Most worm bins run on food scraps and bedding - but, what if adding a form of charcoal could make them work even better? Enter biochar - a carbon-rich material that has attracted considerable attention for its beneficial impact on soils and in composting systems.
What is Biochar?
Biochar is a form of charcoal made through controlled heating of organic matter in a low-oxygen environment - a process known as pyrolysis. It is a surprisingly modern development with roots in ancient agricultural practices.
In the 1980s and 1990s, researchers began closely studying the remarkably fertile, charcoal-rich Amazonian soils known as “Terra Preta”. This renewed scientific attention ultimately led soil scientists to popularize the term “biochar” in the early 2000s, to distinguish intentional soil applications from ordinary charcoal.
Many people associate the word “charcoal” with wood — and woody residues are indeed a very common feedstock. However, biochar can be produced from a wide range of organic materials, including:
- Plant biomass (wastes or biomass grown specifically for this use)
- Sewage sludge
- Livestock manures
- Food wastes
- Other municipal/industrial organic wastes
Still, woody materials remain one of the most common and practical choices. They typically have lower moisture content than many agricultural or municipal wastes, are less likely to contain contaminants, and are rich in lignin - a complex, carbon-dense structural polymer that contributes to the formation of stable, highly porous char during pyrolysis.
NOTE: Among the woody wastes, dry, dense hardwoods are the best choice.
Although much of the interest in biochar relates to its beneficial impact on soil health/fertility, this material has also been used beneficially as a livestock feed supplement, in building materials, and for water filtration, among numerous other applications.
“Are the Ashes From My Fireplace Biochar?”
There are a handful of other materials that can be easily confused with biochar due to some similarities in creation, appearance and properties - but it’s important to get clear on what is and isn’t biochar.
Fireplace/Fire Pit Ashes - This is the material that is formed when wood and other organic matter undergoes combustion (burning in presence of oxygen). In this case the organic matter itself is burned off leaving behind a mineral-based residue. This tends to be very alkaline, and while it does have some potential gardening/composting applications (e.g. when you want to boost pH) it is very different from biochar. NOTE: the chunks of charcoal left behind after a fire are closer to biochar but tend to be a lot more inconsistent in structure and properties.
BBQ Charcoal - This type of charcoal is made in a way that serves its purpose as a low-smoke fuel source in bbqs/grills. Similar to biochar, it is made via pyrolysis and is largely carbon-based, but it often contains fillers and chemicals, and also doesn’t offer the same unique, beneficial structural properties as biochar.
Coal - This is a very dense, carbon-based fossil fuel, formed over millions of years. It doesn’t offer any of the same structural, or fast/easy production benefits that biochar does.
Activated Charcoal - This one is probably the closest thing to biochar. It is made via pyrolysis under highly controlled conditions, and you end up with a similar end product. The key difference is that it requires a two-step process - second being the “activation” step - with higher temperatures, resulting in a vastly more porous structure - one that’s much better suited for filtration than agricultural applications.
The Benefits of Biochar for Worm Composting
The use of biochar in worm composting systems can offer benefits for both the process and the end product.
Biochar can:
- Absorb excess moisture - helping you avoid swampy bins.
- Trap ammonia gas - meaning less nitrogen lost and a safer environment for your worms.
- Provide countless microsites for microbial colonization thanks to its highly porous structure and the vast surface area it offers*.
- Attract and hold valuable nutrients, such as calcium, magnesium, potassium and ammonium.
- Potentially even boost worm productivity and health according to some research!
* To provide some perspective - one widely shared stat suggests that a single gram of biochar can have the equivalent of 9000 sq ft (or more) of surface area!
But before you hop too quickly on the biochar bandwagon, it’s important to make sure you are up to speed on some of the key considerations about this material.
The Quality/Consistency of Biochar Matters:
It’s very important to remember that not all biochar is created equal. The type of starting material you choose, alone, can have a major impact on the product you end up with - but there are some other very important considerations as well.
