DIY Soil Fertility – Part 6: Charcoal and Biochar

Welcome to DIY Soil Fertility For Homesteads!

This post series covers the many ways you can build fertility, create soil, cycle nutrients and take responsibility for “waste” streams on your property – all with materials and substrates you are likely buying already or can be sourced for free or very low cast. These systems all integrate with one another to enable you to in-source your soil fertility inputs instead of relying upon centralized commodity supply chains. By employing these systems together in an integrated system you will improve nutrition (for soil and humans) and save dollars, while making living soil fertility enhancers better than anything you can buy at the store. The posts are written from our own experience and are geared towards the DIYer, though options are provided for ready-to-go purchased systems as well. Check the link tree below or at the bottom of the post to explore the rest of the series!

Charcoal vs. Biochar – What Is The Difference?

Charcoal is the carbonized solid that remains after biomass has had all moisture and volatile compounds removed from it via pyrolysis, the slow heating of wood or other carbonaceous materials in a low to no-oxygen environment.

Biochar, generally speaking, is charcoal that has been “charged” with nutrients and beneficial microbiota (microbes and fungi) before it is used in a growing system as a soil amendment. Charcoal by itself can be a nutrient sink and is known to deprive nearby plants of nutrients in the short term when added to soil. However, when pre-charged with nutrients and beneficial microbes, it can act as a nutrient well from day one, feeding nearby plants and trees for millennia.

Why Make Charcoal And Biochar?

Charcoal can persist in soils for thousands of years, unlike directly buried organic matter or finished compost, which have effective lifespans of months to several years. Not only is the fractal carbon lattice that remains following pyrolysis chemically stable in soils (hence its long effective lifespan), it has a tremendous amount of surface area. The highly porous surface of charcoal makes an excellent substrate for microbial life to colonize – sort of like high-density housing for beneficial microbes in your soil!

Terra preta is the term used to describe the deep, black, stable pockets of soil that have been found throughout the Amazon rainforest that continue to be immensely productive even to this day, in areas where soils are generally thin and very prone to nutrient leaching if left uncovered. Charcoal, and more specifically biologically active charcoal – i.e. biochar – has been found to be a critical component in these persistent fertility pockets. The multitude of ways in which the Amazonian societies of old used and cycled charcoal to cleanse, deodorize, fertilize and ultimately build and stabilize fertile soils in the notoriously fragile rain forest soils demonstrates a deep understanding of nutrient cycling (at a societal and cultural scale) and how critical a generative fertility cycle was to sustaining prosperous and healthy city-sized settlements on top of such fragile native soils.

The history of terra preta and its formation over centuries of human habitation in the Amazon jungle, and its subsequent effects of not only preserving but accruing soil fertility over time, is well documented and represents an exciting new-old edge in creating closed-loop fertility systems that enable truly regenerative habitation. For brevity’s sake, will not do a deep dive into the story of terra preta in this post – for this we recommend browsing the list of articles and resources below:

• Terra Preta – Model Of Cultural Technique – by Hans Peter-Schmidt, Ithaka Journal.

What Makes Biochar Such A Potent Soil Amendment?

Absorptive Capacity (AC) – Well-made “hard” charcoal has an extremely high surface area to volume ratio, estimated to be greater than 2 acres (90,720 square feet!) per ounce of material (28.35 grams).

This incredibly high surface area to volume ratio lends certain properties to charcoal, and ultimately biochar. It is highly porous (all those old xylem and phloem tubes in the plant tissue now re-opened following pyrolysis) and is capable of absorbing up to five times its own weight in water, as well as any nutrients dissolved in that water. Charcoal integrated into soils helps to buffer moisture swings, creating more stable growing conditions and thus healthier plant life.

Cation Exchange Capacity (CEC) – Charcoal has a very high cation exchange capacity – a measure of its ability to bind positively charged nutrients (like ammonia and ammonium) and retain them and later make them available to plants under certain conditions. Charcoal’s high CEC enables it to “hang on” to otherwise soluble and easily leached nutrients, retaining them in the soil horizon where plants can make use of them.

