant differences, and 20% noted negative yield or growth impacts.

Results of some studies have found that biochar’s greatest potential might be in places where soils are weathered or degraded and fertilizer scarce, in part because it helps the soil to better retain any nutrients that it does have. One study of such degraded soils in western Kenya suggests that farms using biochar averaged 32% higher yields than controls.

According to a 2014 World Bank report biochar probably holds the most potential for small farmers in developing countries, not just because they are working with infertile soils most likely to benefit, but because biochar may be a key element of ‘climate-smart’ agriculture — practices that both help to mitigate climate change and reduce vulnerability to its effects.

Biochar additions to infertile soils have also been found to improve soil cation exchange capacity (CEC), the ability of soils to hold onto crucial nutrients. Applying biochar has been demonstrated to improve availability of potassium, for instance. But studies have found that not all biochar–soil combinations cause an increase in CEC. Experts suggest ways to “charge” biochar with fertility before applying it so it doesn’t absorb nutrients and water already in the soil, slowing plant growth. Many farmers soak it for a day or two in compost tea, fish emulsion, aged manure or other nutrients before application. They also recommend inoculating it with microbes before use.
The mechanisms by which biochar reduces vulnerability to climate change effects are somewhat speculative so far. Some researchers suggest the most important feature of the substance is its porosity and the presence of many connected spaces. Clays tend to be composed of flat grains and sand tends to be of circular grains. Even though clays can hold large amounts of water, that moisture has a hard time moving through the grains and reaching plant roots. But biochar is very amorphous so it creates many convoluted pathways that help to slow down drainage in sand and speed it up in clays. These same spaces provide protection for microbes from predation by larger creatures.

Section through a charcoal pile showing wood and soil cover.

Section through a charcoal pile
showing wood and soil cover.

Other scientists are exploring how biochars can mitigate climate change by cutting emissions of nitrous oxide, a greenhouse gas. According to a Chinese study, after biochar had been applied to corn and wheat fields once, nitrous oxide emissions declined over the following five crop seasons, a period of three years. Recent studies have also indicated a complex biochar and fertilizer interaction with respect to yield response. Others have observed that some biochars raise pH, particularly useful in regions like the northeastern US.

Many analysts report, however, that biochar’s effect on agronomic crop yield is variable, with crop production improvements ranging from negative to more than twofold, compared to controls. Some suggest that this inconsistency is because feedstocks, production methods and temperatures have not been standardized. Grass and nonwoody biomass biochar is more easily mineralized than wood-derived biochar, for instance, resulting in longer predicted soil residency times for wood biochar. From a soil fertility perspective, this increased mineralization from non-woody feedstocks could provide nutrient resources to plants. On the other hand, food waste biochar and that with high volatile matter contents have also suppressed plant growth.

Others feel that the material itself changes as it ages. Soil nutrient improvements may take some time to be observed. Delays may occur if the particular element is enclosed in a chemical ring structure that only slowly decays. Most of the existing studies have been limited to less than 3 years, which may not be enough time for the soil nutrient cycle to be fully affected.

Lastly, it is not clear whether biochar use will be economical in agriculture. Recommended application rates vary from 0.1 pound to 1 pound per square foot, or 2 to 22 tons per acre. At $1000 to $3000 per ton, that is a lot of expense for many growers. In the Third World particularly, poor soils and poverty often go hand-in-hand. Studies in Africa have found that few local farmers are willing to buy biochar at the price necessary to create it.

Environmental and Industrial Uses

Biochar’s capacity to bind to heavy metals in soil can keep them from reaching plants or entering water supplies. This interests governmental and other groups concerned about reclaiming land that has been destroyed by mining. The product’s large and numerous pores also give it a large surface area which enables it to tie up contaminants in polluted water and could remove chemical wastes and provide low-cost water treatment where little funding is available.

Scientists are just beginning to explore biochar’s potential for treating fluids in industrial settings. Oil and gas drilling fluids, print toners and paint products all have been suggested as markets for its ability to clean and process materials that flow.

Biochar Production

Plant responses to biochar soil additions are the net result of production conditions (feedstock types and pyrolysis methods) and postproduction storage or activation activities. These processes can confer unique properties on each batch of biochar, even from the same pyrolysis unit and biomass feedstock.

Reactor pyrolyzes biomass yielding charcoal, from which heat has driven off volatile gases. Some gases are condensed for oil, some used to predry the biomass or burned elsewhere.

Reactor pyrolyzes biomass yielding charcoal, from which heat has driven off volatile gases. Some gases are condensed for oil, some used to predry the biomass or burned elsewhere.

The raw feedstock biomass characteristics impart specific properties to the resulting biochar, such as ash content and elemental constituents like density and hardness. Currently char is being made from such diverse materials as chicken manure and nut hulls, wood scraps and plant residue. Such various sources of organic matter will provide quite different kinds of char.

Specific biochar nutrient concentrations may be greater or lesser than what was in the original feedstock nutrient concentration. Occasional volatilization and loss of nutrients during pyrolysis may be linked to higher production temperatures. The large range of operational maximum temperatures common to slow pyrolysis processes determines the extent of volatilization taking place and therefore the final composition of the resulting biochar.

There are also important differences in biochar quality not only as a function of the production process but also linked to postproduction storage or activation. Surface oxidation of black carbon in storage, even at ambient conditions, alters surface chemical groups, which correspondingly influences potential interactions with soil nutrient cycles. Activation can occur by simply cooling the biochar with water or exposing hot biochar to atmospheric oxygen during cooling.

However you make your char, you should cool it afterward – either with water or air. This cooling process, sometimes called “priming”, significantly alters the chemical and physical properties of the char. Unfortunately, researchers are still unclear how to best prime different chars to maximize their benefits.

Whether you own a large farm or tend a kitchen garden, you can produce your own biochar. The key is to heat waste biomass in a low or zero oxygen environment at temperatures ranging 200 to 800˚ C. Many small farms or homesteaders can only afford low-tech systems that generally produce less than a yard of char per burn.

