Home » Biochar Training » Instructions for Making Char in a Trench

Instructions for Making Char in a Trench

What you will need

  • A hoe
  • A sheet of corrugated roofing metal, the longer the better. NOTE: It does not need to be new. Any old piece will work as long as it does not have holes so big that you cannot block them with dirt.
  • A long stick. Two meters is great.

How to make your trench

Lay the sheet of roofing metal on flat ground where you want to make your trench.

Instructions for making char in a trench

Using a stick, outline it by drawing all around its edges.

Instructions for making char in a trench

Set the roofing sheet aside.

Instructions for making char in a trench

Inside the rectangular outline of the roofing sheet you made in the dust, draw a second, smaller rectangle. Make the sides of the second rectangle at least a full hands’ breadth inside the sides of the original outline.

With your hoe, dig out the area inside the inner, smaller rectangle. Dig down approximately 70 centimeters (about the length of your arm). Make the sides of the trench slope in toward the bottom. Not much, maybe 10 cm.

Scrape the dirt you dig from the trench outside of the original outline of the roofing sheet. Leave it piled around all four sides.

When you are finished digging, test your trench by laying the roofing sheet over it. You have a good fit it:

  • The roofing sheet covers the trench with no gaps around the edges.
    • If the trench is too wide or too long and you cannot cover it completely with the roofing sheet, dig a new trench. The gaps will allow air to enter. Your biochar will burn up!
    • The roofing metal should lie flat on the ground. Scrape all the loose dirt from the hole back so that the roofing sheet lies flat on the ground all the way around. No bumps!
      • If the roofing metal does not lie flat, scrape back any loose dirt. If that is not enough, use your hoe to level the ground around the trench.
Instructions for making char in a trench

How to use your trench

  • When you are satisfied that you have a good trench, set the roofing sheet aside again. You will not need it until the end.
  • Start a small fire at the bottom of the trench and feed it. When it is going well, spread the fire the length of the trench.
  • When you have a good fire going along the bottom of the trench, begin to add feedstock.

Good trench use practice

Anybody can make a fire. You are not making a fire. You are making biochar. You are making biochar (1) to stop the smoke that just burning the feedstock would produce and (2) because you want as much biochar as possible from the work you are doing. To succeed at (1) and (2) takes practice and know-how.

  • Fires smoke when they are not hot enough.
    • If you add feedstock too slowly and your fire begins to die down, it will smoke unless you feed it better.
    • If your feedstock is wet, it will not char until the moisture has evaporated. This cools the fire and your trench will smoke. Do not use wet or green feedstock.
    • If your feedstock is straw or anything else that comes in bundles, it will smoke because it partially smothers the fire. Do not throw bundles of stuff into the trench. “Sprinkle” them in.
  • Really big fires do not smoke, but they burn up a lot of your biochar, too.
    • It is very satisfying to build a bonfire in your trench. Unfortunately, bonfires burn lots of carbon leaving you with less biochar. Restrain yourself.
  • You want balance when using a trench and balance takes practice. You have to watch your fire and think about your two goals. You want a fire big enough not to smoke but small enough not to waste carbon.
  • Here are three  other tips.
    • If you are using a low-density feedstock – say, millet or maize stalk, rice or wheat straw – poke the burn regularly with your long stick. The char will crumble and you will be able to make much more char per trench.
    • If you are using tree branches or bamboo, always use pieces that are roughly the same diameter. If you do not, either the small stuff will be consumed by the time the big stuff is ready or the big stuff will be only half done when the small stuff is ready.
    • If you have a bit of clay handy, especially nice reddish clay, sprinkling it onto your fire will improve the quality of your char. Just toss a handful or so on every few minutes.
Instructions for making char in a trench

Finishing your burn

  • You can stop adding feedstock at any time; you must stop when the trench is completely full of biochar.
  • When you want to stop, simply stop adding feedstock.
  • Watch the flames. For a minute or two, the surface of the char will be covered with dancing orange flames. When these die down to almost nothing, your char is ready.
  • Place the roofing sheet over the trench.
  • Scrap the loose dirt over the edges of the roofing metal and pack it down with your feet.
    • If you see any smoke escaping from gaps under the roofing sheet, put dirt over them and pack it down. Your object is to seal the trench so that no air can get in.
  • Now wait – a long time. Biochar is stubborn stuff. A trench can take five hours to go out.


