Making Biochar to Improve Soil

By making biochar from brush and other hard to compost organic material, you can improve soil — it enhances nutrient availability and also enables soil to retain nutrients longer.

| February/March 2009

making biochar - improve soil

One method of making biochar: pile up woody debris in a shallow pit in a garden bed; burn the brush until the smoke thins; damp down the fire with a one-inch soil covering; let the brush smolder until it is charred; put the fire out. The leftover charcoal will improve soil by improving nutrient availability and retention.


Last year, I committed one of the great sins of gardening: I let weeds go to seed. Cleaning up in fall, I faced down a ton of seed-bearing foxtail, burdock and crabgrass. Sure, I could compost it hot to steam the weed seeds to death, but instead I decided to try something different. I dug a ditch, added the weeds and lots of woody prunings, and burned it, thus making biochar. It was my new way to improve soil—except the technique is at least 3,000 years old.

What’s biochar? Basically, it’s organic matter that is burned slowly, with a restricted flow of oxygen, and then the fire is stopped when the material reaches the charcoal stage. Unlike tiny tidbits of ash, coarse lumps of charcoal are full of crevices and holes, which help them serve as life rafts to soil microorganisms. The carbon compounds in charcoal form loose chemical bonds with soluble plant nutrients so they are not as readily washed away by rain and irrigation. Biochar alone added to poor soil has little benefit to plants, but when used in combination with compost and organic fertilizers, it can dramatically improve plant growth while helping retain nutrients in the soil.

Amazonian Dark Earths

The idea of biochar comes from the Amazonian rain forests of Brazil, where a civilization thrived for 2,000 years, from about 500 B.C. until Spanish and Portuguese explorers introduced devastating European diseases in the mid-1500s. Using only their hands, sticks, and stone axes, Amazonian tribes grew cassava, corn, and numerous tree fruits in soil made rich with compost, mulch, and smoldered plant matter.

Amazingly, these “dark earths” persist today as a testament to an ancient soil-building method you can use in your garden. Scientists disagree on whether the soils were created on purpose, in order to grow more food, or if they were an accidental byproduct of the biochar and compost generated in day-to-day village life along the banks of the Earth’s biggest river. However they came to be, there is no doubt that Amazonian dark earths (often called terra preta) hold plant nutrients, including nitrogen, phosphorous, calcium and magnesium, much more efficiently than unimproved soil. Even after 500 years of tropical temperatures and rainfall that averages 80 inches a year, the dark earths remain remarkably fertile.

Scientists around the world are working in labs and field trial plots to better understand how biochar works, and to unravel the many mysteries of terra preta. At Cornell University in Ithaca, N.Y., microbiologists have discovered bacteria in terra preta soils that are similar to strains that are active in hot compost piles. Overall populations of fungi and bacteria are high in terra preta soils, too, but the presence of abundant carbon makes the microorganisms live and reproduce at a slowed pace. The result is a reduction in the turnover rate of organic matter in the soil, so composts and other soil-enriching forms of organic matter last longer.

In field trials with corn, rice and many other crops, biochar has increased productivity by making nutrients already present in the soil better available to plants. Results are especially dramatic when biochar is added to good soil that contains ample minerals and plant nutrients. Research continues (track it at The International Biochar Initiative), but at this point it appears that biochar gives both organic matter and microorganisms in organically enriched soil enhanced staying power. Digging in nuggets of biochar — or adding them to compost as it is set aside to cure — can slow the leaching away of nutrients and help organically enriched soil retain nutrients for decades rather than for a couple of seasons.

4/29/2016 12:46:04 PM

Interesting article, but why must we continue to invent new words for things that have been practiced and have gone on for ages. Just because it is being rediscovered does not mean we need a new word to make it sound like a new breakthrough. Farmers have been using ash and charred wood for ages, it just went out of fashion when all the chemical adiditives were being marketed which did not require for thought and knowledge that was possessed by the "ancients". Its good to rediscover past knowledge, don't give a new name to make it seem new.

6/4/2013 3:02:56 PM

Sorry about the extra posts,MEN has it backwards on posting.