The International Biochar Institute provides free open-source instructions for constructing many low-tech, small-scale biochar production systems. Materials range widely in cost. You could build a simple burner out of scavenged materials, spend $300 on a cone kiln, or pay upwards of $5,000 plus labor for an Adam Retort. Instructions and designs are available at www.biochar-international.org as well as www.backyardbiochar.net

Biochar in the Future

The possibility of engineering “designer biochars” for improving a specific soil deficiency is one direction in which biochar could develop. There have been efforts, for example, to impregnate biochar with various inorganic fertilizers to serve as diverse slow-release nutrient sources. Biochar could also be blended with specific composts, which could increase its value for both fertility and microbial inoculation.

Biochar is expensive as a carbon sequestration agent or as a soil supplement for crop yield improvements. The high production cost for biochar, however, could be offset if these specialty or boutique markets are more fully developed. Also, if biochar applications to other sectors besides agriculture are expanded, it could result in reduced costs of production.

Critics of biochar are quick to point out, and rightly, that the ultimate future of biochar depends on the sustainability of the feedstocks. In a completely ecological world there is no waste and every product or process must be evaluated for completing cycles efficiently. We need to carefully analyze use of biochar by this standard and see if there are more efficient uses for organic matter than charring and burying them.

Landscaping & Biochar

I’ve been asked more than once recently how landscapers could incorporate the use of biochar into their businesses. Not being in the biz myself, I decided to do some investigating to understand a bit more about the specific services landscapers provide to better understand how to answer this question. Obviously services will vary significantly by region, but in my neck of the woods services generally seem to fall into a few basic categories: lawn care, tree care and some are now offering environmental services such as rain & roof gardens. (Hardscaping is also a big service area, but I’ll leave that one out for now.) Below are some ideas on how biochar might be used in various landscaping services:

pg 7 no more sawdust pix 1On the lawn care front there are at least two services where biochar could be very useful. Establishing new lawns is required for newly built homes and office buildings. Unfortunately many times builders scrape away the topsoil to facilitate building and then sell it off, leaving poor quality subsoil that contributes to poor lawns and significant runoff in some places. Adding biochar prior to establishing new lawns will provide much needed carbon, improve water management and reduce leaching and erosion.

Aeration services are provided for already established lawns that suffer from compaction caused by heavy lawn equipment, heavy rainfall, foot traffic, etc. Compaction reduces the soil’s ability to absorb water and oxygen resulting in thatch, rapid drying, rain run-off and other issues. Typically this is dealt with by pulling out soil plugs to allow for improved air and water penetration. Instead of leaving these new holes empty, filling them with highly porous biochar would likely prevent holes from caving in while still allowing for air and water to enter.

Rain gardens and Bioswales

Rain gardens and Bioswales

Tree care and biochar is a great closed loop opportunity. Many times when trees are pruned or removed the debris is chipped and transported offsite, incurring increased cost for the homeowner and sometimes logistical headaches for the landscaper if there are no local places that will accept chips. Thus charring leftover biomass on-site could not only make debris management cheaper, but could provide high quality biochar for various uses for the homeowner or the landscaper. Planting trees with biochar has been shown in various trials to improve survival rates as well as to improve growth rates.

Environmental services such as rain gardens and bioswales seem to be increasingly popular, at least in the Finger Lakes region. No doubt this is in an attempt to better manage the increasing number of heavy precipitation events and reduce costly flooding impacts. In contrast to using sand in rain gardens and bioswales, biochar makes an excellent light-weight, highly porous filtration medium.

Overall I’d have to say biochar production and use within the landscaping industry makes for a great closed loop scenario! One model that has been popping up in different locations is for landscapers to purchase portable kilns such as the Kon-Tiki, and either rent these to homeowners for a few days so they can char on their own, or to provide charring services in lieu of chipping & shipping debris. (Note: it is recommended to wait a few days after pruning to lower the moisture content.)

Kathleen blogs at http://fingerlakesbiochar.com/blog/

pg 7 What is a rain garden_

How Biochar Works in Soil

Biochar first came into broad public awareness through the example of the Amazon, where the hypothesis is that Amazonian inhabitants added biochar along with other organic and household wastes over centuries to modify the surface soil horizon into a highly productive and fertile soil called Terra Preta, which is in direct contrast to the typical weathered Oxisol soils in close proximity. Biochar is exciting to many people because of its role in such soil-building processes. Those who have used biochar for several years may obtain tangible positive results, but they may not have solid concepts and theories about how it works. Biochar is a heterogeneous and chemically complex material and its actions in soil are difficult to tease apart and explain mechanistically.

The Role of Carbon in Soil

The evolution of soil shows how the soil building process works. Before photosynthetic bacteria transformed Earth’s atmosphere by filling it with oxygen, soil was nothing more than a mineral mixture of anoxic green clay. After oxygen entered the atmosphere, minerals started reacting with the oxygen, and red iron oxides appeared in the soil. Good organic, rich, productive soils developed slowly only after algae and arthropods crawled from the sea to dry land and plants took root. Life colonized land and began shedding its wasted, used up and discarded parts onto the earth where they formed a carbon-rich banquet that allowed new life to feed and grow, using photosynthesis to pump ever more energy into the system.

Soil building is the product of a self-reinforcing, positive feedback loop. But soil decline is also a self-reinforcing loop that can result in catastrophic soil loss. Most forms of agriculture tend to deplete soil carbon by reducing the amount of natural organic inputs from leaf and fruit fall as well as from woody debris as it is found in native ecosystems. However, modern, chemically-based agriculture depletes soil carbon much more drastically. Nitrogen fertilizers combined with tillage accelerate microbial respiration, burning up soil carbon faster than it is replaced. Due to the loss of organic carbon reservoirs, many soils have become nearly lifeless substrates that must be continually fed with irrigation water, mineral nutrients and pesticides to produce a crop. Although productive in the short term, this practice is not sustainable. Soil scientist Rattan Lal estimated that “Most agricultural soils have lost 25% to 75% of their original soil organic carbon (SOC) pool.”

Is it possible that biochar can substitute for some of this missing soil carbon? Some of the most productive and resilient soils in the world contain significant quantities of “natural” biochar. Nature makes megatons of biochar in the process of naturally occurring wildfires in forests. Prairie fires can also generate a lot of biochar. Tall grasses burn quick and hot. Close to the ground, however, where the roots start, air is excluded. So the base of the grasses will pyrolyze and not burn. This kind of natural charcoal is present in some of the most valuable agricultural soils in the world, such as the carbon-rich Chernozems of the Russian steppe and the Mollisols of the US Midwestern prairie states. Recently scientists 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 (40–50% of organic C).”

