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What is Biochar?


First lets clear any confusion and make it really simple – biochar is charcoal. It’s only called “biochar” because it is used for biological purposes. Therefore it’s not like the cooking charcoal or briquettes you buy with all the chemicals in them, it’s just good clean wood-derived charcoal. Now it’s true not all biochar is the same, but this is an introduction so let’s just keep it simple. You can make awesome biochar without worrying about the finer points. 


Another thing to remember, while biochar has been gaining a lot of popularity in recent years due to carbon sequestration benefits and new research on it’s biological benefits, using charcoal in agriculture is an ancient practice dating back thousands of years. This is a great natural farming technique that wise old farmers have known about forever. Therefore it has a nice place here on The Unconventional Farmer.


What is Biochar Good For?

I like to cover the high level description and then just dive right into the good stuff. Let’s discuss benefits. Biochar’s benefits come from it’s unique physical and chemical attributes. It is insanely porous – a single gram can have over 1000 sqm of surface area. It is inert, meaning it doesn’t interact with other compounds in the water and soil. It is also indigestible to soil microbes. So basically it doesn’t break down in the soil – it persists in the soil for a long time(thousands of years!) continuing to add it’s benefits, and doesn’t change over time. Those are the two big ones. Let’s look at the factors biochar enhances:

  • Incredible nutrient holding capacity (as in, biochar+fertilizer outperforms fertilizer alone by 60%)

  • Aids moisture retention (micropores hold water, dry slowly)

  • Aids drainage (biochar is inert, so it doesn’t interact with the polarity of water. Drains excess freely)

  • The above two points together means that biochar soil is buffered against dramatic changes in moisture level (floods/droughts)

  • Awesome microbe habitat! Bacteria, fungi, and other microbes love populating the moist pores

  • Brings up the pH of acidic soils, reducing the need for Lime

  • Reduces fertilization requirements (since it traps and holds fertilizers in the soil)

If that list isn’t enough to convince you, let’s look at a few of it’s environmental benefits:

  • Carbon sequestration: Biochar traps carbon in the soil for thousands of years, that would normally be pumped back into the air in a very short time through decomposition. This is huge and is a big factor in biochar’s recent popularity

  • Reduces need for petroleum based fertilizers

  • Studies show biochar soil produces 50-80% less greenhouse gas emissions than non-biochar inoculated soil

How to Make Biochar

First we need to look at what biochar is: charcoal. Charcoal is just a carbon source that has been heated in the absence of oxygen. Most things in the source like oils, tar, chemicals burn off leaving only carbon. For the Carbon to burn you need Oxygen. But the carbon is what we are after so that’s why we burn it in the absence of oxygen.

Any carbon source should work fine. Some carbon sources that you might find cheap, nearby would be:

  • Wood chips

  • Saw dust

  • Rice hulls

  • Wheat hulls(?)

  • Oat hulls(?)

  • Horse/cow/sheep manure

  • Wood pellets

  • Down timber in the woods

  • Weeds and pest plants

  • Last year’s garden remnants (corn stalks, etc)

  • Dried grass clippings

  • Coconut husk

That gives you an idea what you’re looking for. Any kind of carbon you can think of. Not all carbon sources are the same though. For example, wood pellets are a nicely sized, uniform carbon source. Wood chips would be roughly the free equivalent. Saw dust or dry grass clippings however are very flammable. You’ll have to get creative to make good biochar from those. I haven’t personally made biochar from those sources – I imagine it would help to pelletize them – soak with water and then compress with the heaviest weight you can manage, let dry. 


Your carbon sources you are carbonizing need to be dry! Otherwise you’ll have to burn some to dry the rest out to make charcoal – waste.

Now that you have your carbon source you need to cook it. For this part just remember the basics – make biochar by firing a carbon source in the absence of oxygen. This confused me for a long time. “But fire doesn’t burn without oxygen. How are you supposed to build an oxygen-less fire?” Well that is the common mistake of explaining biochar. It does require oxygen, but you need to manipulate the oxygen input a particular way to make good biochar. 

Here is how fire works. You have a carbon source like wood. You light it starts to burn. With flames. Consuming oxygen. The burning starts charring the carbon, firing out all the volatiles and leaving carbon only. Now we’re getting charcoal, nice good biochar. But oh no, the fire is still sucking in oxygen, the charcoal is continuing to burn with oxygen! Now its turning to ash, no more nice biochar.

So we need to arrest that burning before it turns everything to ash. So we build the fire, and this time it starts charring but we quickly put it out before it becomes ash. But now just the outside is charred and there are still lots of volatiles in the wood, lots of not really ideal biochar there.

So round 3. We start the fire, and it starts charring the carbon source. But this time we don’t put it out and we don’t allow air in. We put high walls all around the fire so that it doesn’t pull oxygen in, but the smoke still has somewhere to go. We allow oxygen to come into the fire, but only from a single point. This way, the oxygen comes in, the carbon source starts to burn. It gets very hot and uses up all the oxygen. The flames start chasing the oxygen back towards the source, so now its not combusting anymore, but it is so hot still because all the volatile, hot gases are passing over it and out the top. Now it is undergoing pyrolysis – burning in the absense of oxygen.