6/4/2013 3:01:01 PM

Climate Change?

6/4/2013 2:59:12 PM

Climate Change?

6/4/2013 2:57:58 PM

Climate Change?




6/4/2013 2:57:13 PM

Climate change?


6/2/2013 7:03:27 PM

I say KUDOS to you who bring us further reseaarch and more science. Thank you!

6/2/2013 3:16:54 PM

My gardenin g friends will like this.

5/14/2013 11:18:39 PM


5/14/2013 11:11:40 PM

The last thing I would believe is the so could "scientific results". Most of it, more than 90 % are funded by business interest to make us believe what they want us to. The truthhowever is just around for us to observe. The poorest amongst us are exposed to smoke the most, however observation will convince all of us that they are not prone to cancer more than any one else. This talks about PAH is another of those funded self interest.

Another example is the global warming scam and the reason for this to exist is to convince humanity that CO2 is bad thus they (the elite) can implement CO2 capture (read about it) with our support. In reality and everyone knows it, CO2 makes plants grow. CO2 capture will stunt plant growth, will result in less food. This elite group then with all their funds can make synthetic food to sell us. You see control of food is control of humanity. Global warming scam is all about global control. wake up before its to late.

brian cartwright
5/11/2012 12:32:26 PM

Hi folks, I want to second the concerns about improperly burned biomass and emissions such as PAH's, also the important difference between biochar and ashes. Biochar is all about properly burning off the gases from biomass so that only the basic carbon structure remains. If your soil needs potash from ash, that's a different issue. I've put together a lot of links to good biochar info on the web at and hope people will find them useful.

meredith lancaster
12/16/2011 6:14:05 AM

Peter_28 ...Do you know of any tests which show the presence of PAH's in Terra Preta? Where can I get more information on that topic? Wikipedia does not address Terra Preta under their articles on PAH and they do not address PAH in their articles on Terra Preta. Something I read made it sound to me like the microbes might 'eat' the PAH... where is a good source for more information?

12/21/2010 7:10:57 AM

I heard about biochar a few months ago from a friend of mine. I never thought that something as simple as charcoal could do so much for the soil and the environment. I was amazed after reading "The Biochar Revolution" from They have a great discount for Christmas on the book at the moment. Check it out. It was a great help in opening my mind to issues that affect us all.

john bonitz_2
3/1/2010 4:51:06 PM

This is a very interesting article. Thank you to Barbara for writing it and doing the experiment! I should note that the science of biochar production has evolved during the past year since this was written. Please note that THIS method of charcoal production is NOT climate-friendly. Any combustion process that releases un-burned gases will actually exacerbate the greenhouse-effect, AND pollute the local air-shed. The simple pyrolysis/gasification effect of incomplete combustion during open-burning will release gases called "volatile organic compounds" or VOCs, including methane. This looks like smoke or fumes, and may be gray or yellowish in color. The same goes for those methods using a primitive kiln or retort like a steel drum. Methane is a gas that is 20 to 25 times more potent than CO2 in trapping heat in the atmosphere. In other words, the effect of the gases you create while making biochar could exceed the carbon-capture benefit of biochar in soils. To fix this, you have two options: 1) make certain that any gases you create (i.e., "smoke" or "fumes") are burned or flared, thus reducing the VOCs to CO and CO2 (less potent GHGs than methane). 2) even better, engineer a system to make use of these gases for thermal energy. Waste-not-want-not! Cheers!

kris johnson
1/4/2010 5:31:00 PM

Ashes are not the same a biochar! They contain lots of potassium and are very alkaline so you have to be very careful with them. Many soils have too much potassium and not nearly enough calcium. Biochar is long-lasting carbon, which is what is valuable, but the carbon has all burned off with ashes. Adding wood chips to your garden will tie up nitrogen until the wood chips break down into compost, but they don't give you biochar.

kris johnson
1/4/2010 12:03:26 PM

I might add that in tropical areas the carbon in plant residues and compost burns off very fast because of the heat (hence the thin soils in tropical jungles), so the biochar was very valuable as a long lasting form of carbon.