Biochar – the Electric Carbon Sponge

Figure 1. Eight allotropes of carbon:  a) Diamond, b) Graphite, c) Lonsdaleite,  d) C60 (Buckminsterfullerene or buckyball), e) C540, f) C70, g) Amorphous carbon, and h) single-walled carbon nanotube or buckytube. Design created by Michael Ströck from: en.wikipedia.org-Allotropes_of_carbon

Figure 1. Eight allotropes of carbon:
a) Diamond, b) Graphite, c) Lonsdaleite,
d) C60 (Buckminsterfullerene or buckyball), e) C540, f) C70, g) Amorphous carbon, and h) single-walled carbon nanotube or buckytube. Design created by Michael Ströck from: en.wikipedia.org-Allotropes_of_carbon

To understand biochar, we must first appreciate the role of soil carbon. Soil carbon comes in many forms and the terminology used to describe it can be confusing. There are two main pools of carbon — organic and inorganic. Organic forms can be further divided into “recalcitrant carbon” or that resistant to decay, like humus, and “labile carbon.” Labile carbon will be quickly consumed by soil organisms because it is both bioavailable (in the form of easily degraded compounds such as oils, sugars and alcohols) and physically accessible to microbes (not bound up with minerals). These labile compounds include hydrogen and oxygen in the form of hydrocarbons and carbohydrates. The organic carbon pool includes both the living bodies and the dead, decomposing bodies of bacteria, fungi, insects and worms, along with plant debris and manure. Inorganic carbon includes the carbonates such as limestone, and even though some life forms use carbonates to make their shells or skeletons, these compounds are still termed “inorganic”. The main distinction of the inorganic carbon pool, however, is that it does not fundamentally provide microbes with energy for feeding the soil building reactions.

Mineral carbon refers to carbon solids like diamond and graphite as well as the gases of carbon (CO2, CO and many others). There are numerous ways a carbon atom can be arranged in a solid which leads to different physical structures, which are called allotropes. Allotropes of mineral carbon, include diamond, graphite, graphene, buckyballs and carbon nanotubes (Figure 1).

So what is biochar then? Organic or mineral carbon? Actually biochar is a mixture of both, depending on the conditions of formation. But let’s first look at how biochar is produced. Biochar is made by heating biomass under the exclusion of air. This process is called pyrolysis, which includes the drying of the biomass and the subsequent release of flammable vapors. Technically this can be done by many different methods. Some methods use a retort, which is a closed vessel that is externally heated. Heat is transferred through the metal vessel and vapors pass out of a vent where they can be burned and help heat the retort. Gasification is another method that supplies enough air to burn the vapors, but prevents the complete combustion of the biomass material by excluding air from the charcoal zone, thus preserving the biochar. Many other methods of charcoal making exist that range from simple pit kilns to multi-million dollar machines producing energy in gas or liquid form from the vapors.

ompare terra preta with normal tropical soil From presentation by Steve Diver at April 2013 Resilient Farmer workshop http://www.slideshare.net/MauraMcDW/emimo-kcsa-resilient-farmer-april-2013

ompare terra preta with normal tropical soil
From presentation by Steve Diver at April 2013 Resilient Farmer workshop

The resulting charcoal resembles a blackened, shrunken version of the original biomass. But it now has very little hydrogen and oxygen. Microscopically, it inherits much of the structure of the original biomass. The only difference is the material now has been converted from lignin, cellulose and hemicellulose to many of the allotropes of carbon shown above (Figure 1); however, you will not find any diamonds in biochar! What you will find is a collection of disjointed graphite crystals based on hexagonally-shaped carbon rings, with some leftover hydrogen and oxygen attached, along with minerals (ash) that were in the original feedstock. These hexagonal carbon compounds are fused carbon rings. Fused carbon rings are also called “aromatic” carbon, (another confusing chemistry term – it does not mean that the compound has a strong aroma, although some of them, like benzene, do. In chemistry it refers to the molecular structure containing a planar unsaturated ring of atoms that is stabilized by the bonds forming the ring.) They are very stable and it takes microbes a long time to degrade them. The more you heat the biomass, the more of these fused carbon rings are created. The rings hook up with each other to form layers and layers of discontinuous, rumpled sheets – the graphite crystals. Biochar’s jumble of carbon crystallites is an important source of its porosity – imagine all the tiny spaces in the wrinkles between sheets.

Biochar starts out as organic and becomes more mineral-like with heating. This mineral transformation creates the skeletal structure that looks like a carbon sponge (Figure 2). While the mineral, fused–carbon ring structure is hardly biodegradable, the recondensed vapors that can be found in the biochar pores and on its surfaces are less aromatic and more biodegradable and can thus be considered organic phases of the biochar.

The fused carbon rings are also responsible for the electrical activation of the biochar carbon sponge. Fused carbon rings form a special bond with each other that allows electrons to move around the molecule producing electrical properties like those that are found in engineered carbon materials such as graphene sheets and carbon nanotubes. Depending on the pyrolysis temperature and resulting arrangement of atoms, biochar can be an insulator, a semi-conductor or a conductor of electricity. Electrically active fused carbon rings also support “redox” or oxidation and reduction reactions that are important to soil biochemistry, by acting as both a source and sink of electrons. In soils, microorganisms use aromatic carbon both as an electron donor and as an electron acceptor during metabolic chemical reactions. Biochar seems to not only serve as an electron buffer for redox reactions, but it also helps bacteria swap electrons among themselves, improving their metabolic efficiency as a microbial community.

Terra Preta region -- Amazonian region of South America with terra preta soil sites shown as black circles, white circles indicate sites where there there are no terra preta soils Credit The Royal Society

Terra Preta region — Amazonian region of South America with terra preta soil sites shown as black circles, white circles indicate sites where there there are no terra preta soils
Credit The Royal Society

With its pores and its electrical charges, biochar is capable of both absorption and adsorption. Absorption (AB-sorption) is a function of pore volume. The larger pores absorb water, air and soluble nutrients like a normal sponge. Adsorption (AD-sorption) depends on surface area and charge. The surfaces of biochar, both internal and external, adsorb materials by electro-chemical bonds, working like an electric sponge.

