This fact sheet is meantto provide farmersand gardeners the

background informationrequired to make in-formed choices aboutbiochar use in their soils.This will include a discus-sion about:• what biochar is and what itis made from;

• how biochar stores carbon and how thisdiffers from other methods of carbon se-questration;

• how biochar acts on agricultural soils, in-cluding a discussion of how different“feedstocks” affect biochar’s influence onsoils; and

• application recommendations.

What is Biochar?Biochar is the product that remains whenbiomass is heated at a relatively low temper-ature (<700O C), with little to no oxygen. Thebiomass used to create biochar often is re-ferred to as the “feedstock,” and can comefrom any number of organic materials, in-cluding plants and trees, animal wastes, andorganic wastes from industries like the paperand cotton industry.

While one might be inclined to think ofthis process as “burning” biomass, thebiochar production process is actually called“pyrolysis.” The key difference betweenburning and pyrolysis is that pyrolysis ex-

cludes oxygen from the reac-tion. While burning resultsin the production of ashthat doesn’t contain car-bon, pyrolysis results inthe production of charcoalthat is very high in carbon.This “thermal decomposi-

tion” of the organic materialtraps much of its carbon in the

charcoal.(5)The difference between charcoal and

biochar lies in the end use of each product.Charcoal is produced with the intention ofburning it for fuel, thus releasing the carbonit contains back into the atmosphere. Biochar,on the other hand, is a charcoal product thatis produced for carbon capture and is subse-quently buried in soil.(6)

Biochar and Carbon CapturePhotosynthesis is the process by whichplants absorb carbon dioxide (CO2) from theatmosphere and convert it into carbon (C)and oxygen (O2). The plants use the carbonfor growth and, through respiration, releasethe oxygen back into the atmosphere. Whena plant dies and decays, much of its storedcarbon reunites with oxygen and is returnedto the atmosphere as carbon dioxide.(2) Thecarbon that remains forms humus, a stableform of organic matter that is very resistantto further decay. It is estimated that only 10-20% of the carbon stored in a plant’s tissue isconverted to humus.(10)

AGRONOMY FACT SHEET:BIOCHAR

(updated 12/2010; © Garden Gate University)

Like photosynthesis, biochar productionalso converts some of the carbon once storedin plant tissue or other parent material into avery stable form. However, biochar produc-tion is much more efficient at stabilizing car-bon than photosynthesis. Because thepyrolysis method producing biochar intro-duces very little oxygen, it does not produceas much CO2. Instead, up to 50% the carbonin the parent material is stabilized in theform of biochar.(7) This carbon is locked up inthe biochar for a very long time — estimatesrange from hundreds to thousands of years.Therefore, biochar is considered a “carbonnegative” product — it actually removes car-bon from the atmosphere permanently.

Since carbon dioxide is one of the green-house gases implicated in global climatechange, its removal from the atmospherewould help reverse the warming trend. Fur-ther, because its production generates addi-tional materials (gases, oils, and heat) thatcould replace some fossil fuel use andthereby further reduce CO2 emissions, manyscientists are embracing biochar as a solutionto the climate crisis. Biochar offers anotherbenefit that is contributing to its rising popu-larity: its usefulness as a soil amendment.

Biochar as a Soil AmendmentMost discussions about biochar’s effects onsoils begin with a reference to the “TerraPreta” (literally, “black earth”) soils of theAmazon. These soils were first studied bythe late Dutch soil scientist Wim Sombroekin the 1950s, and since then by countless oth-ers who agree that their dark color is a resultof the ancient practice of making and bury-ing char. Studies of these remarkably fertilesoils reveal that they contain as much as300% more phosphorous and nitrogen thannearby soils that were not amended withchar, and contain 9% carbon compared to0.5% of surrounding soils.

Archaeologists have not yet establishedhow many centuries (or milennia) of buryingcharred wastes it took to achieve such fertil-ity. It is likely that small-scale applications ofbiochar would not achieve such results.However, given that these soils are thoughtto be up to 7,000 years old, these fertilityrates provide an excellent example ofbiochar’s persistent nature.(8)

Since the discovery of the Amazoniandark earths, numerous studies have demon-strated that most biochars provide both di-rect and indirect benefits when applied toagricultural soils. In field experiments con-ducted on a wide variety of soils, plantsgrown in biochar-amended soils have repeat-edly (although not exclusively) demon-strated positive responses compared to thosegrown in soils without biochar additions.Some of these responses include increasedyields, increased biomass, and decreased fer-tilizer requirements.(1)

However, it is difficult to establishwhether this apparent advantage thatbiochar extends to plants can be attributed tobiochar’s nutrient content. In fact, in addi-tion to supplying nutrients, biochar acts ina number of other ways that may also be

credited for positive plant responses. Each ofthese areas of influence are discussed in de-tail below.