It is best to use biochar that:
-
Comes from clean, consistent biomass - When biochar is made from a range of different materials (and different forms of the same materials) you can end up with an inconsistent biochar. Using a single source of clean, dry woody wastes (especially hardwoods) for production is usually the best approach.
-
Is free of chemical additives - It is best to avoid biochar made from treated wood products and other contaminated feedstocks.
- Is made using a controlled, consistent process - Again, we’re not just talking about grabbing some left-over charcoal from your fire pit. For best results, biochar should be made in a system designed for proper pyrolysis. This is not to say it needs to be a high-tech, super expensive set-up (we’ll look at some DIY methods a bit further along).
Some indicators of quality biochar can include: 1) being lightweight, 2) easy to crush, 3) dark black colour (white/gray material usually indicates a higher % of ash).
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The Quantity of Biochar Matters
With a material as beneficial as biochar you might assume that more = better, but that’s not usually the case. Interestingly, the amounts of biochar typically recommended actually seem quite similar to the recommendations for worm castings.
In other words, a little goes a long way!
Aiming for 5-10% biochar by volume in soils is a pretty good rule of thumb.
The application rate we recommend for PC BioChar in worm composting systems is approximately 1 cup per sq ft / month. This assumes that the system will be harvested according to a pretty typical schedule (every 3 to 6 months).
IMPORTANT: biochar is extremely stable - so the % in your end product can greatly increase as wastes get converted into castings (which involves major volume reduction) and as you add more biochar to the system over time.
This concentration effect shouldn’t create any issues, though, since (as touched on) the best application rates for castings are also quite low.
All of this ties in with the topic of “charging” and “inoculating” biochar, which we will be looking at a bit further along.
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The Particle Size of Biochar Matters
Biochar is often ground up into a pretty fine powder. This greatly increases the surface area, facilitating increased microbial colonization (and thus activity) and nutrient uptake. The fine texture makes the material even easier to work with in composting systems, potting mixes etc - and provides a much more uniform, aesthetically pleasing appearance.
But finer isn’t necessarily always “better”.
Bulkier biochar can be beneficial for situations where additional aeration and drainage are needed - e.g. in very dense, clay-based soils…and potentially even when being used as a worm composting habitat material (as we’ll come back to in a minute).
Just keep in mind that you may not be getting quite the same “bang for your buck” with the larger chunks since much more of the potential will be locked away than with an equivalent amount of smaller-particle biochar.
An Interesting Exception to the Rule(s)?
While conventional biochar wisdom seems to suggest that we should be using this material more as an amendment (i.e. in moderation) than as a primary substrate - both in soils and in composting systems - as always in vermicomposting, there can be some exceptions to the “rules”.
We have been working closely with someone involved in some very interesting work with biochar that kinda flies in the face of what most people suggest in terms of application rates.
Our friend has been making large quantities of biochar from locally-available lumber wastes and using it very successfully as a primary habitat material in his worm composting systems.
The systems (Hungry Bins) were started in a normal manner, with typical bedding materials, but over time, more and more biochar was added until it essentially replaced all other habitat materials (these are flow-through systems so finished material is periodically harvested from the bottom).
The biochar being used is quite coarse in texture - with average particle size of ~ 1 cu inch (or higher).
The worms from these systems are some of the biggest, most vibrantly coloured Red Worms we’ve seen! In other words, it’s abundantly clear that this rather unconventional approach is creating a very healthy environment for the worms.
Are we suggesting that readers start setting up worm bins with biochar as the main habitat material? Absolutely not!
For one thing, this would be very expensive unless you were making your own. Secondly, there are a lot of variables to consider with scenarios like this. Our hunch is that the particle size is playing an important role in this particular case, but further study of these methods are needed (something we are hoping to do - along with our own related experimentation).
As an interesting sidenote - the leachate from these systems is apparently very clear, not the usual “tea colour”. This may have something to do with the cation exchange capacity and filtration properties of biochar.