Porosity comes in many scales, from the relatively large vascular and cellular structures preserved from the original biomass, to the nano-pores formed by tiny molecular dislocations. The amount of porosity depends mostly on the feedstock material, particle size, and the highest treatment temperature (HTT). Temperature determines how much of the volatiles (hydrogen and oxygen containing compounds) will be driven off and how much pure carbon graphite is formed. Generally, porosity increases as more volatiles are driven off, clearing the pores (although the pores can re-clog when vapors are incompletely driven off and condense on the forming biochar surfaces). Also, at temperatures approaching 1000 degrees C, pores begin to collapse or melt. For this reason, HTT is a key variable to know when specifying a biochar for a particular purpose. Porosity will also depend on the feedstock, with high ash feedstocks like grass reacting quite differently to heating than low ash feedstocks like wood or bamboo. For wood feedstocks, porosity typically peaks at an HTT of about 750 degrees C.

A Well-Aged Cheese

Biochar is not soil. The electric carbon sponge is only an ingredient in the mineral and organic stew that makes up soil. The dish is usually potluck, composed of whatever the local geology and biology provide. However the Terra Preta soils are different. The fertility of these black, humus-rich soils is many times greater than the surrounding, highly leached red soils. They may have been deliberately created over centuries by people living on densely settled high bluffs along the Amazon River. It is thought that the ingredients included charcoal, ash, food scraps and human excrements, but how they actually combined to form Terra Preta is unknown. Explaining the formation of the Terra Preta is like determining the recipe for a fine Camembert cheese. You can analyze all the ingredients and still have not the faintest idea how to make one if you don’t learn it from the artisans.


Figure 2. The skeletal structure of biochar looks like a carbon sponge.

Figure 2. The skeletal structure of biochar looks like a carbon sponge.

One thing that is becoming obvious after a decade of biochar scientific research and the first results from multi-year field trials is that, just like a good cheese, the time dimension is critical. From the moment that biochar is pulled from the kiln, its surfaces begin to oxidize and form new compounds. These changes result in different molecules attached to the surface, called “functional groups,” composed primarily of oxygen, hydrogen and carbon. The functional groups are able to bond with nutrients and minerals, while the underlying fused carbon rings support redox reactions (reactions that move electrons) and shuttle electrons around the microbial community attached to biochar surfaces, potentially enhancing microbial metabolism and the cycling of nutrients. The end result of this ferment could be any one of many “terroir”-distinct Terra Preta flavors, depending on what kind of soil, organic matter, minerals, water and life forms come into contact with the biochar, and how long it has to ripen. But, if you sample the cheese before it is mature, it’s just sour milk.

Raw biochar placed in soils before it has a chance to collect a charge of nutrients can actually reduce crop yields because 1) it reduces the availability of plant nutrients by binding and immobilizing them and/or 2) it may add volatile organic compounds (labile carbon) that feed a bloom of microbes that use up nitrogen in the soil, depriving plants. These problems are easily corrected by adding nutrients to the charcoal application to compensate for this effect. Once the labile carbon fraction is used up, biochar enters a new phase – a deep time dimension where its carbon matrix is stable for hundreds to thousands of years and may become the core of humic substances that crystalize around the fine biochar particles; at least this is what the existence of ancient fertile black earth soils suggests.

In fact, biochar, whether naturally created or man-made, may be the base of many humic materials found in soils (Hayes, 2013). Very little humus naturally forms in tropical soils, where high temperatures and moisture accelerate microbial decomposition, yet Terra Preta soils have a high content of humus. To understand why, scientists added new organic matter to both a Terra Preta soil and an adjacent, poor natural soil. They found that more of the organic matter was retained as stable humus in the Terra Preta soil. A combination of factors may lead to this result. Biochar surfaces adsorb carbon and retain it in compounds with minerals, supporting at the same time a large microbial community that potentially makes more efficient use of organic debris containing carbon and other nutrients. The existence of this mechanism raises the possibility that Terra Preta soils are thus able to accumulate additional carbon more efficiently than adjacent soils.

If tropical soils need biochar to make humus, what about compost? Well balanced compost, with the optimum C:N ratio, will contain lots of humus. However, if there is not enough stable carbon (from wood, straw or other lignin sources), then the easily degradable sugars, fats and proteins will be completely consumed by microbes leaving very little substrate behind. This is what happens in tropical soils where heat, moisture and high microbial activity will decompose a fallen leaf nearly as soon as it hits the ground, allowing very little soil to form.

A number of studies have demonstrated that biochar has value as an ingredient in compost that can help capture nutrients and form humus. In the next section, we review some of these results and explain why biochar is valuable in compost. The answers will also tell us a lot about how biochar behaves in soil, because compost accelerates many of the processes that occur in healthy soil.

Kickstarting Compost with Biochar

If you look at a list of things biochar is supposed to do in soil, you’ll find it is very similar to lists you see for compost. Both biochar and compost are said to provide these benefits, taken from various claims made by biochar and compost manufacturers:
• Improves tilth and reduces soil bulk density
• Increases soil water holding capacity
• Becomes more stable by combining with clay minerals
• Increases cation exchange capacity (CEC – the ability to hold onto and transfer nutrient cations: ammonium, calcium, magnesium, and potassium)
• Improves fertilizer utilization, by reducing leaching from the root zone
• Retains minerals in plant available form
• Supports soil microbial life and biodiversity
• Helps plants resist diseases and pathogens
• Helps plants grow better in high salt situations
• Adds humus carbon to the soil carbon pool, reducing the atmospheric carbon pool

If compost really can do all these things, why do we need biochar? The answer is twofold:

First, unlike biochar, compost is quickly broken down by microbial action in soil over months to at most, decades, depending primarily on climate. Biochar lasts at least ten times longer in most soils. Recently, I called a California agriculture extension agent with a question about adding compost to fields to improve water holding capacity. I was told that because of the hot climate, at least two applications a year are needed to maintain enough soil organic matter to make a difference in water holding capacity. Aside from the expense of applying that much compost, there is simply not enough compost available to support such large application rates.

