Biochar as a soil amendment and carbon sequestering tool

Photo courtesy http://2.bp.blogspot.comBy Vuthisa

It is surprising how many people cart away their yard waste (renewable biomass) to landfill sites or dispose thereof in open burns until only ash remains.  I say: Make your own biochar instead.

What is biochar and what is the difference between biochar and charcoal?

They are identical in many respects, but the telling difference is in how they are used.  Charcoal is used as a fuel.  Crushed charcoal mixed into soil as a soil amendment is biochar.

As a soil additive, biochar offers numerous potential benefits

Unlike fertilizers, biochar has an extremely long life in soils. Charcoal is carbon-rich and gives it the ability to persist in the soil indefinitely by not being susceptible to biological decay.  Biochar also attracts microbes and beneficial fungi, holds on to nutrients that are put into the soil. i.e. biochar works better the second and third year than it does the first.  One of the major challenges in agriculture is to make the nutrients in the soil available to the plant when the plant can benefit from them.  Fertilizers can often only be applied early in the growing season, before the crop canopy closes and field operations are no longer feasible.  Unfortunately, between the time the fertilizer is applied and the crop takes it up, fertilizers can be leached out of the soil by excess rainfall, consumed by weeds, or metabolized by microbial activity in the soil.  Biochar helps conserve plant nutrients by storing them within its matrix and making the nutrients available when the crop needs them.  This happens because of a property in biochar, certain clays, and soil organic matter known as Cation Exchange Capacity (CEC).  CEC is a measure of the capacity of biochar to retain ions, such as ammonium and potassium cations, in an exchangeable form that is available to plants.  CEC not only helps conserve the fertilizers added to the crop during the growing season, but also improves the ability of the soil to capture and retain nutrients from other sources available at other times.  For example at the end of the growing season crop residues are often left in fields to decompose.  When this organic matter decomposes, biochar captures some of the nutrients released, leaving those nutrients for the next growing season.

Biochar in soil also has the ability to hold moisture and save on irrigation costs.  Biochar modifies the soil’s performance by retaining moisture and making it available during periods of low precipitation and hot, dry soil conditions.  This is possible because biochars have very large internal surface areas – typically over 100 square metres per gram.  This internal surface area adsorbs moisture when water availability within the soil is high and releases it back into the soil when water availability is depressed.  Some may think that biochar being black in color would heat up in the sun, but biochar helps the soil stay moist even in full sunlight.  Biochar also has significant impacts on soil drainage.  Clay soils which are typically poorly aggregated are too tight and do not drain effectively.  Ineffective drainage results in extended periods of inadequate soil aeration.  Other soils, especially sandy soils may drain too efficiently.  Overly efficient drainage can shorten the benefit of periodic wetting.  In both cases, the addition of biochar compensates for the native soil deficiency in the following ways:

Clayey and poorly aggregated soils become less compacted and provide better aeration

Sandy soil acquire additional bulk moisture storage capacity

Biochar also makes a significant contribution to mycorrhiza by promoting microbe populations.  Mycorrhiza is a fungi that has a symbiotic relationship with plant roots and contribute to a healthy soil-plant nutrient exchange.  Biochar increases the availability of mycorrhiza by:

Detoxifying soil water by adsorbing compounds that inhibit microbe growth

Providing a  protective habitat for microbes

Improving soil moisture management in which mycorrhiza thrives


Biochar can mitigate climate change

By reducing consumption of fossil fuel and

Capturing CO2 and sequestering carbon in the soil

In a world dependent on fossil energy, it is easy to see the carbon capture benefits of biochar as offsets against current and future fossil fuel emissions.  Many scientists believe there is already an unsafe excess of carbon dioxide in the atmosphere, this obligates the nations that caused the excess to abate it.  It is notable that from the year 1850 to 2000, 34% of carbon dioxide emissions have been attributed to land clearing.  Therefore, in a sense, the first goal of biochar is to restore the carbon lost from the soil due to the past 150 years of agricultural practice.  After that, the particular durability of biochar will enable the build-up of more carbon in soils, with further fertility benefits as the existence of Terra preta soils have shown us.  Terra preta soils are fertile, black biochar-rich soil found in scattered tracts throughout the Amazon basin, also, the pre-Columbian civilization responsible for creating that soil, dating back to 450 to 8,000 B.C.  Charcoal has the potential to sequester gigatonnes of atmospheric carbon per annum, making it the most potent engine of atmospheric cleansing we possess.  Approximately 8 percent of all atmospheric CO2 is absorbed by plants each year. If just a small proportion of the carbon captured by plants can be pyrolysed and transformed into charcoal, humanity’s prospects will be much brighter, for this will buy us time as we struggle to make the transition to a low emissions economy.

How do you make biochar?