Charging/Inoculating Biochar
As beneficial as biochar can be for soils, it does require a little ‘TLC’ in order to be ready for use. The pyrolysis process leaves you with a raw form of biochar - something you can think of as an amazing “blank canvas”. But like any blank canvas, it requires some work in order for you to end up with a masterpiece! 😉
This is where “charging” and “inoculating” come into play!
Charging - Biochar is a negatively charged material that has a high cation exchange capacity (CEC). In plain-ish english this basically means that it is really good at attracting and holding positively charged molecules (cations) - including many important nutrients (e.g. potassium, calcium, magnesium, ammonium). But it needs to be in an environment where it is exposed to nutrient cations before this can happen.
Inoculation - You can think of this as the same idea as charging but involving microorganisms instead of nutrients. Biochar doesn’t actually attract and hold microbes based on opposite charges in this case, but based on the incredible surface area of its porous structure. All of those countless nooks and crannies serve as very attractive sites for microbial colonization.
NOTE: Keep in mind that some people use the term “charging” as a blanket term for both of these processes. We prefer to keep them separate since they are quite distinct, and they don’t necessarily happen at the same time (although they often do).
When purchasing biochar, unless it is clearly stated, you should assume that the material is raw and in need of charging/inoculating.
This is one of the great things about adding raw biochar to a worm composting system! You are effectively charging and inoculating the material in one go. In fact, you are ending up with a trifecta of awesomeness in the form of:
- Charged biochar
- Inoculated biochar
- Worm castings
(To be clear it’s like a 3-in-1 end product - not 3 distinct end products)
Which brings us to another important point about charging and inoculating.
Some people may just want biochar that’s charged and inoculated - not all mixed up with another material like compost or worm castings. Obviously, a different approach will be needed in this case.
The most popular choice seems to be immersing biochar in quality compost extracts/teas. How long does this take? Some sources say as little as “a few minutes”, while others suggest it takes “days” or even “weeks”, but our recommendation would be to err on the side of “days”, and to make sure your brew has a source of aeration. If you have a fine mesh compost tea/extract bag, this is a great way to contain your biochar during the process, but even just putting it in the bottom of a bucket and then adding your tea/extract (again, with aeration) should work fine. Just make sure to pour it through some type of fine mesh sieve once it’s ready to be separated.
Another approach involves “incubating” quantities of biochar in another solid substrate like compost or rich soil. In this case you would definitely need some form of fine mesh bag…and likely a lot of patience, since it could take months in order for the material to become fully charged/inoculated.
Why Does Biochar Need to Be Inoculated / Charged Before Use in Soil?
Some may wonder why you can’t (or at least shouldn’t) just dump bags of biochar straight into your garden. Well, the inoculation side of things likely won’t be such a big deal, but on the charging front, this can result in your raw biochar sucking up a lot of available nutrient cations initially, likely resulting in a reduction of plant-available nutrients for a period of time (potentially as long as one or two growing seasons).
Bottom-line, it is far better to add biochar that already has microbes and nutrients bound to it! And again - adding it to your vermicomposting systems (combining it with castings in the process) offers a fantastic way to accomplish this!
Tips for Success
- Use high-quality biochar (based on criteria discussed earlier, and some additional considerations listed further down)
- Aim for a “less is more” approach when adding biochar to your worm bin (especially if planning to add it multiple times).
- Don’t harvest your castings too soon after your last biochar deposit - give the system at least a few weeks in order for the material to get charged/inoculated sufficiently.
Making vs Buying Your Biochar
Hopefully by now we’ve made it fairly clear that simply digging some charred material out of the bottom of your fire pit or bbq isn’t a good way to end up with a steady supply of quality biochar. Still, we don’t want to completely scare you away from the idea of making your own. This can be a very rewarding endeavor that doesn’t require an industrial-grade pyrolysis reactor or an engineering degree.
Before we explore some DIY options, let’s first chat about purchasing biochar. This can be an excellent option - often the best option - for those who don’t have the space/time/resources/desire to attempt making it themselves, and also who want a top notch, consistent product.
On that note, there are some important questions/considerations to keep in mind before you run out and buy some:
- Does the supplier have an excellent reputation?