kris johnson
1/4/2010 11:37:11 AM

Sorry for the late comment, but I just ran into this issue of MEN in a stack of unread material. I experimented with simple biochar burning last year with an old garbage can inside an old leaky barrel, using the woody trimmings from my wild flower meadow. It worked pretty well, though I didn't make any attempt to test the results as Barbara did. I'd like to comment on Barbara's experiment with squash. The layered bed had one advantage that the biochar bed did not have - that is the compost. The burning of the biochar would have tended to kill off the soil microorganisms in the vicinity, while the compost would have added humus and soil microorganisms as well as food for them. Since the life in the soil is crucial to healthy productive soil, the biochar bed started out at a disadvantage. As the season wore on the biology came back thanks to the nourishment from the organic fertilizer, but if compost and fertilizer had been added to the biochar bed it might have grown like gangbusters. Biochar is a very stable long lasting form of carbon, which no doubt attracts mineral ions as does good clay soil, but it is not the same as the humic acids (also carbon) formed by the microbes in good compost which hold minerals in the form most readily available to the plants. Kris

barbara pleasant_3
8/22/2009 7:11:56 PM

If you're interested, I just posted the results of my experiment growing winter squash in a pit burned using basic biochar techniques. See it here:

7/16/2009 5:35:37 PM

Sorry about the multiple posts, maybe admin can remove them for us? The end of the post was cut off so.. However The USEPA has offered a lifetime cancer risk estimate of 62 per 100000 exposed people per µg benzene emission per m3 ambient air from burning, with incomplete combusion producing the highest levels of the most dangerous PAH. The guidelines values for Benzo(a)Pyrene corresponding to an excess lifetime cancer risk of 10-5 was estimated as 0.7mg/litre in Guideline for drinking water quality (WHO, 1998). The comment about charcoal removing lead is not correct and lead is rarely in a bioavailable form in non tropical temperate zone soils where the soil geochemistry in any case, unless it is associated with the breakdown products of leaded gasoline from leaking underground tanks of former gas stations. I am a director of science of an environmental consultancy with a special interest in PAH. I hope that you find this helpful and that it encourages you do do some reading of your own. I think you will be shocked.

7/16/2009 5:19:50 PM

The incomplete combustion of organic matter produces extremely toxic polycyclic aromatic hydrocarbons, (PAH), which have no place in agricultural soils. These compounds are so dangerous that some of the partial burn products such as pyrene are regulated here in Europe to the point that the presence of ANY of these proven carcinogenic toxins above laboratory detectable levels, (in some cases just a few parts per million), triggers the legal requirement to clean the land up to remove PAH or remove the pathway between the compound and humans. The presence of these compounds will also mean that in the UK, where the values exceed our soil guideline values, then the soil contaminated with levels of PAH easily exceeded by this proposed process would actually be classified as hazardous waste and have to be sent to a special landfill or treated to remove the PAH contamination. Also, PAH contamination of groundwater is a serious problem in much of the industrialised world. I would not want to see farmland polluted in this way. Also, given the probable increase in N2O output from treated soils and the release of CH4, (nitrous oxide and methane, both many time more powerful greenhouse gasses), I doubt that there are any net benefits overall in the release of greenhouse gasses. It is good to see alternative ideas and thinking about how we can do better, but this has to be properly thought about. I for one would not eat any food laced with PAH. Much as I dislike wikipedia, a good introduction to the chemistry of PAH can be found here Owing to its carcinogenic properties no safe level of PAHs can be recommended. There is no known cancer threshold for Benzo(a)Pyrene the most thoroughly studied PAH. (World Health Organisation, 1987). All levels are associated with increased cancer risk However The USEPA has offered a lifetime cancer risk estimate of 62 per 100000 exposed people per µg benzene em

4/8/2009 6:42:31 PM

I have an outdoor wood stove that heats my home. I am going to start a vegetable garden this season and was wondering if ashes from my stove would provide good soil amendment for my garden? Any thoughts about doing this?