Second, biochar has important synergistic effects when added to compost. Researchers find that biochar makes faster, more nutrient rich, more biologically diverse and more humified, stable compost. Below, I examine several of the most important biochar effects and summarize some recent research results.

1. Biochar keeps compost moist and aerated, promoting increased biological activity.

The composting process is governed by various physical parameters that are subject to alteration by the addition of biochar materials as bulking agents. Some of the parameters that most affect compost are: aeration, moisture content, temperature, bulk density, pH, electron buffering and the sorptive capacity of bulking agents. Water and air are both held in biochar pore spaces and voids, and the spaces between particles. Moisture is also the vehicle for bringing dissolved organic carbon, nitrogen and other plant nutritive compounds into contact with biochar surfaces where they can be captured. Biochar’s stable carbon matrix accepts electrons from decomposing organic compounds, buffering electric charges that might otherwise impair microbial activity and be responsible for the production of greenhouse gases like methane and hydrogen sulfides.

All these properties of biochar promote microbial activity in compost. Researchers tested 5% and 20% additions of pine chip biochar to poultry litter compost and found that the addition of 20% biochar caused microbial respiration (measured as CO2 emissions) to peak earlier and at a higher level than either the 5% or 0% biochar treatments.

2. Biochar increases nitrogen retention

When nitrogen-containing biomass materials decay, they can release large amounts of ammonia. Ammonium (NH4+) is the aqueous ion of ammonia. Ammonium is generated by microbial processes and nutrient cascades that convert nitrogen from organic forms found mainly in proteins and nucleic acids into mineral forms (ammonium, nitrate and nitrite) that can intermittently be converted by nitrifying and denitrifying microbes to gaseous emissions that include volatile ammonia gas (NH3), nitrogen gas (N2), nitrous oxide (N2O) and other reactive nitrogen gases (amines and indoles). At neutral pH the aqueous ammonium (NH4+) and the gaseous ammonia (NH3) are in equilibrium. Higher pH forces more of the aqueous ammonium into the gas phase that can escape to the atmosphere.

Numerous studies have shown that biochar is effective at retaining nitrogen in soils. Several studies have also shown that biochar enhances nitrogen retention in compost, reducing emissions of ammonia and increasing total nitrogen retention by as much as 65%. The ammonia retention ability of biochar can actually improve during the composting process. Adding 9% bamboo charcoal to sewage sludge compost tested sorption of ammonia on biochar during composting and found that while ammonia retention was correlated with saturation of binding sites in fresh bamboo biochar, this did not hold for composted bamboo biochar. During composting the biochar is subjected to an accelerated aging process. That means that biochar surfaces get oxidized and enriched by carboxylic (acid) functional groups. The latter more than doubled at the end of the composting period, improving the capacity to exchange cations like ammonia.

3. Biochar improves compost maturity and humic content

Several studies have looked at effects of biochar on the timing and results of compost maturation and found that adding biochar to compost reduced the amount of dissolved organic carbon (labile carbon) in mature compost while increasing the fraction of stable humic materials (stable carbon).

4. Biochar compost improves plant growth

Biochar seems to improve the composting process, but how do plants like those biochar-composts? Several researchers have experimented with various combinations of compost and biochar added as separate amendments. These studies found improved plant growth response when biochar was added to soil along with compost. A 2013 study in Germany looked instead at biochar composted together with other materials. It tested six different amounts of biochar in compost, from 0 to 50% by weight, and also three different application rates of each compost type. Using oats in greenhouse pots on two different substrates (sandy soil and loamy soil), researchers found that plant growth increased with increasing application rates of each type of biochar compost, which is not surprising since the amount of deliverable nutrients was increased, at least by the compost fraction. They also discovered, however, that plant growth was increased as the amount of biochar in the compost increased. The biochar may either have improved nutrient retention during the composting process with subsequent enhancement of nutrient delivery to plants, or it promoted plant growth through some other mechanism. However, the researchers confirmed that synergistic effects can be achieved by adding biochar to composts.

How could we put biochar to work in soils?

One of the basic principles of good compost production is that the wider the variety of materials you use, the better the compost. The ideal biochar compost system is based on a speculative reconstruction of the Terra Preta soils. According to this model, these areas began as garbage dumps where accumulation of food wastes, ashes and manure were deposited. However, as populations grew, it is possible that they began to realize that the waste sites were developing into very fertile and productive areas. They may have begun to deliberately manage the material flows of plant biomass, mammal and fish bones, ash, biochar, and human excreta that likely resulted in the Terra Preta soils we see today.

For maximum conservation of resources, it is important to remember another principle: use the less degradable carbon sources like biochar to help preserve the more easily degradable but nutrient-laden sources like manure and food waste. I believe there is much exciting work ahead to determine optimum recipes for biochar-based organic composts and ferments, exploring the effects of different kinds of biochar in combination with other compost ingredients.

From past and on-going research, we realize that biochar has numerous possible mechanisms for its action in soils that can occur on a variety of different scales. But if the results from recent biochar compost research prove to be consistent, we now have the beginnings of a recipe book for biochar-enhanced super compost that can kickstart the process of returning carbon to soils today. Our industrial legacy has left us with a rapidly deteriorating climate, and soils that are dying and eroding. Biochar, as a form of recalcitrant carbon, may be just the medicine that degraded and unproductive soils need.

Kelpie Wilson is a writer and a mechanical engineer who has worked in the biochar field since 2007. She was a project developer and writer for the International Biochar Initiative (from 2008-2012) and now works as editor of the Biochar Journal and with her company Wilson Biochar Associates. She has been a tree hugger, an auto mechanic and a science fiction author, and has lived off-grid in the Oregon woods since 1990. Have a look at her valuable backyard biochar website with many low budget biochar production devices developed by Kelpie & others.