Nutritional properties of biocharThere is growing evidence that the feedstockused in biochar production determines, tosome extent, the nutritional properties of theresulting biochar. Because biochar can beproduced from any number of feedstocksunder any number of production methods,there is a very broad range of variabilityamong biochars’ characteristics, includingtheir nutritional profiles.(3)

A feedstock that is high in nitrogen (N)tends to produce a biochar that is also highin N, relative to that from a low-N feedstock.For example, Chan et al measured 11 timesmore N in biochar made from (high-N) poul-try litter than in biochar produced from(low-N) green wastes.(3) However, evenbiochars produced from the same type offeedstock may have very different nutri-tional profiles. For example, Lima and Mar-shall (2005) also measured the nutritionalcontents of biochar from poultry litter, butfound a much lower N content than thepoultry litter biochar in the Chan et al study.The contrast can most likely be attributed tothe temperature difference used to producethe two products. The char with the higher Ncontent was produced at 450o C while thelower-N char was produced at 700o C.(3)Other studies imply that the nutrients foundin biochar are not necessarily responsible forthe positive plant responses so often ob-served. For example, Graber et al (2010)studied tomato and pepper plant growth insoilless media either amended with biocharor not. Those grown in biochar-amendedmedia demonstrated advantages in a num-ber of characteristics including leaf size,number of nodes, and yields. However, the

leaf nutrient content of the biochar plantsand the unamended plants were the same,indicating that the benefits observed in thebiochar plants were not the result of addi-tional nutrient access. One hypothesis pro-posed by the authors is that the biochar’sinfluence on soil microorganisms could becredited for the superior plant growth.(4)

Biochar and soil microorganismsIt is important to understand how biochar af-fects the microbial life of soils, since soil mi-croorganisms play a critical role in a soil'sproductive capacity.(9) Although the use ofagricultural charcoal dates back thousands ofyears, scientific research demonstrating itsinfluence on soil microbiology is in its in-fancy. However, the limited number of stud-ies make increasingly evident the notion thatbiochar exerts considerable influence on soilbiota. Also increasingly evident is that feed-stock choice plays a part in the response ofsoil microorganisms to biochar amendments.

Unlike other forms of organic matter,biochar does not provide energy or reducedcarbon to microorganisms — at least afterany residues on the char have decomposed— because it is so resistant todecomposition.(9) Instead, it changes the

Arbuscular mycorrhiza fungal hyphae growing intobiochar pores from a germinating spore. Ogawa, 1994.

physical and chemical soil environment in away that tends to be favorable for microbialpopulations. For example, its physical struc-ture provides a protective habitat for the soilmicroorganisms that contribute to the healthof soils. Bacteria and other microbes findprotection from predators and dessicationthroughout its highly porous structure. Myc-orrhizal fungi, which form mutualistic rela-tionships with plants through which theyprovide increased access to water and nutri-ents, often (though not always) seem tothrive on the addition of biochar, showingroot colonization increases as high as 2900%(Harvey et al, 1976, as cited in Warnock et al,2007). Warnock et al (2007) propose thatbiochar additions benefit mycorrhizal fungiin four ways(12):

• by making nutrients more available to fungi;• by providing protection from predators;• by affecting other soil biota in a way that

benefits mycorrhizal fungi (like supportingthe increase of mycorrhization helper bacte-ria); and

• by altering the signaling processes betweenplants and mycorrhizal fungi

Biochar’s ability to improve aggregation insoils may also promote soil microorganismby improving the balanced water/air envi-ronment in which they thrive.(11)

It is also important to understand thatdifferent feedstocks used in the productionof biochar effect soil biota differently. For ex-ample, Steinbeiss et al. (2009) found thatsoils amended with glucose-derived biocharpromoted Gram-negative bacteria in soils,while yeast-derived biochar promoted fun-gal populations.(10) The implication that dif-ferent sources of biochar promote differentsoil-dwelling populations means that grow-ers will have to make informed choicesabout their biochar sources based on theneeds of their soil and crops.

Additional benefits: pH, water reten-tion, and nutrient holding capacityOther agricultural benefits of biochar can beattributed to its complex surface area. In fact,a single gram of charcoal can have the sur-face area equivalent to 5000 ft2, or the size oftwo tennis courts!(2) This complexity enablesbiochar to retain nutrients, thus increasingthe efficiency of applied fertilizers and re-ducing leachate pollution.

These same properties contribute to im-proved water-holding capacity of soils. Thewater retained in the abundant pores helpsplants and soil biota alike meet theirhydration needs.

The one characteristic of biochar that ap-pears to be consistent regardless of parentmaterial or production factors is its pH. Mostbiochars are alkaline, with a pH greater than7.0. Some scientists suggest that this aspectof biochar can be credited, at least in part, formany of the positive plant responses demon-strated in biochar-amended soils.(3) An alka-line addition to the soil will raise the soil pH,which increases the bioavailability of nutri-ents to plants.