Plain charcoal (not biochar) is a very effective nutrient sink when added to soils – by itself it is not a fertilizer. Some of the early critics of charcoal’s use as a soil amendment were correct in identifying that it would rob plants of available nutrients, at least initially, when incorporated into the soil. Over time, however, this charcoal would become a nutrient well (nutrient carrier) that plants could access once it had been fully charged with nutrient and colonized with beneficial microbes. A high CEC also enable charcoal to bind to many pollutants very effectively, thus charcoal is also effective at remediating damaged soils and mitigating toxic inflows (see Biochar In Cattle Farming, Biochar In Poultry Farming, and Treating Liquid Manure With Biochar).

The term biochar describes charcoal that has been pre-charged with nutrients and beneficial microbes and fungi prior to its application in a growing system. There are many different methods, recipes and ingredients that can be used to make biochar, however all aim to achieve essentially the same thing – to skip over the nutrient sink phase and go straight to the nutrient well phase after incorporating char into soil. Both the high AC and CEC of char make it a perfect habitat for microorganisms to flourish, which nourishes all the surrounding soil biota and promotes symbiotic relationships and a diverse, resilient soil ecology.

DIY Methods For Making Charcoal

There are many ways of making charcoal at a home or small community scale with inexpensive infrastructure, most of which can be sourced or made from scrap materials. All methods work to achieve the same functional condition for pyrolysis – the low to no-oxygen heating of biomass, which yields impure carbon residue (the charcoal) once all volatiles have been driven from the material. No matter where you are in the world and what you have available to you, there is a way to make charcoal!

NOTE: For those of us living in fire ecologies, making biochar from the vast amounts of brush and forest thinnings that are otherwise burned to ash each year represents a tremendous opportunity to restore healthy carbon cycles, heal dysfunctional hydrological cycles, improve our soils (and thus our nutrition) and air quality, drought-proof our landscapes and otherwise shift our societal trajectory from ecological degeneration to regeneration.

Read our Living With Fire Series for more on changing our relationship with fire (and making/using biochar!).

Generally speaking, when making charcoal, the goal is to maximize heat and minimize oxygen, which will serve to limit the loss of char to ash. Charcoal that is made from pyrolysis reactions held between 840 and 1300 degrees Fahrenheit (450 and 700 degrees Celsius) will have the highest absorptive capacity (AC). To create such temperatures a clean, hot burn needs to be created. For all the systems illustrated below, there should be little to no smoke from a well managed burn.

1. Top-Lit Up-Draft Pyrolysis Chambers (TLUDs)

Top-lit up-draft pyrolysis chambers (TLUDs for short) are an excellent low-tech approach to creating large amounts of charcoal quickly and safely at a homestead or small farm scale. Most designs out there make use of repurposed 55 gallon steel barrels. At their simplest, TLUDs can be a single 55 gallon barrel with air holes drilled in the bottom, a lid and a chimney. Watch Jon Jadai of Pun Pun Organic Farm give an 8 minute video walkthrough of a straightforward charcoal making process using a single chamber TLUD.

For a more in depth discussion on the construction and operation of this TLUD design watch Aqueous Solutions in-depth video covering the entire process start to finish, or, if you’d prefer a shorter video without the details, just the steps to getting a TLUD built, check out the 5 minute video below.

John Rogers has an excellent video (below) detailing how a single person can operate four 55 gallon kilns in rotation to produce up to a yard of char in a single afternoon.

You can also make much smaller TLUDs if you have access to smaller barrels – even cookie tins and old gallon sized paint cans will work!

2. Forced-Air TLUD + Retort

For increased charring capacity beyond a typical single chamber TLUD detailed above, consider a two-tier, double chamber TLUD design like that demonstrated in the video below . Instructions for constructing a two-tier “Jolly Roger” style biochar oven can be found here.