Biochar in Temperate Agricultural Soils

Introduction and Background

Biochar temperature versus characteristics Some researchers believe that the best biochar is formed by low temperature pyrolysis at about 500˚ C, with higher temperature pyrolysis producing a more traditional charcoal. Five hundred degrees C seems to be high enough to achieve maximal surface area but also low enough to retain some bio-oil condensate. Credit: Temperature effects on carbon recovery, CDC, pH and surface area, Lehmann (2007), Front. Ecol. Environ. 5:381-387

Biochar temperature versus characteristics
Some researchers believe that the best biochar is formed by low temperature pyrolysis at about 500˚ C, with higher temperature pyrolysis producing a more traditional charcoal. Five hundred degrees C seems to be high enough to achieve maximal surface area but also low enough to retain some bio-oil condensate. Credit: Temperature effects on carbon recovery, CDC, pH and surface area, Lehmann (2007), Front. Ecol. Environ. 5:381-387

As the world’s population rises, people will continue to put more pressure on their terrestrial landscapes in order to extract food, fiber, and fuel to meet their growing needs. At the same time that populations drive a need to find methods of more efficient agricultural land use the climate is changing and land degradation is an increasing problem. In the industrialized world, modern agriculture relies on heavy chemical inputs and creates pollution problems in our waterways and our air. Carbon dioxide, methane, and nitrous oxide are byproducts of modern agriculture that exacerbate climate change. In the developing world farmers often do not have access to the expensive chemical inputs of modern agriculture and thus rely on ‘slash and burn’ techniques. These practices volatilize most of the nutrients accumulated in biomass and cause air quality concerns. Alternatively, farmers may incorporate organic wastes that have short residence times in soil, thereby releasing large amounts of methane and other greenhouse gasses as they decompose. In both systems of agricultural production, nutrient cycles are very leaky with nutrients entering the system and then quickly leaving through leaching, volatilization, erosion, and crop removal.

Agricultural systems are thus faced with two problems. They are in constant need of nutrient additions, while simultaneously leaky nutrient cycles cause nutrient overabundance elsewhere. In the case of carbon, agricultural soils no longer store nearly as much carbon as undisturbed soils. After conversion to agriculture, soils loose 89% of their stored carbon. Land converted to agriculture thus acts as a carbon source, both directly by releasing stored soil carbon and indirectly by requiring heavy nutrient inputs that release carbon during their production.

In natural ecosystems 90% of soil organic matter turns over on decade timescales, much of it being very labile.

There are numerous sites, however, among the old and highly weathered tropical Amazon-basin soils where Amazonian Dark Earth, or Terra Preta, is found (see map on page B-10). These Terra Preta soils contain highly stable organic black carbon waste, or biochar, that was added to the soils by people during the pre-Columbian period, 500-6,000 years ago. These dark colored soils have 70% more carbon in them than surrounding soils, and demonstrate unexpectedly low nutrient leaching and high primary productivity, even with extensive agricultural use.

These unusual Amazonian Dark Earth soils often originate in ancient human middens, or trash pits, where the charred organic waste was mixed with other organic soil amendments such as bones and manures. These mixed-waste Terra Mulata soils are also stable on millennial time scales, suggesting that ancient people used biochar along with other organic wastes to convert poor-quality soil into agriculturally productive soil that could maintain long-term productivity. There is a clear historic use and benefit of biochar addition to agricultural soils, but the applicability of biochar addition to today’s agricultural systems, especially in temperate regions, is largely unknown.

Biochar Production

Biochar is produced by baking organic materials in the absence of oxygen, a process known as pyrolysis. The resulting biochar is a polycyclic aromatic hydrocarbon and is very stable due to its structure. Pyrolysis of agricultural waste products such as crop residues and manures can offer a better way to dispose of these wastes, avoiding the air pollution associated with burning them and the green house gas emissions of methane, carbon dioxide, and nitrous oxide associated with leaving them to decompose.
Biochar is a term used to describe any organic waste that has undergone the pyrolysis process. The specific physical and chemical composition of biochar depends on the starting material it was made from. All biochars are carbon-rich, aromatic rings that have high surface areas and porous structures. The addition of biochar affects physical properties of soil texture, structure, porosity, particle size, distribution and density that alter the movement of air, water, microorganisms, and roots through the soil. Additions of biochar increase the soil’s cation exchange capacity and provide protection for many living organisms in the soil, such as mycorrhizal fungi that can intercept leachable nutrients. Thus, biochar can act to hold nutrients in soil chemically, physically, and biologically.

Biochar thus offers three attractive benefits. First, it can sequester carbon in agricultural systems better than no-till agriculture because the recalcitrant form of carbon remains in soils for thousands of years. Second, energy can be harvested during the pyrolysis process. Third, biochar can improve agricultural soil through its beneficial chemical and physical properties. This article will focus on the benefits that biochar can provide to agricultural soils, specifically how it affects the availability of nitrogen in temperate agricultural systems.


Nitrogen is one of the nutrients that most often limits primary production in ecosystems. Recently disturbed sites, such as those cleared for agriculture or subject to burning, are especially nitrogen limited. Biochar, however, has great potential to be used as a soil amendment in temperate agriculture and of particular interest are the effects biochar addition has had on nitrogen cycling. From the limited scientific research that has been conducted on the effects of biochar on nitrogen there is general agreement that it offers a three-pronged benefit to soil nitrogen.

Biochar additions 1) increase plant-use efficiency of nitrogen, 2) decrease leaching of nitrogen, and 3) decrease nitrous oxide emissions from soil. In addition, biochar appears to alter the microbial community, favoring fungi, and thus providing opportunities to manipulate nutrient cycling through altering the microbial mediators of those nutrients.

Biochar’s three observed benefits to soil nitrogen may be explained by its ability to increase cation exchange capacity in soil, enabling soil to hold on to nutrients for long-term plant growth. By increasing the soil’s ability to hold on to available nitrogen, biochar can reduce nitrogen losses from the system from ammonium leaching and nitrous oxide emissions. Biochar’s physical structure, rich in microsites, coupled with its cation exchange benefits that help hold nutrients in the soil for future plant use favor a microbial community dominated by fungi and rhizobial bacteria that capture and store more carbon and nitrogen. Thus biochar simultaneously increases carbon storage and nitrogen directly available for plant use.