  • When you open your trench, use your hoe or long stick to turn the biochar. Watch it carefully. If it begins to go grey – anywhere – or to smoke – even just little wisps – close the trench up again and wait some more.
    • If you do not, you biochar will all burn up.


There are great stories about guys who left their truck overnight with a load of new char in the back only to return to a gutted, melted ruin in the morning.



The best thing about using a smothered trench is that the char is bone dry. If you are charring anything soft/flexible (not bamboo or wood), the char also does not require grinding, but will powder under foot. If you are charring wood or bamboo and need a fine, regular grain, you may need a grinder. If you do not, spread the char on hard ground and stomp on it. Kids on bikes, motorbikes and even pickups also do the job. Warning: The dry char will produce a lot of fine dust that is very bad to breathe. Unless you need bone dry char, consider spraying some water about. If you are in a confined space (a shed or barn), absolutely spray some water about to avoid an explosion risk.

Instructions for making char in a trench
  • How much char you get from a trench depends on your feedstock. We assume about 5 kg of dry char from a trench fed corn stalk, more for tree branches, less for rice straw.
  • If you have a ready supply of feedstock (straw, stalk), it should take no more than 10-15 minutes to fill a trench.
  • We, therefore, recommend that you prepare as many trenches as possible during or just after the rains when the ground is soft. If your feedstock is naturally centralized (for example, straw piles left by threshers), dig there. If you feedstock is on a steep slope (for example, mountain corn), dig a row of trenches across the bottom of the field.
  • Consider your only real limit to be the number of roofing sheets you can afford. After all, if you have 30 sheets, two of you can probably do 15 trenches in the morning, 15 in the afternoon and then refill the 15 from the morning to leave to smother overnight with the 15 from early afternoon.
  • We have found the easiest way to move char is to shovel it into old rice bags. A full bag of dry char should weigh no more than about 10 kg, which makes it easy to carry out of the field.
  • If possible, wear an N95 rated mask. We do not recommend the basic N95 without a side vent because you will find it hard to breathe. An N95 mask with one or two side filters should do fine and will allow you to pant.

But stop!

What if the people you are working with are too poor to be able to afford roofing sheet? (This is not a joke. Warm Heart’s partner in Malawi wrote to say that the first draft of this set of instructions was very nice, but useless in Malawi – because farmers could not afford roofing sheet.)

Panic not. There is a good solution.

If you cannot afford roofing sheet, do not want to use it or simply want to make more trenches than you have sheets for, here is an elegant solution from southern Africa.

Just add dirt.

Fill your trench to the very, very top with biochar and beat the char down with your long stick. When the trench is absolutely full, push the dirt you dug out of the trench back on top of the hot char. Spread it out carefully to leave no char exposed. Look for any smoke. If you see any smoke, put dirt on it. Pack the dirt down with your feet. (Be careful!) Add more dirt. Look for smoke. If you see any…. And so on.

Leave for at least five hours before you open the trench up.

Remember, if it turns gray or smokes, you must cover it up again or else…!

Don’t worry if you get a bit of dirt mixed in with your char. It will make no difference.

Best biochar tech for the poor

©️ D. Michael Shafer 2019

#biochar #biochartrench #makingbiochar

Easy to Share!

9 Responses

  1. Joram Mathenge
    | Reply

    Thankyou for the guide on how to make biochar.I have tried this and its amazing since you can produce biochar in large quantities.The question is? My organization is working with rural farmers in Kenya to enhance their food production through farmschools.How can we partner with Warm Heart Worldwide.
    Joram Mathenge
    Executive Director
    Kiangure Springs Environment Initiative

  2. Jillian Lemmond
    | Reply

    I live in a small town and regularly have local companies that cut down and chip trees, dump their chips in my driveway to use as much in the garden and for making pathways through the garden. I’m wondering if I can also use these chips for making biochar small scale in my back yard? it’s wet fresh wood so has to be used asap in my garden and spread thinly in the driveway asap until all used up so it doesn’t heat up and ignite. Would I need to spread it out and dry first to use in a small pit?