- What is the starting material used?
- What is the production method used?
- Does the product come in raw form or is it already charged/inoculated?
- What is the particle size (generally speaking)?
The answers to these questions don’t necessarily need to be deal-breakers - but the more information you can gather about the product and seller, the greater the chance of you ending up with a quality biochar that suits your needs!
The DIY Route
There are quite a few different ways to make biochar effectively. It is well beyond the scope of this article to explore them all, or even explore the common ones in a great deal of detail - but we at least want to leave you with a decent overview of some of the more common DIY methods.
Before you head down this path just make sure to review the important quality/consistency considerations discussed earlier.
Basic Trench/Pit Method - One of the simplest (and most inexpensive) ways to produce biochar is the pit or trench method. A shallow pit or trench is dug in the ground and filled gradually with dry plant material such as branches, crop residues, or other woody biomass. The material is ignited and allowed to burn while additional layers are added as the lower layers begin to char. The goal is to allow the material to heat and carbonize while limiting the amount of oxygen reaching it, which prevents the biomass from burning completely to ash. Once most of the material has turned black and brittle, the fire is extinguished - typically by covering it with soil or dousing it with water, leaving behind charcoal that can then be crushed (optional, depending on goals) and used as biochar.
Kon-Tiki Kiln - This is similar to the pit method but uses a conical metal kiln, often referred to as a Kon-Tiki kiln. In this system, a fire is started at the bottom of the metal vessel, and additional amounts of dry biomass are added gradually. Each new layer burns quickly at the surface while the lower layers are protected from oxygen and converted to char. The sloped walls help reflect heat back toward the center, creating very high temperatures and a relatively clean burn. When the vessel is nearly full of glowing char, the process is stopped by quenching with water or covering with soil.
Top-Lit Updraft (TLUD) Barrel - A common small-scale method uses a metal drum or barrel designed as a top-lit updraft (TLUD) stove. The barrel is packed with dry biomass and ignited from the top. As the upper layer burns, it produces heat that drives gases out of the material below. These gases rise and burn above the fuel bed, while the lower material is heated in a low-oxygen environment and gradually converted into char. Once most of the fuel has carbonized, the airflow is restricted or the material is quenched to stop the process before the char burns away.
Nested Barrel (Retort) System - Another fairly simple DIY approach uses two metal barrels arranged so that biomass inside an inner container is heated by a fire burning in an outer barrel (or some other structure - e.g. one made with bricks). The outer fire provides the heat needed to drive off gases from the biomass in the inner chamber, but because oxygen is largely excluded, the material carbonizes instead of burning completely. This type of setup mimics the basic principles of industrial pyrolysis systems on a very small scale and can produce relatively consistent biochar when managed carefully.
Again, we are just (barely) scratching the surface here as far as DIY methods go! If you are really interested in this topic - especially if you have a strong desire to make your own biochar - we highly recommend looking up different methods on YouTube, and getting involved in discussions in various online biochar groups. There is a wealth of top notch biochar information online.
Final Thoughts
If you are now wondering if biochar is mandatory for successful vermicomposting, we can assure you that the answer is no. People have been successfully composting with worms well before the use of biochar became popular. And many continue to have great success without it.
Think of biochar as a potential enhancement, not a requirement.
Like various other amendments, biochar can offer some great benefits for the worm composting process and the quality of your end product - so we do feel its use is at least worthy of your consideration.
Here at PC, we’re fascinated by the topic of biochar, and eager to dive in further with even more research and experimentation. We hope you’ll join us on this journey of discovery! Please don’t hesitate to drop us a line any time and continue the discussion - we always love hearing from you!
Helpful Related Resources
Common Questions About Harvesting Worm Castings
How to Separate Worms from Their Castings
How to Use Worm Castings & Liquid Extracts
How to Feed Your Worms to Get The Best Results From Your Worm Bin
Tips and Tricks for Worm Food Optimization
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Bedding - The Most Important Material in Your Worm Bin?
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