Application RecommendationsThe wide range of variability amongbiochars’ various characteristics makes verydifficult the task of developing a recom-mended rate of biochar application for agri-cultural soils. In studies that document

Amagnified view of biochar shows its porous structure.Source: S. Joseph (left) and Yamamoto (right).

positive plant responses from biochar addi-tions, application rates ranged from 0.5-7tons per acre.(2) Therefore, farmers and gar-deners should approach biochar applicationswith some caution, by perhaps applyingonly small amounts of biochar at first, andcarefully monitoring vegetation after addi-tional applications for any signs of overdose.Growers with alkaline soils would be wise toavoid biochar applications, due to its alka-line nature.

Biochar is very fine and can easily be dis-placed by wind, which would turn it into aparticulate pollutant instead of a soil amend-ment. Therefore, surface applications are notrecommended. Instead, biochar should betilled in, or added to the soil concurrentlywith transplants. In no-till operations wheresurface applications cannot be avoided,biochar should be mixed with compost be-fore applying to reduce the chance of winddispersal.(2)

ConclusionsAs the world’s population swells and itsagricultural lands give way to development,less land will be available to feed more peo-ple. Therefore, the quality of the remainingagricultural soils — in this case measured bytheir productive capacity — will become in-creasingly important.(2) Equally important,however, is that the solution to this challengedoesn't contribute to further environmentaldegradation. Since biochar and its produc-tion captures carbon, offers a solution tofarm-waste management, and generates en-ergy, it is being promoted by some as the“silver bullet” solution.

The study of biochar’s effects on soils isin its infancy. The only thing we can be sureof is that biochar is a widely variable productwhose influence on soils is also widely vari-able. However, a growing body of research

indicates that biochar is generally beneficialto agroecosystems. Farmers and gardenerswho adopt biochar use as part of their soil-management practices can only help clarifyits usefulness by contributing more real-lifedata (even if only anecdotal) to the body ofevidence about this fascinating substance.

References and Further Reading1. Blackwell, P., Riethmuller, G. and M. Collins. “Biochar Appli-

cation to Soil,” in (Lehmann, Johannes and Stephen Joseph)Biochar for environmental management: science and tech-nology, 207-226, Virginia: Earthscan, 2009.

2. Bruges, James. The Biochar Debate. Vermont: Chelsea Green,2009.

3. Chan, Y. Yin and Zhihong Xu, 2009. “Biochar: Nutrient Prop-erties and Their Enhancement,” in (Lehmann, Johannes andStephen Joseph. Biochar for environmental management: sci-ence and technology, 67-84, Virginia: Earthscan, 2009.

4. Graber, E., Harel, Y., Kolton, M., Cytryn, E., Silber, A., David,D., Tsechansky, L., Borenshtein, M., and Yigal Elad. “Biocharimpact on development and productivity of pepper andtomato grown in fertigated soilless media.” Plant Soil 337(2010):481–496.

5. Jin, Hongyan. “Characterization of microbial life colonizingbiochar and biochar-amended soils.” Diss. Cornell Univer-sity, 2010.

6. Lehmann, Johannes and Stephen Joseph. Biochar for Envi-ronmental Management: An Introduction,” in (Lehmann, Jo-hannes and Stephen Joseph. Biochar for EnvironmentalManagement: Science and Technology, 1-12, Virginia: Earth-scan, 2009.

7. Lehmann, Johannes. “A handful of carbon.” Nature 447 (10May 2007): 143-144.

8. Marris, Emma. “Black is the New Green.” Nature 442 (10 Au-gust 2006): 624-626.

9. Sohi, S.P., E. Krull, E. Lopez-Capel, and R. Bol. “A Review ofBiochar and Its Use and Function in Soil.” Advances inAgronomy 105 (2010): 47.

10. Steinbeiss, S., G. Gleixner and M. Antonietti. “Effect ofbiochar amendment on soil carbon balance and soil micro-bial activity.” Soil Biology and Biochemistry 41:6 (June2009): 1301-1310.

11. Thies, Janice and Matthias C. Rillig, 2009. “Characteristics ofBiochar: Biological Properties,” in (Lehmann, Johannes andStephen Joseph) Biochar for environmental management:science and technology, 85-105, Virginia: Earthscan, 2009.

12. Warnock, Daniel D., Lehmann, Johannes, Kyuper, ThomasW., and Matthias C. Rillig. “Arbuscular Mycorrhizal Re-sponses to Biochars in Soils: Concepts and Mechanisms.”Plant Soil 300 (2007): 9-20.

13. Wolf, Duane C. and Wagner, George H, 2005. “CarbonTransformations and Soil Organic Matter Formation,” in(Sylvia, David M., Fuhrmann, Jeffry J., Hartel, Peter G. andDavid A. Zuberer), Principles and Applications of Soil Mi-crobiology, 285-332, New Jersey: Pearson/Prentice Hall,2005.