The production of charcoal and biochar has a common root.  Before fossil coal emerged during the Industrial Revolution (18th Century) the word coal meant charcoal – the black fuel made from wood.  The basis for all charcoal and biochar production is pyrolysis: essentially, breaking wood down into its chemical constituents by heat, with little or no oxygen.  We do not use the same archaic methods of yesteryear, with the development of cylindrical metal vessels and high temperature refractories.  Good biochar has high porosity, extensive micro-structure, and adsorption capacity that enable beneficial interactions between microbes, nutrients, and water in the soil.  The so-called 55/30, a simple closed retort, is popular with biochar enthusiasts.  In a typical configuration, the “55/30” consists of a 55-gallon (200 litre) outer drum containing the fire around a 30-gallon (100 litre) inner drum acting as the enclosed retort.  A 30-gallon barrel, open at one end standing with the open end down on flat surface, makes a simple and serviceable closed retort.  Click here for a collection of all types of biochar making kilns:

Vuthisa opted for the Portable Metal Kiln Method for biochar production, which differs from conventional open burn methods in five ways:

The steel plate is very thick and not only lasts longer than 55 gallon drums, but allows the contents of the kiln to “cook” in the absence of charcoal by means of a tight sealing lid

The drum can be made oval to fit in two sealable 55-gallon drums, yet also allow burning material to be added to heat the drums

Small sized feedstock, typically found in yard waste, such as twigs and branches are ideal for this system as it will not turn to ash, as with larger horizontal type kilns that can take up to 3 days to cool down, reducing small diameter feedstock to ash

Large quantities of biochar can be produced as opposed to the 55/30 type closed retort that produces small quantities

The kiln can be rolled long distances to adjacent feedstock piles without any difficulty, yet can still fit on the back of pickup trucks.  More information can be found here:

Resources The Biochar Revolution – Transforming Agriculture & Environment – Edited by Paul Taylor Make charcoal in your own backyard Biochar for Environmental Management: Science and Technology Edited By Johannes Lehmann and Stephen Joseph Mycorrhiza Biochar producing kilns Biochar discussion list website Improved Biomass Cooking Stoves

To obtain access to more URLs relating to the above subject matter (not for public viewing) send your request through to:


Make charcoal in your own backyard

…with a Portable Charcoal Kiln.

By Vuthisa

Herewith a step-by-step guide to making charcoal from yard waste in your own backyard. This specific design has been used extensively since 1996 to clear Namibian encroachment bush. Having tested this system recently we see no reason why this technique cannot be applied to clear yard waste or any other biomass, including corn cobs. For more information on how this charcoal can sequester CO2 back into the soil instead of being vented into the atmosphere visit our Biochar page. What’s great about this design is that it can be used for making charcoal for extended periods, due to its durable design AND its portability – you simply have to roll the drum to where the brush pile is located.  Even though this is a direct method of making charcoal, it does NOT require complicated opening and closing of air vents or chimneys, due to the vertical stack effect of the narrow drum. If you’re looking for a way to transform your yard waste into a fuel briquette (with or without charring), visit our Fuel briquettes page. **Update**  We secured a contract to remove invasive alien tree species and return the area back to pristine grassland and to increase streamflows. See more on that here. Due to the steepness of the terrain we decided to construct the kiln in sections to be assembled on site, but more on that later. See this new video slideshow outlining the production process: 

A single burn will produce between 120 kg to 160 kg (350 lb) of charcoal from 0.7 m3/1000 kg (25 ft3/2200 lb) of medium-sized hardwood, but yields are dependent on many variables, such as your geographic location, moisture content-; type-; size of material and the experience of the operator, to name a few. It is possible to char wet timber with this system, but we don’t advocate this due to the increased time it takes to vent the extra water vapor. Colder, wetter climates yield less charcoal. In the test described below we achieved a yield of 16% charcoal, a conversion ratio of approximately 6:1  (6 weeks air-dried).  By fine tuning your burn method and identifying the visual clues more accurately, yields closer to 30% can easily be obtained. Instead of increasing your carbon footprint by transporting large amounts of timber to the kiln (as with large industrial kilns) the kiln is simply rolled (by one person) to the already prepared and air-dried brush-piles. What you will need:

  • A kiln consisting of a cylindrical drum and lid
  • 120 kg of river grade sand (optional)
  • General purpose soft potters-clay
  • Industrial purpose gloves
  • Large polypropylene bags or paper charcoal bags
  • Small ladder
  • Wheelbarrow
  • Water or Fire extinguisher
  • Shovel (optional) and Spade
  • A few bricks
  • Garden fork
  • Tinder and matches
  • Axe and/or bow saw and/or chainsaw
  • A stockpile of yard waste, preferably within wheelbarrow distance of burn site
  • A Sieve constructed from 15 mm chicken wire or expanded metal sheets
  • Black plastic sheeting to prevent bags from getting wet
  • If you are on a small holding or farm you will need a truck and or trailer to transport polypropylene bags to the packaging area
  • In some cases it may be necessary to hire/purchase an Army type tent and small generator (for stitching machine) if you don’t have a shed
  • A stitching machine and extra cones (thread)