Structure of Biochar

biochar with nitrogen fixers Biochar porosity encourages fungal proliferation. In this picture the nitrogen fixing bacteria Azospirillum spp. can be observed on the biochar as well.  Photo credit to carbolea, http://www.carbolea.ul.ie/project.php?=sfichar

biochar with nitrogen fixers
Biochar porosity encourages fungal proliferation. In this picture the nitrogen fixing bacteria Azospirillum spp. can be observed on the biochar as well.
Photo credit to carbolea, http://www.carbolea.ul.ie/project.php?=sfichar

Biochar’s physical structure increases the ability of soil to retain moisture, while simultaneously increasing porosity and permeability that allows excess water to move through the system. This favors aerobic microorganisms as opposed to denitrifying bacteria that can produce nitrous oxide. Biochar porosity also offers habitat for microbes that aid in making nutrients available to plants. Thus, a unified understanding of how biochar benefits soil nitrogen chemically, physically, and biologically can help us understand the interactions between these factors and mechanisms.

Some studies find that biochar additions alone can increase production, likely due to its ability to absorb and retain water. Biochar additions alone to temperate rice fields in China showed yield increases of 14%, likely due to its water-holding ability. The greatest benefit, however, is in the addition of biochar alongside fertilizers. One study examined the effects of biochar additions on barley yield alongside different amounts of nitrogen fertilizer. Researchers found that additions of biochar with nitrogen fertilizer could increase barley yields by 30% above nitrogen fertilizer alone. Another field study highlighted the role of biochar in increasing nitrogen fertilizer use efficiency. This field study found that coupled biochar and fertilizer additions produced higher yields of radishes than nitrogen fertilization alone.

One laboratory study measured nutrient losses in three different soils from the US Midwest amended with poultry manure with varying amounts of biochar. As biochar rates increased, the amount of N, P, Mg, and Si leached from the soil decreased, even though the biochar itself contained ‘substantial’ additions of these elements. Over their 45 week trial, even at the low biochar addition rate of 20 g/kg soil, total nitrogen and phosphorus leaching decreased by 11% and 69% respectively. These results suggest that biochar additions to productive Midwest soils could help manage nutrient leaching problems.

These studies provide encouraging evidence that biochar can be used as an additive in current conventional and organic agricultural practices in order to increase the use-efficiency of nutrient addition. Some of these studies were short-term experiments, however, conducted in laboratory settings, and thus do not include all of the varied conditions that can affect processes in the field.

Microbes and Biochar

The increased surface area, microsites, and soil structure that biochar provides create environments from which microorganisms can benefit. Surprisingly, the initial addition of biochar can reduce microbial activity, at least in laboratory experiments, while in general biochar additions increase long-term microbial activity. The initial decline in microbial activity followed by a long-term increase may be explained by shifting community dynamics. The original microbial community in highly fertilized agricultural systems is likely dominated by copiotrophs (organisms which predominate in nutrient-rich environments), which may decline when biochar is added to the soil, binding available nutrients in cation exchange and changing the physical properties of the soil. The replacement community may be more oligotrophic (capable of surviving where there is little to sustain life) and competitive when leachable nutrients are less readily available.

The microsites of biochar are particularly favored by mycorrhizal fungi, which form symbiotic relationships with plants and help extract nutrients and water for the plant in exchange for photosynthesized carbon compounds. The fungal hyphae of these mycorrhizae intercept leachable nutrients and thus we observe significantly lower nutrient losses when biochar is added to agricultural land. Fungal dominated soils generally store more carbon than bacterial dominated soils. Biochar’s ability to favor fungi may serve as an additional mechanism by which biochar can increase the carbon-storing capacity of soils and thus even offer humans a tool by which to manipulate microbial communities and carbon storage. Additionally, the fact that microorganisms create soil aggregates may provide an explanation of the observed increase in soil structure found in soils amended with biochar applications.

One of the key microorganisms in the soil is the group of Rhizobium spp bacteria that fix atmospheric nitrogen into bioavailable forms and develop symbiotic relationships with plants. One study found that at maximum nodule development alfalfa amended with biochar in field experiments had 227% greater nitrogen than plots without biochar additions. It appears that biochar stimulates biological nitrogen fixation, and can thus directly alter nitrogen addition to soils by altering the microbial community.

Differences in Biochars

Biochar variability graph Biochars produced from different feedstocks (shaded circles) and at different temperature vary in their properties. Credit: http://www.soilquality.org.au/factsheets/biochar-for-agronomic-improvement

Biochar variability graph
Biochars produced from different feedstocks (shaded circles) and at different temperature vary in their properties. Credit: http://www.soilquality.org.au/factsheets/biochar-for-agronomic-improvement

Not all biochar is the same. The way it is made can strongly affect the end biochar. The temperature at which pyrolysis occurs also shapes the end product. In general, as the temperature of pyrolysis increases, the yield of biochar decreases. As the organic material is heated to create biochar, many compounds volatilize, leaving only the most recalcitrant material such as aromatic hydrocarbons and basic elements. Nitrogen is one of the first materials to volatilize at about 200˚ K (Kelvin or -73˚ C). Researchers have found that the higher the temperature of pyrolysis, the greater the loss of nitrogen relative to the starting material. It is clear that not all of an element volatilizes at a certain temperature. Biochar made from sewage at 450˚ K (177˚ C) still contained 50% of its original nitrogen and all of its phosphorus, though not in bioavailable forms. A study that examined the amount of the elements P, K, Ca, and Mg in biochars produced at 400˚ C and 500˚ C found that the slightly higher temperature acted to concentrate these elements. Thus, it is clear that the temperature at which pyrolysis occurs has a huge effect on what materials remain in the biochar. Additionally, the form of the nutrients found in the original material affects what elements will remain in the biochar product. In general, biochar produced under low-temperature pyrolysis yields a greater quantity of material that is more labile and has a shorter residence time in soils. Biochar produced at high temperatures, however, is more recalcitrant with a longer resident time in soil.