4/4/2009 3:28:45 PM

We have just started selling a precharged biochar product for gardeners in Marin County California called Growth Charm. Biochar such an important thing to do for so many reasons. Reducing the use of fertilizers and improving soils nutrient holding capacity are well known. Few seem to appreciate biochar's ability to protect food crops from soil toxins including allelopathic chemicals from certain trees like black walnut and even charcoal's ability to extract and hold lead so that plants don't take it up.

3/6/2009 1:00:16 PM

From a reader: The sentence about making charcoal from wood scraps in metal barrels should have been followed immediately by a very strong caution not to use wood that has been treated with chemicals to retard rot and/or insects. Inhaling smoke is very hazardous to health. Each piece of new stock treated wood has a caution not to burn stapled onto one or both ends - Mother

chris singer
2/9/2009 10:05:34 PM

Barbara, Thank you for your comments and suggestions. I may bring this up at our local fish club as a possible project also. Thank You again, Chris

2/8/2009 9:40:55 PM

My family burns corn for heat in the winter. I wondered if the waste from the corn stove would be suitable for use as biochar. Does anyone know if this is so?

barbara pleasant_3
2/8/2009 9:55:37 AM

Carolyn, Your alkaline conditions would benefit more from using woody materials as mulches or soil amendments than from burning them, and I agree with your agent about the wood ashes. Woody materials in general have an acidic pH, which is exactly what you want. Mulch and compost is the way to go. Good luck... Chris, One of the leading researchers at Lehmann's lab at Cornell also keeps fish, and uses her old charcoal in soil mixes for houseplants. I think you do have a valuable resource! If I had your scummy activated charcoal, I'd rinse out salts and then mix it with finished compost that's being set aside to cure. Or, just dump it in your garden. My I suggest keeping track of its possible effects? Lots of people would like to know more about its value in the garden.

chris singer
2/6/2009 9:57:04 AM

I have a question. I raise tropical fish and use activated charcoal as a filtration agent. Would this work as Biochar after its effectiveness as a filter medium? I already use some waste water from my tanks for my container garden and wonder if I am throwing out a valuable resource. Any thoughts?

carolyn overbo
2/5/2009 7:47:04 PM

The pH of my soil ranges from 8.0 (in the garden) to 8.5 (on my lawn). My county agent has told me not to add ashes to my soil because ashes will raise the pH. What is the pH of biochar? Can I just shred material with a high carbon content and bury it, or does it really need to be burned first?

craig knock
2/4/2009 11:03:55 AM

Bio Char is great stuff , but as with anything else it can be overdone. Keep in mind that the nutrients in the char do add to the total nutrient ballance in the soil. To much of a good thing is seldom good. About global warming or Global Climate Change as it has morfed to since things have started to cool off, you are not going to save the planet with bio-char. You can improve your soil and the quality of your food a lot if done in moderation. As we crusade to "save the planet " from the CO2 monster, just remember that your house, your wood table, the trees, your garden and almost everthing you have ever eaten was once free CO2 floating around in the atmosphere. Yes, it all came out of the air. I have asked a lot of CO2 Chicken Littles what is the Ideal PPM of CO2 we are targeting and "NO ONE" knows! Would the climate adjusting God like person who is capable of knowing this and managing the climate please step forward and present their credentials. Careful though, a false prophet with a good line of BS and an ego to match could cause unbelivable dammage while making themselves rich beyond immagination. Didn't the Goracle make just over 100 million selling carbon indulgence thus far. May need to get some of those before starting up the char pit.

barbara pleasant_3
2/2/2009 2:47:04 PM

It is so great to have so many sharp minds weighing in on biochar. There is much to think about, both globally and at home. The latter is my specialty, and we gardeners have an opportunity to look not only at how biochar might benefit our soils when used as an amendment, but also at the effects of on-site burning on the soil itself. It is possible that as low-oxygen burning takes place IN the ground, soil biota respond by staging a wave of renewal that is somehow captured and slowed by biochar. But there is much we don’t know. As I researched biochar for the article, I kept coming back to mental pictures of the early agricultural society that created terra preta. Archaeologists think they enjoyed a lot of leisure time. When a crop did come in that required hard work, the people probably moved to the field and turned harvesting into a party. For example, cassava (also called manioc or yucca) was dug, stripped, cooked and dried in the field, over a period of days. Replicating that process means burying the fire, which is why I started with an excavated site. Please join me in thinking up ways we can make or use biochar in our gardens without releasing unnecessary smoke. To show you more of my experiment, we added some photos to this article’s IMAGE GALLERY today. Other staffers at Mother Earth News will be trying their ideas, too, but the proof will be in our gardens. Stay tuned! Thanks for helping us think big, and in this case, thanks for helping us think small, too.