This design of TLUD utilizes the heat released from the TLUD (lower barrel) to heat a second retort that is filled with additional material. Once this material is heated to a high enough temperature, the woody material within will begin to off-gas remaining water and volatile gases. These gases will self-pressurize and be injected into the secondary air inlet just above the top of the lower TLUD, mixing with the pre-heated air to create a secondary combustion that serves to further heat the retort and increase the unit’s efficiency.

This design employs forced air at both the primary (bottom) and secondary (top) air inlets using small motorized fans. This “forced draft” enables better optimization of the burn conditions depending upon the density, moisture level, and size of the various materials in both the TLUD and retort. TLUDs with forced air enable successful conversion of wetter materials into charcoal than those without, however it is always better to burn completely dry material.

In our experience, double chamber TLUDs like the Jolly Roger do require forced air in order to function well. The placement of the retort above the lower TLUD creates some interesting challenges with managing the chimney draft, and the forced air provides the extra finesse that may be needed in order to get the best burn and therefore highest yield.

3. Portable 55 Gallon Open-Top Design

This style of using 55 gallon drums to make charcoal is very low tech – all that is needed is an angle grinder to cut open your barrel. The barrels are easier to transport than TLUDs due to them being a single piece and lighter, and are helpful in conducting burns in areas without vehicle access and where you may not wish or be unable to dig cone pits or trenches (see below for these methods). This design must be regularly attended during the burn process, as new fuel wood will need to be added frequently in order to maintain the oxygen deprived pyrolytic state required to maximize char and minimize ash.

4. Cone Pit / Cone Kiln Method

Cone pits are circular “upside down cone” pits dug into the ground. The “point” of the cone is the deepest part of the pit, and the diameter increases the closer to the native ground level you get. Cone pits create an oxygen deprived environment at the bottom of the pit. Cone pits need to be attended to regularly during the burn process – as soon as all existing fuel has caught and is combusting, new fuel should be added to prevent the carbon from turning to ash.

Cone pits are create because you don’t need anything more than a shovel to create them. Assuming you’ve got something to burn and the place and means to dig a hole, you can create a cone pit.

5. Charcoal Trenches

Charcoal trenches are well suited for burning long pieces of wood that are common when thinning forests of crowded undergrowth. They function based on the same principle as the cone pit and portable open top 55 gallon kiln, with the newly added material on top excluding air from the already combusted material below. These trenches are also easy to quench upon completion of a burn (the water has nowhere to go and so sits in the trench bottom).

One potential expansion of the charcoal trench method that may be of interest to those wanting to produce biochar at farm or enterprise scale would be to dig/construct one of these trenches just wide enough for a small farm tractor to drive into them. Provided the trench was just wide enough for the tractor bucket, large amounts of material could be pyrolized in a single large burn and then the charcoal could be unloaded using machinery instead of shovels. Formed into windrows, the machine could be used to quickly crush the charcoal down to uniform small sizes until a tow-behind roller.

6. Top-Lit Open Piles

In many instances it may not be feasible or energetically worthwhile to transport and process woody material from where it has been cut to a pre-dug trench, cone pit, or TLUD. However, you can still construct and manage burn piles to maximize the amount of carbon and minimize the amount of ash. These piles can be left in place to enrich their immediate area for a thousand years to come, or can then be transported to another point of use (much easier to pack and move charcoal than the woody material it comes from). Below is another excellent video from SkillCult demonstrating how and when burning in piles may make the most sense depending on your context.

Several things to note when constructing and managing an open burn pile for charcoal yield:

• Pile should have vertical sides as best as possible (make a wall, not a mound).
• Brush should be laid in parallel if possible for dense stack.
• Pile should be top lit for cleanest, hottest burn.
• You will need to manage the pile – occasionally pushing unburned pieces that fall to the outside back into the center of the burn to completely char them.
• Read Learn To Burn, And Make Biochar, Not Smoke by Kelpie Wilson for an excellent, short primer on making charcoal from burn pile.