Kiln construction

You need to construct this kiln with the help of your local light engineering works. Expect to pay around $600 USD which includes the cost of labor, so get more than one quotation. It can be a D.I.Y. job if you have good welding and bending skills. Otherwise consider having the kiln built in three sections, which is simply bolted together, no welding required and it is easier to transport. We have dubbed the new system the “Trans-Portable Kiln“. The plans for both types of kilns are contained in our Trans-Portable Kiln Construction Plans and User Manual.

Click here to download the plans. Also included in the manual is the specification sheet of a wooden Sieve Chute to screen your own charcoal. Join our participatory discussion group called the Portable Kiln Google Group. Joining it is optional, and is exclusive to Portable Kiln Owners for sharing and discussing their experiences OR enter our DESIGN CHALLENGE to design a biochar producing kiln that emits less greenhouse gases. Just print out the plans and hand it over to your local engineering works.

DIY_Biochar_KitWe have also launched a new manual. This manual describes how to build our new Biochar kiln called the ‘3-drum Biochar Retort‘. The 3-drum Biochar Retort is more clean burning (with the added lid and chimney) and the burn is more controlled, and the user has an option to create either Biochar or conventional charcoal. Click here to view the development of this kiln and how the Design Challenge helped to draw in biochar experts around the world.

We are bound by a license agreement for use of the kiln in Southern Africa, so if you are from the southern part of Africa and wish to use the Portable Kiln commercially you’re required to pay a license fee.  For more information on licensing fees, made-to-order options or simply wanting to do backyard trials without paying the license fee, kindly visit our Order page.

The plans will outline the exact dimensions- and type of materials to use as well as the guidelines set out below.  The kiln basically consists of a:

a) Lid.  The lid sits loosely on top of a cylindrical drum. **Update** We have developed several new lids to suit different applications.

b) Open ended cylindrical drum/barrel.  A length of mild steel sheet, rolled into a drum shape and welded on the seam.  Please note: an ordinary 200 litre (55 gal) oil drum will not work – or last!  This drum has a 1400 litre (370 gal) capacity and weighs 37kg (81 lb).

Some garden waste!

The thickness of the branches or stems that you use need to be more than 30 mm (1-1/4 inch), but preferably not more than 120 mm (7-3/4 inch).  The larger pieces tend to not pyrolyze completely in the center.

Yard waste

Organize the waste into a separate pile.  The volume of waste is not important, although you have to try and fill the drum up with as much yard waste as possible, but typically one should have more than 1 m3 (35 ft3) of waste ready.  The kiln can accept 0.7 m3 (25 ft3) of carbonous brush.  The branches and stems must be cut in lengths not exceeding 1 m (3.2 ft) long with the use of an axe, bow-saw or chainsaw.

Site preparation

Before anything else, carefully decide on the location of the kiln.  Make sure there are no overhead hanging branches or power- or telephone wires nearby.  Have running water (preferably a hose pipe) available or if it’s in a remote area, bring a fire extinguisher along, in case of unforeseen emergencies.  The kiln is portable in the sense that it can be rolled into position.  Care has to be taken not to position the kiln near the brush pile.  Choose a level site and clear an area of approximately 3 x 3 metres (10 x 10 ft) of any flammable material.

If you don’t intend moving the kiln to other brush piles we recommend pouring river grade sand evenly over the site, but make sure it is at least 10 cm (4 inches) thick.  The sand insulates the ground, but also allows one to dig vent holes underneath the kiln.  If you find you don’t have sand or enough volume of sand, you can place 3 wooden pegs underneath the kiln, each approximately 100 mm (4 inches) in thickness, evenly spaced, to keep the kiln slightly elevated.  This will allow for enough ventilation or allow one to create the holes in the sand more easily.

Making charcoal

(Also see alternative method posted as a blog comment)

  • Burn “out of the fire season” and if you feel it necessary alert the neighbors if they are the jittery type – it is just common courtesy to respect other people’s rights.
  • Pick a clear morning or a quiet windless afternoon.  This is very important because in the first hour you are venting copious amounts of smoke and the neighbors will learn to hate you all over again!  Theoretically, strong winds can also cause tiny pieces of burning material/embers to start spot fires far away from the kiln.  I don’t believe this to be a serious possibility, but it’s always better to be on the safe side.
  • The burn site must be cleared of dry grass and any other flammable material.  A normal fire must be lit.

  • The kiln is then rolled into position over the fire.  If you’re using pegs – insert them now.  Make sure the drum is completely round, not oval, as the lid will not be able to close up the opening at the top.  Four air vents are then dug under the bottom of the kiln with the use of a spade.