2/2/2009 1:02:18 PM

Great article! I had never realized the connection. We have an open burning law in my city so it's kind of frowned on to make fires in town. But the idea of checking campgrounds is great. There are actually a couple of close state camping grounds within driving distance! Nice article, great information with a huge impact for us all!

james miller_1
1/31/2009 10:49:45 PM

i thought this was a great article. But I was wondering how much biochar was enough and can you get too much in a garden. Also how fine should the charcoal be when it is applied to the soil.

1/31/2009 3:19:53 PM

Whoa. Biochar is a great thing, but let's not get carried away. Read page 3 of this article again carefully. When you make biochar you release a lot of CO2 into the atmosphere -- just not as much as open-burning of wood with unrestricted oxygen produces, but you're still putting out plenty of CO2 into the air. You only make up for that when you put the biochar into the garden and it does its job in the soil over decades. So it takes a long time before the making of biochar becomes "carbon neutral" if it ever does. Certainly making biochar will do nothing to reduce or "balance out" the amount of carbon going into the atmosphere from other sources such as autos and coal-fired power plants as one reader seemed to be suggesting.

ron larson
1/31/2009 10:34:22 AM

I thought Barbara's comments on biochar were almost perfect. I say "almost" hoping there can be more next time on the climate benefits of biochar - a topic that is driving many Mother Earth readers to this magazine. If we all (6.5 billion) were to produce just a ton of biochar each year, we could offset our entire annual global atmospheric input of fossil fuel-based carbon. Of course most of us will have to rely on the local utility or biofuels suppliers to do much of that on our behalf. But as Barbara has demonstrated, we can have better gardens if we do more ourselves as well. In addition to the two very different charcoal production techniques offered by Barbara and the blacksmith - let me recommend closing down one's wood burning stove each night. I am now getting about a pound or two of nice cold lump charcoal each morning. Granted the room temperature drops a bit lower - but I am getting more into home gardening as I try to lower my carbon footprint through biochar - that I have to put somewhere.

1/30/2009 7:55:13 PM

For those who don't have open ground where they can make biochar -- and those who live where open burning is prohibited -- what about something as simple as using a stand-alone BBQ grill (the kind intended for Sunday grilling which are allowed everywhere)? Branches and other scrounged wood/weeds, even a bag of coarse "pine chip" mulch from the garden center, etc. could be control-burned in the grill. BBQ grills have a built devices for controlling the amount of oxygen that can get to the fire. While probably not as good as the retort kiln mentioned in a previous comment, it certainly seems like a viable way to create modest amounts of biochar -- and a new, round BBQ grill can be had for as little as $20 bucks in a Mega Mart or Big Lot store.

fred baginski
1/30/2009 7:12:45 PM

Interesting article, but I live in the city and open burning is illegal. I'm thinking Lump Charcoal might be a good alternative. It's generally charred hardwood, (molding cutoffs and other hardwood scraps) unlike charcoal briquettes which can contain coal, petroleum byproducts and starches which act as binders. I'm involved with competition BBQ. The cooks who don't use wood pellets use either Lump Charcoal or wood chunks, so you might look into cleaning up behind a competition near you.

folke gunther
1/30/2009 6:15:29 PM

The charring methods described in the article are certainly not environmentally friendly, sonce smoke from charring contains methane, PAH and othe nastiness. Use a retort kiln instead. They are really simple. See Lots of other methods to burn the fumes during charring exists. See for example the Anila! FG

1/30/2009 12:48:09 PM

I hear of this biochar and it seems great. I worry about adding heavy metals to the soil. But the point I wanted to ask was whether this is a case where the by product of "wood gas" or heating a biomass without oxygen and capturing the volatile gases. Those volatile gases can be used to produce electricity and the "charcoal" that is the by product could be added to the soil!! It bothers me when a "Green" approach is not a "Wholistic" approach.