7. Hookway Retort

This style of charcoal retort employs a central chimney within a 55 gallon drum (or equivalent vessel) in which a fire is lit and stoked. The barrel is loaded with wood to be charred. The fire in the central chimney creates the heat that begins to drive volatile gases from the wood in the barrel (the wood in the barrel itself is not combusting, it is pyrolizing). An additional open pipe leads from the pyrolizing zone into the combustion zone so that, when the temperature rises high enough and substantial amounts of wood gas are begin driven from the wood being charred, that gas is re-injected into the combustion chamber – this reduces smoke and uses the wood gas to help heat the container, which greatly improves efficiency.

There are versions of this retort that can be made from largely earthen materials, though most models are perhaps more suited to those with access to and skill with metal working. For more this design we recommend exploring the videos at James Hookway’s YouTube Channel.

8. Flame Cap Kilns

Flame cap kilns are portable and can be brought to the biomass to set up burns without hauling material. They are called flame cap kilns because the flame burning on top of the cylinder effectively de-oxygenates the air that would otherwise turn the char into ash, thereby maximizing char preservation. With the addition of a second layer of metal around the outside, offset from the main burn chamber walls, these kilns can be approached and tended safely and comfortably, even when a roaring fire is going in side. They are capable of producing large quantities of biomass in a short time. They are best operated with multiple people.

Pre-made kilns can be purchased from Wilson Biochar.

How To Make Biochar From Charcoal

Charcoal by itself, when added to soil, will act as a nutrient sink – binding up nutrients and depriving whatever plants are growing in it from accessing adequate nutrition. This will eventually self-correct, but why miss a gardening season (or decrease your crop yields) when you don’t have to?

Enter biochar. Here charcoal is made biologically active and nutrient rich – essentially pre-charging it with the nutrition and beneficial microbes needed to make it an asset in the soil right away.

There are many ways to charge charcoal and create biochar, or terra preta. Additionally, there are hundreds of uses for charcoal prior to turning it into the soil (see next section), and it is worth considering how to maximize the use of charcoal before it reaches its final resting place in the soil. In this section we will focus on the most approachable ways to charge charcoal and turn it into biochar for a homesteader or small farmer.

1. Incorporate Charcoal Into Thermophilic Compost

Charcoal can be added as one of the ingredients in a thermophilic compost pile, up to 10-25% by volume. Charcoal should be pulverized prior to adding to get the most benefit (large chunks won’t harm anything, but more numerous, smaller pieces – think pea sized, will provide the greatest benefit in the soil). Provided the pile is well tended (no bad smells, gets up to 140-160 degrees and stays there, turned every 2-3 days) the microbial community that does the work of composting that pile will also take up residence within the vast pore-network in the charcoal pieces, effectually inoculating them and creating “microbe hotels” ready to go into your soils and nourish the entire soil food web. See DIY Soil Fertility – Part 4: Thermophilic Compost for a complete guide to creating your own thermophilic compost.

2. Incorporate Charcoal Into Bokashi Fermentation Buckets

Charcoal can be added to bokashi buckets and fermented with household food wastes. It will come out the other side loaded with nutrients and colonized with a diverse range of Lactobacilli. For home scale bokashi buckets, we recommend adding some pulverized charcoal at the start of each new bucket. The charcoal will act as a sponge and help to limit odors and capture leaching nutrients (the liquid that comes out the tap at the bottom). Charcoal can be sprinkled in throughout the layering process as well. For a complete guide to home scale bokashi composting see DIY Soil Fertility – Part 2: Bokashi Compost Systems.

Large scale bokashi composting can also be done (at tractor scale) for farms or composting facilities, and charcoal can be mixed in with the food wastes and other organic material to be fermented. Large piles are formed, inoculated with EM (effective microbes – in this case Lactobacilli that like to ferment under anaerobic conditions), mixed, then reshaped and driven over repeatedly with heavy machinery to force the air out of the material. Large suction covers or well-weighted plastic sheeting is then used to cover the pile (excluding air) while it ferments for 14-21 days.