    • admin
      | Reply

      Dear Ms. Lemmon,

      This is a great question. I want to think about it some more. My immediate reaction, however, is (1) it is really important to dry the chips thoroughly on the driveway. Fires smoke when they are not hot enough. Damp chips will require a lot of heat to dry before they char. This will be an even bigger problem in a trench where there is little oxygen. (2) Once the chips are totally dry, I would start a good fire at the bottom of your trench and then feed the chips in fairly slowly, keeping an eye on the smoke. A good trench fire should not be too small (it will smoke) or too large, a bonfire (you will be wasting – combusting – carbon rather than keeping it as char).

      Michael Shafer

      PS My other thought would be to char the chips in a sealed retort. If you are interested, I can send you a description if we don’t have the new video up yet.

  3. Crisencio M. Paner
    | Reply

    Can we measure the CO2 release during biochar production? This is to find out if it does not really release CO2 in the process and does not contribute to GHG? There must be the right amount of CO2 released in relation to the Biochar production. I asked this question because I want to have assurance that by making biochar is not really contributing GHG in the atmosphere.

    • admin
      | Reply

      Yes, there are good measures of CO2 and other gas emissions from pyrolysis. What is released during pyrolysis depends very much on what technology you are using to make biochar but making biochar is carbon negative irrespective. If you use a TLUD, then the exhaust gases will be CO and H2. If you use a trough or trench, according to some, they will include considerable amounts of methane (CH4) that will offset the emission reduction achieved from making biochar for twenty years. The trade off is this: the biochar itself i(which is 40% of the total carbon originally contained in the biomass) s 80% carbon such that the mass of the biochar is one half the mass of the CO2 removed from the atmosphere. The question is then this: To what extent do CO and CO2 equivalents such as N2o and CH4 released in the pyrolysis process offset this “gain”? The answer seems to be that pyrolysis is carbon negative. (An open field burn, for example, releases 1,585 kg of CO2 per 1,000 kg of biomass burned, 102 kg of CO, 0 kg of N20 and 5.82 kg of CH4. Pyrolysis in a TLUD leaves 40% of the biomass carbon in the form of biochar, releases essentially no CO2, 102 kg of CO, 4.85 kg of CH4 and no N20.) (Note, the 1,585 kg of CO2 is the result of the added weight of the O atoms.)

  4. D. Michael Shafer
    | Reply

    Dear Dr. Pazhani, Thank you very much for taking the time to write. Your kind words make the work at Warm Heart lighter! Our purpose is, in fact, to simplify, to take the highly over-“complexified” world of biochar and make it available to those who need it most. It may be true that biochar is immensely complex and scientifically interesting, but we should not lose sight of the fact that what the world’s 2.54 billion poor, tiny farmers need is not customized, supplemented biochar. The core qualities of all biochars meet the basis requirements of the soils that these farmers must contend with. Their soils are acid, prone to aluminum toxicity, hard, often impenetrable clays that shed water and do not retain what penetrates, infertile (low CEC), and virtually if not completely without soil life. Basic biochar will raise pH, thereby lessening the threat of aluminum toxicity and increasing plants ability to take nutrients from the soil, it will render soil more porous and increase water retention, it improves CEC and attracts and supports soil microbiota. No less important, farmers can make it themselves for free from their own crop wastes that they would otherwise burn. And, by biocharing, not burning, they reduce the emission of climate change gases and health damaging particulates.

    | Reply

    Dear Sister,
    I went through too much of literatures related to biochar. The biomass used and the methods followed for its production determines the quantity of biochar produced Your method is simple, economical and easy to adopt by the needy poor, which the need of the hour. In our area (southern Tamilnadu, India), the negative effect of prosopis juliflora has been a topic of discussion and its positive impacts as a biochar and its uses as a generate of employment and livelihood to the rural poor, source of foreign exchange earner through export and also persistent improvement in soil fertility, which improves soil PH and growth of plants and yield. But most of the writeups are missing one part, ie., the quantity to be applied for better yield. I wish to do this. You are my motivator. I will contact you whenever I need any clarity of thought in this respect.
    Thanking you,
    With prayers,
    K. Pazhani

  6. Dr. K. Pazhani, Ph. D.,
    | Reply

    Dear sister,
    Your service is a boon to the needy poor farmers. This is the matter I have searched for a long time.
    Thank you for your timely service. The world will remember you forever. Will you me to have a visit to your place.

  7. Sr. Miriam Paulette
    | Reply

    This is very practical. Thank you.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.