The temperature at which biochar is formed can also affect its pH. Higher pyrolysis temperatures lead to biochars with higher pH. There is potential for biochar to provide a service to soils by increasing the pH of agricultural soils that have a low pH (often because of long-term inputs of nitrogen fertilizer and acid rain). Changing the buffering effect of biochar through its pyrolysis process is one way that biochar could be created for a specific agricultural use to mitigate a specific agricultural soil problem.

fava beans Fava beans grown in soil amended with 0, 10, and 50 tonnes per hectare of poultry litter biochar. Biochar has the potential to boost the natural ability of legumes to fix nitrogen to the soil. From a story originally published in Agriculture Today, June 2010. Full story at: www.dpi.nsw.gov.au/aboutus/news/agriculture-today/june-2010/biochar-boosts-nitrogen-fixation---study

fava beans
Fava beans grown in soil amended with 0, 10, and 50 tonnes per hectare of poultry litter biochar. Biochar has the potential to boost the natural ability of legumes to fix nitrogen to the soil. From a story originally published in Agriculture Today, June 2010. Full story at: www.dpi.nsw.gov.au/aboutus/news/agriculture-today/june-2010/biochar-boosts-nitrogen-fixation—study

The starting organic material for biochar can also have a large effect on the end product, as well as the conditions under which it is formed. The initial relationships of the starting materials will correlate strongly with the end product. One study measured the element contents of biochars made from different substrates. It found that the final carbon content ranged from 40% in biochar made from poultry litter to 78% in biochar made from pine chips. Interestingly, the amount of nitrogen conserved in the biochar was inversely proportional to the source material nitrogen, with poultry litter and pine chip biochars conserving 24.7% and 89.6% of their starting nitrogen respectively. Additionally, the moisture and lignin content will affect the pyrolysis process and thus the biochar product. For example, due to the high lignin content in nutshells, biochar from nutshell wastes has a higher surface area and more developed micropore structure than many other organic agricultural wastes.

Another way that biochar can aid in nutrient retention is to be added to decomposing materials as a bulking agent before they are composted and applied to fields. In one comparative study scientists mixed poultry manure in a 1:1 ratio with biochar, coffee husk, and sawdust respectively. They found that despite the inert nature of biochar, the biochar manure mixture underwent the highest level of humification of the three trials. The conversion of organic matter to humus (70% in this study) marked the transformation of labile organic matter to more recalcitrant humic acids that have a long-term soil improvement benefit. Thus, biochar appears to have acted as a catalyst by aiding in the transformation of labile organic matter to recalcitrant humus. Other studies have also demonstrated the ability of biochar to enhance the nutrient status of the compost products and reduce nitrogen losses during decomposition of organic matter. While this was a short-term study, it demonstrates how biochar can help enrich the long-term recalcitrance of organic matter in soils.

Biochar can also aid in the retention of nitrogen in the soil through reducing nitrous oxide emissions. Soil nitrous oxide emissions are primarily a function of moisture content and tillage regime in agricultural soils. Biochar alters the physical location of water within the soil matrix by providing increased surface area, porosity, and a more developed soil structure. Denitrifying bacteria produce nitrous oxide in low oxygen conditions, such as waterlogged soil, as a leaky product along the pathway to forming atmospheric nitrogen (N2). Biochar additions in rice fields in China decreased nitrous oxide emissions in nitrogen fertilized fields by 40-51% and in unfertilized fields by 21-28%. This same study found that biochar significantly increased rice yields and reduced methane emissions by as much as 41%. This is significant, as rice cultivation is notorious for emitting greenhouse gases from the soil and biochar significantly reduced the emission factor of nitrogen fertilizer in this conventional cultivation system. In addition rice is a major food staple for much of the world, especially the developing world where agriculture will need to be intensified and more efficient in the future in order to feed growing populations.


Biochar is expensive to manufacture at large scales such as those needed to sequester considerable amounts of carbon, and requires considerable infrastructure to do so. For the near future, biochar will most likely be created for small-scale agricultural benefits. Even small amounts of biochar additions to soils have been demonstrated to provide real and significant benefits. Small-scale biochar additions incorporated into the rooting zone of plants can help retain phosphate, ammonium, water, and microorganisms that can help rebuild organic matter stocks quickly in denuded landscapes and low-yield agricultural land. Biochar has a particular appeal for small-scale subsistence farmers who can benefit economically and socially from improving their land for the long-term gains that are not often at the forefront of corporate farming interests. Such long-term, small-scale experiments provide the greatest benefits for the lowest cost and are a starting point for biochar use.

In order for broader scale applications of biochar to be feasible, for example on industrial farms, production will have to be cheaper. Currently production costs, infrastructure costs, and transportation costs of such a high bulk-density product, as well as the health hazards that dusty materials pose, make the industrial application of biochar infeasible. Some investigators suggest that biochar could be manufactured into a pelleted product that can be transported easily. They also conclude that if pyrolysis occurs at 550˚ K or less no crystalline materials form and thus there is little health hazard associated with breathing biochar dust during use. Some argue that, given the political and economic will to sequester carbon, biochar may become a cost-effective way to do so. Whether the broader-scale application of biochar to terrestrial ecosystems is driven by the need to sequester carbon or the need to improve agricultural soils, it will provide multiple benefits to soils and communities across the globe.

One chief concern with biofuels and biochar is where the organic substrate material will come from. With growing populations there are increasing pressures on terrestrial ecosystems. Often biofuels are grown on former food cropland, or are made directly from food crops. But biochar can be made from any organic waste material and is a good way to turn crop residues into long-term soil improvements. Crop residues can be charred (pyrolysed) and added back to the soil in agricultural systems. In addition, forestry waste could be charred and added to forest ecosystems, thus providing a ‘slash and char’ rather than a ‘slash and burn’ system for creating agricultural land that has the ability to sustain long-term production. Abandoned and denuded agricultural land also has the possibility to be improved and reclaimed as productive cropland through ‘slash and char’ biochar addition. Though biochar can offer a more sustainable way to clear land for agricultural growth, biochar ought to first and foremost be applied to existing agricultural land for its ability to increase yields and nutrient use efficiency. In addition, there is great potential for biochar to increase the carbon storage ability of any terrestrial ecosystem through its ability to tighten leaky nitrogen cycles in agricultural soils and make nitrogen more available to plants.

One of the great difficulties in better understanding biochar is that there are so many factors that affect its characteristics. Biochar is not one substance, but a family of black carbons that each has its own properties and interactions with the local environment where it is applied. It is this manipulability that makes biochar an appealing additive for agricultural soils that may benefit from its various and diverse characteristics.