3. Bubble Charcoal In Compost Tea

Perhaps my favorite way to inoculate large amounts (at least on a homestead scale) of charcoal and charge it up with nutrients is to create a compost tea and charcoal slurry and bubble it for 24-48 hours. Not only does this ensure that the charcoal will be fully hydrated, it also ensures complete and rapid colonization of the charcoal pores with beneficial microbes and fungi. Additional nutrients can be added to the brew as well – like urine, kelp, kelp extract or kelp meal, finished thermophilic compost, blood meal, worm castings, mycorrhizal fungi spores and more – creating a super-charged soil amendment that will be of great benefit immediately in the soil. For a complete DIY guide to getting started brewing your own compost tea, read DIY Soil Fertilty – Part 3: Compost Tea.

Ways To Use Charcoal And Biochar In Your Landscape

In order to maximize charcoal’s value it is worth exploring its myriad different uses cases (and sometimes even use cascades). Arrange the relevant uses in your context so that after each use, as many other possible uses remain. In this way, the yield and efficiency of a single batch of charcoal can be maximized.

1. Livestock Operations And Manure Treatment

Using charcoal at numerous intervention points within a livestock operation provides perhaps the clearest example of a highest use cascade.

1. Silage Addition – Char added to silage at around 1% by volume helps to bind mycotoxins, absorb toxic chemicals in (pesticides, herbicides, fungicides – all the –ides), and suppresses butyric acid formation during fermentation, which optimizes nutrient availability in animal feed.
2. Feed Supplement – Once in the silage, the charcoal finds its way into the animal’s rumen where it helps to bind toxins, increase roughage and enhance digestibility (less cow farts!).
3. Litter Additive – Charcoal added at 10% by volume to animal litter helps to control smell by binding ammonia and absorbing liquid nutrients. Charcoal helps to prevent putrefaction (anaerobic decomposition), thereby limiting the growth of pathogenic bacteria and improving hygiene within animal housing facilities.
4. Manure Slurry Additive – for operations large enough to be generating manure slurries, charcoal added at 1-5% by volume helps to bind volatile nutrients, thereby limiting noxious odors while improving the nutrient composition of the slurry, which improves its effectiveness when added back to crop fields.
5. Manure Composting – Once liquids are filtered out, manure solids can then be composted (again with added charcoal) to create a fine, high value soil amendment.
6. Soil Amendment – When this compost is returned to the soil, the carbon will aid nutrient retention, improve soil water holding capacity and aeration, and serve as scaffolding for a healthy soil food web which nourishes healthy crops (perennial and annual alike).

For additional reading on ways to incorporate charcoal and biochar into livestock operations, we recommend reading the articles in the Ithaka Journal by Hans Peter-Schmidt on Biochar In Poultry Farming and Biochar In Cattle Farming.

2. Water Purification / Decontamination

Charcoal and other activated carbon water filters are very effective and cleaning watering of chemicals (intentionally added or not), heavy metals and biological pathogenic organisms. Most water filters found in contemporary homes have some sort of activated carbon filter element. Activated carbon, while incredibly effective, is also energy intensive to produce. Charcoal produced in high temperature pyrolytic reactions and then “fractured” at the moment of quenching (quickly drench all charcoal with large volumes of water, which causes a mini steam explosion within the pores and “fractures” the char) has nearly the same level of performance as activated carbon when it comes to eliminating agricultural and atmospheric toxins. The Slow-Sand Charcoal Water Filtration System is one such example.

Slow-Sand Charcoal Water Filtration For Potable Water

Slow sand filters are a well studied and trialed water purification technology for creating potable water even in settings with highly polluted influent water sources (pesticides, herbicides, other agricultural chemicals, heavy metals etc). The set up detailed below is particularly useful in situations with low pressure, variable flow and when a large supply buffer is required. The figure below illustrates the internal components of four tanks, each comprising one step in the filtration process.

The first tank is filled with large, coarse stones at the bottom (2-4”). On top of these stones a 3 foot deep layer of pea gravel is added. This tank pre-filters inlet water of any larger particulate by introducing inbound flow at the bottom of the tank (in the coarse stones) and forcing it to percolate up through the pea gravel in order to exit the tank. This slows the water down, allows for large particulate to settle out, and the roughness of the stones also works to act as an initial bio-filter.