  • The air holes will provide much-needed draft through the kiln.  Make sure holes are dug at least 30 cm (1 ft) across, even up to 50 cm (1.6 ft) to allow as much air in as possible.  The kiln is then fed with material until the burning slash-pile reaches the top.  The general rule is to keep adding timber until it no longer sags below the rim of the kiln.  Be careful not to have pieces hang over the edge.

  • Soon, all the wood/brushwood in the kiln will be ablaze and the smoke issuance will cease almost immediately.  The temperature in the kiln will consolidate  (and sometimes fluctuate) at between 380 to 400°C (716 to 752°F) anything from 1 hour to 1.5 hours after the last piece of timber has been loaded.

  • Once white ash is seen spilling out of the vent holes (four openings under kiln), they must be sealed, by simply kicking them closed with the surrounding sand or by removing the wooden pegs.  Blue/grey smoke will start appearing which means that the alcohols and phenols are burning off.  This phase is closely followed by the tar producing phase whereby yellow flames are now visible.  We advise that the lid be closed before the end of this phase as the tar given off assists in sealing off the lid against the drum.  This phase can last for up to 30 minutes.  Waiting too long before closing the lid can reduce the smaller sized wood to ash and closing the lid too quickly, won’t allow larger pieces of wood to pyrolyze completely on the inside.  One is usually left with a small proportion of semi-charred pieces, but these can simply be added to the next load waiting to be turned to charcoal.  Seal off the remaining openings with clay or strategically placed bricks, where one can see smoke spilling out.

  • Care must be taken against accidental skin burns when applying the clay, as temperatures inside the kiln can exceed 500°C (935°F).  Absolutely ALL the air supply to the kiln must be choked off, otherwise you’ll end up with too much ash.

  • No further oxygen is therefore allowed into the kilns and the cooling down period commences.  Under these conditions hydro-carbons are absorbed by the charcoal.  In larger horizontal type kilns, temperatures remain hot for almost 72 hours and larger pieces of timber (usually the trunks) are able to sufficiently “cook” during this period.  The same cannot be said for large pieces of wood contained in our kiln.  On the positive side that is why this kiln works so well with small sticks and twigs.  The kiln must only be opened once the metal sides are cool to the touch, usually the next morning (24 hours later).

  • Charcoal is then sieved and smaller pieces are separated from the larger pieces by use of a wire mesh screen or simple garden fork and placed into polypropylene bags.  In this exercise we produced 16 kg (35 lb) of charcoal which amounts to a conversion factor of 5.5 : 1.  Take care to store the charcoal in a dry environment as moisture can lower the calorific content (heating value) of charcoal.  The average heating value of charcoal is around 29 MJ/kg, almost twice that of wood.  We used logs with an average diameter of approximately 100 to 120 mm (4 to 4-3/4 inch) and as a result produced very little fines and ash.

The process – explained

Now that we know how to make charcoal, what thermodynamic reactions are actually taking place during the different stages of charcoal making?  Knowing this might help you to gauge your own operation and fine tune it.  It is a good idea to invest in an industrial purpose, digital thermometer and Type K thermocouple rod ±0.5 metres (1.6 ft) long.  This will cost approximately $200 (USD).

The pyrolysis of wood in such apparatus basically resolves itself into 3 different stages:

  1. Evaporation of the moisture present in the wood takes place as the initial phase, up to an average temperature of 170 °C (338°F). During this period scarcely any gas is produced.
  2. As the fire builds up inside the kiln, the temperature rises and at approximately 280 °C (536°F) the exothermic reaction begins to take place. Gas consisting almost entirely of carbon monoxide and dioxide is evolved and a certain amount of acetic acid is formed together with small quantities of wood-naphtha and tar.
  3. The exothermic reaction then continues during which concentration of carbon in the charcoal takes place. Large quantities of hydro-carbons, acetic acid and wood-naphtha and tar are produced while the temperatures rise to 380 to 400 °C (716 to 752°F).

What is charcoal generally used for?  The main use of charcoal in the households of the developing world is to heat water either to cook food or provide hot water for washing, but elsewhere charcoal also has many applications.  You could use the resultant charcoal and turn it into Biochar (also known as Terra Petra) as a soil amendment.  Charcoal can enhance plant growth, reduce fertilizer requirements, reduce the soil’s acidity and offer a host of other benefits as can be seen in this post: – OR – combine the char with a non-woody biomass fuel-briquette – OR – A good way to celebrate the fruits of your labor would be to test some charcoal in a barbecue, and since you reduced your carbon footprint by not buying commercially manufactured charcoal and converted waste biomass that would otherwise have decomposed at the landfill site, which would contribute to global warming – its eco-friendly too :-).