Now that you know the principles, you can create your own method to make this awesome soil amendment for the garden. But in case you would like some inspiration, check out these different methods to produce biochar:

Fire barrel: take a metal oil drum, turn it over, and cut a 10″ hole in the bottom. Build a fire, then upend the drum on top of it so the hole is in the top. This will be where the smoke exits. Leave 1/4″ gap in the bottom to allow just a little oxygen to feed the fire from below. Now feed your carbon source through the top hole. The whole middle section of the drum will be making charcoal as it burns without oxygen. This would be good for the loose things like loose grass clippings, dry manure, weeds, brush, etc

Philippine chimney: This is how carbonized rice hulls are made in the Philippines. Start with a cylindrical medal bucket like a metal coffee can. Perforate the sides of the bucket with small holes about 1 inch apart. Cut a hole in the bottom and attach a chimney – long cylinder that fits flush to the hole. Build a small fire and put this chimney on top of it so the fire is inside the bucket. Now pile your carbon source all around the bucket+chimney that is sitting on top of the fire. It will slowly char as it burns from within. I suppose the carbon source here is charring due to heat but not combusting because of the lack of open flame.This method is how the small farmer produces this valuable soil amendment here in the Philippines:


Great infographic on the Philippine method of making biochar (carbonized rice hulls)































Thanks to for the picture



Earth pit method: This method is useful when you have a ton of wood to make into biochar. There are a myriad different styles of earth pit biochar production. The simplest involves digging a hole, putting a bunch of wood in it, starting the fire, then covering it with soil. From there it gets much more complicated. The method I’ll describe here is pretty simple and hopefully a little more efficient than just digging a hole and starting a fire in it. Dig a trench pit, a rectangular shaped pit. Give it a slope lengthwise so it slopes up to the ground surface. Lay the first layer of logs down lengthwise in the pit, the whole length of the pit. Stack the the second layer horizontal to the first. This creates channels that run the length of the pit on the bottom. Now fill up the pit with the rest of the wood. Once filled, cover the pit with a layer of leaves, then a nice thick layer of dirt. At several places in the pile, you’ll want to install stove pipes through the dirt, to allow the smoke to exit. One way to do this is dig out little channels, about 6in wide, at several places in the side of the pit. Then install pipe where the channel meets the surface (to keep oxygen from getting in). These vents are needed for an efficient burn. Where the sloped pit meets the surface, don’t cover with dirt. That’s where you start the fire. Start a nice bonfire there. It will burn down into the pit, the channel will allow air to come in, and the vents will allow the smoke to escape.


ATTN: Don’t be an idiot and walk on top of the soil while the burn is underway. It could collapse and you’ll fall into a pit of burning coals suddenly exposed to oxygen! Bad bad bad bad bad bad bad…

The keys to efficiency in this system are: try to have uniform sized wood, try to distribute it fairly uniformly with a nice matrix so there is airflow underground as it burns, and don’t skimp on vent number and distribution (just make sure the vents are enclosed where they meet the pile, you don’t want oxygen getting in, just smoke coming out). 

Modern barrel method: There are many modern methods but I’ll discuss the TLUD method : Top-Lit Up-Draft. Take a metal can – can be a paint can or an oil barrel, whatever size you need. Punch a lot of holes in the bottom, smaller than whatever you are using as your carbon source. Enclose the top of the barrel in a chimney, but punch some holes in the chimney to help increase the airflow through it. Now prop up the barrel so air can come in through the holes in the bottom. Fill the can with your carbon source right up to the top. Light the material at the top of the barrel. Once you have a little fire going, put the chimney on. The chimney helps keep the top of the barrel low oxygen, and gets the air moving in one direction. The fire will pull oxygen from the bottom, and as it burns, all the area above the flame will be oxygen depleted. The hot gases will continue cooking the char as the burn line continues down to the bottom of the barrel. I’ll include a link to an excellent video demonstrating this method in action. It comes from one of our unconventional farmers, Bryan McGrath. It is one of the best videos I’ve seen explaining how to make biochar.

Watch Bryan McGrath’s Video on Biochar here.

Make your biochar infinitely more effective:

Phew, you’ve finally made some awesome biochar. If you add it to the soil right now though, its not going to do much. In fact, it might even leach nutrients from the soil because it is so effective at trapping molecules in all it’s nooks and crannies. Then over time it will get progressively more effective. Studies have shown dramatic results become apparent on soils that were inoculated with biochar 2 years prior. But we don’t want to wait 2 years! Jeez. 

So we’ll give our biochar an amazing head start, and make it effective from day 1. Once you’ve made your biochar, brew up a really nice compost tea. At the very least, you’ll want to pour the compost tea over the biochar. For example say you have an earth pit where you just burned a few cords worth of wood to make a cord or more of biochar. You’d make a batch of compost tea and pour that over the whole thing before doing anything else. I’d make the tea strong then dilute it to make sure you get all the biochar soaked. If you’re working with smaller amounts, I’d get all the biochar in a very fine-mesh bag. Then suspend that bag in the compost tea while it’s brewing. An alternative to compost tea would be BIM. Soak the biochar in a diluted BIM mixture. 

Another way to ‘charge’ the biochar is to add the nutrients while it is still hot. Have your nutrient bath ready. It should have BIM along with some fermented extracts for their nutrient value. For example a nutrient bath might contain fish hydrolysate, grow fertilizer, bloom fertilizer, and BIM. Have this bath ready, and when your biochar is finished but still hot, throw it in the nutrient bath. The initial sizzle seals nutrients onto the biochar, making it an excellent slow-release fertilizer. Meanwhile it cools to a point that allows microbes to start populating. Good stuff.

Once we inoculate the biochar with our nutrient/microbe mix, it can be referred to as bio-activated biochar. In the Philippines the common form of this is BCRH – Bio-activated Carbonized Rice Hull. It is incredible stuff!

Biochar Summary

What a great product for the garden and for the environment. Like most natural farming techniques, this has been around for thousands of years. It’s success is proven in the legendary “terra preta” soils of the amazon basin, where biochar created thousands of years ago is still adding tremendous fertility to otherwise nutrient-poor soils. So try this out, have fun, grow some monster veggies and save the planet. Job well done.

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