The water then enters into the second tank, the slow sand filter, where it is spread out and dripped evenly over the entire surface area using a flow disperser. The reason for this is to keep the flow very light and spread over a wide area, and thus not disturb the fragile yet very potent biofilm (a layer of bacteria) that grows over the top of the sand and acts to filter out pathogenic organisms and other pollutants. Upon passing through the biofilm, the water moves through 3 feet of sand, then into pea gravel, where it enters a perforated pipe before making its way to the third tank.

The third tank has 3 feet of charcoal on top of pea gravel. The inbound water passes through the charcoal, which helps to further bind pollutants, heavy metals, semi-volatile organic compounds, pesticides  and foul smelling and tasting compounds that are undesirable in drinking water. The water then completes the transit to the fourth tank, the holding tank, from which point it is ready for cooking, cleaning, washing and drinking. For an excellent video walkthrough of this system, watch Jon Jadai of Pun Pun Organic Farm in Thailand as he talks through the various system components and demonstrates how easy it is to maintain and clean it.

For additional in depth construction guidance for this slow sand charcoal water filtration system, visit the Aqueous Solutions website where manuals are available for free download.

Critical to the function of the water purification system detailed above is high quality charcoal for use as the final filtering agent before the water is ready to drink. Fully-charred charcoal is a carbonaceous matrix with a vast amount of surface area through which the water must transit. It has been shown to be nearly as effective as the very expensive, energy intensive ceramic filters at removing pollutants from water, heavy metals in particular. 

3. Soil Conditioning

• Biochar can be used to replace peat moss (unsustainably mined from bogs), vermiculite and perlite (energy intensive production process) in potting soil mixes. It performs as well or better with regards moisture retention, and is loads more effective as a scaffolding upon which healthy soil biology can develop.
• Can be added into thermophilic compost, bokashi compost, even vermicompost systems (provided it is fully hydrated and finely pulverized).
• Added to soils as a fertilizer buffer – prevents leaching of fertilizers (thus reducing their use and potential harm to surrounding ecology).

4. Use In Construction / Buildings / Air Purification

• Insulation And Humidity Regulation – Charcoal’s ability to absorb up to 5-6x its own weight in water, along with its highly porous structure make it both an excellent humidity regulator and insulator when incorporated into earthen construction. Up to 50% of the sand used to make earthen plasters can be replaced with finely ground char to improve insulation and breathability of the material.
• Electrosmog Absorption – Charcoal incorporated into earthen wall construction effectively mitigates electromagnetic radiation – i.e. “electrosmog” – an increasingly important consideration given the hypersaturation of our living environments with unprecedented amounts of EMFs from devices, not to mention the coming wave of 5G technology.
• Odor Reduction – Applied to the walls (inside or outside) as part of a spray on plaster mix, charcoal makes a very effective odor reducer.

5. Waste Water Treatment

Charcoal is an effective waste water treatment. It can be utilized to clean up inbound toxic effluent flows, trapping pollutants and excess nutrients. It can be used to help reclaim already toxified waters, binding pollutants that would otherwise prevent biology from establishing and ultimately cleaning the system.

6. And Many, Many More…

Charcoal has applications in industrial materials, health and wellness, paints and colorings, metallurgy (blacksmithing), medicine, exhaust filtration, energy production and much more. For a more extensive list of uses, as well as additional in depth explanations for some listed here, we recommend Hans Peter-Schmidt’s article 55 Uses Of Biochar.

In Summary

Regeneratively produced charcoal and biochar has tremendous potential to heal our landscapes via restoring a healthy carbon cycle. Carbon is always being cycled – it is the very backbone of life on this planet. Our current carbon cycling is out of balance and we are losing our topsoils (the literal canvas upon which complex human civilization depends completely for its existence), and restoring it may perhaps be the most impactful thing we can do for our own and future generations welfare. Capturing carbon and putting it to work in our landscapes has so many direct benefits they are too numerous to count. The key is to start!

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