1/30/2009 11:03:33 AM

That is a really good artical, but would regular chacrol, like the kind we use for grilling work in the same manner, even it it's been used like ashes?

1/30/2009 8:17:41 AM

I would like more specifics, please. How deep does the trench need to be, and how high can I pile the organic material (above the top of the trench?)? I will be digging through clay and shale, so I need a specific depth to work towards.

erich j. knight
1/29/2009 8:19:02 PM

I also have been corresponding with Michael Pollan ( NYT Food Columnist, Author ) to do a follow up story. Since the NGM cover reads "WHERE FOOD BEGINS" , I thought this would be right down his alley and focus more attention on Mann's work. It's what Mann hasn't covered that I thought should interest any writer as a follow up article; Biochar data base; NASA's Dr. James Hansen Global warming solutions paper and letter to the G-8 conference, placing Biochar / Land management the central technology for carbon negative energy systems. The many new university programs & field studies, in temperate soils; Cornell, ISU, U of H, U of GA, Virginia Tech, JMU, New Zealand and Australia. Glomalin's role in soil tilth, fertility & basis for the soil food web in Terra Preta soils. Given the current "Crisis" atmosphere concerning energy, soil sustainability, food vs. Biofuels, and Climate Change what other subject addresses them all? This is a Nano technology for the soil that represents the most comprehensive, low cost, and productive approach to long term stewardship and sustainability. Carbon to the Soil, the only ubiquitous and economic place to put it. In a recent National Public Radio interview, Michael Pollan talks about how he was approached by a Democratic party staffer about his New York Times article, The "Farmer & Chief", an open letter to the next president concerning U.S. agriculture/energy policy. The staffer wanted Pollan to summarize the article into a page or two to get it into the hands of Barack Obama. Pollan declined, saying that if he could have said everything that needed to be said in two pages, he wouldn't have written 8000 words. Michael Pollan is well briefed about Biochar technology, but did not include it in his "Farmer & Chief" article to Pr

erich j. knight
1/29/2009 8:15:47 PM

I thought these updates and endorsements may interest you, Senator / Secretary of Interior Ken Salazar has done the most to nurse this biofuels system in his Biochar provisions in the 07 & 08 farm bill, Below are my current news & Links to major developments; Cheers, Erich J. Knight 540 289 9750 Biochar, the modern version of an ancient Amazonian agricultural practice called Terra Preta (black earth), is gaining widespread credibility as a way to address world hunger, climate change, rural poverty, deforestation, and energy shortages… SIMULTANEOUSLY! The IBI Announces Success in Having Biochar Considered as a Climate Change Mitigation and Adaptation Tool; POZNAN, Poland, December 10, 2008 - The International Biochar Initiative (IBI) announces that the United Nations Convention to Combat Desertification (UNCCD) has submitted a proposal to include biochar as a mitigation and adaptation technology to be considered in the post-2012-Copenhagen agenda of the UN Framework Convention on Climate Change (UNFCCC). A copy of the proposal is posted on the IBI website at The International Biochar Initiative (IBI). Modern Pyrolysis of biomass is a process for Carbon Negative Bio fuels, massive Carbon sequestration,10X Lower Methane & N2O soil emissions, and 3X Fertility Too. Every 1 ton of Biomass yields 1/3 ton Charcoal for soil Sequestration, Bio-Gas & Bio-oil fuels, so is a totally virtuous, carbon negative energy cycle. Charles Mann ("1491") in the Sept. National Geographic has a wonderful soils article which places Terra Preta / Biochar soils center stage. Please put this (soil) bug in your colleague's ears. These issues need to gain traction among all the various disciplines who have an iron in this fire. I also have been corresponding with Michael Pollan ( NYT Food C

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