What Is Biochar? Charcoal In Gardening

What is biochar? Let's discuss what this carbonized material is, what it's not, and how it's made!

What Is Biochar


The term “biochar” has been going around gardening circles a lot. It’s touted as the next amazing amendment for the garden, a powerhouse amendment. But what is biochar, anyway, and what’s it supposed to do?

In its purest form, biochar is pyrolyzed organic material. The pyrolysis process is a form of thermal decomposition in a low-oxygen environment. Extreme heat with low airflow prevents the material from catching fire. Instead, it chars, forming a carbonaceous material that can be utilized as renewable energy for reduced carbon emissions or reduced nitrous oxide emissions.

There are benefits and drawbacks to using biochar in your garden. Let’s start with information on different types of charcoal, including more detail on biochar itself. Then we’ll talk about the benefits and drawbacks in a garden setting. We’ll even discuss ways to produce charcoal if you’d like to try your hand at it.

The Different Forms Of Charcoal

What Is Biochar
Biochar is carbonized wood. This handful has been crushed to a fine size. Source: Green Energy

In and of itself, “charcoal” is the term used to describe charred wood that has turned to coal. But what we’re really trying to describe here is carbonized wood.

As it burns, wood releases all the volatile compounds that were in it, starting with water. Plant saps are burned off. Toxins are often neutralized. What’s left behind is pure carbon and ash.

It’s the pure carbon which is potentially beneficial in garden use, and which should be your goal. The ash can have benefits too, but the carbon itself is what you’re striving for.

You will know it’s good charcoal if it’s light in weight, clinks when it’s dropped, and breaks easily. Material that feels heavy or that doesn’t break easily often still has unburned parts.

Grilling Coals: Can They Be Used?

Assuming you’re like me, you hate waste. And if you’ve got a charcoal grill, that leftover used coal may be looking quite appealing right about now.

But not all grilling charcoal is created equal.

Charcoal briquettes, the most commonly used charcoal, are permeated with additives. Lighter fuel, binders, and accelerants have been added to make them easy to use. Unfortunately, that stuff isn’t great for the garden.

If you use lump charcoal instead of briquettes, you’re in luck. Lump charcoal doesn’t have any of these additives. It produces less ash, and it burns hotter than briquettes do.

Lump charcoal is available in both hardwood and softwood forms, and is usually made from lumber mill refuse. It’s not fully carbonized yet, and often still has some unburned material inside.

I highly recommend using your lump charcoal to make fantastic food on your charcoal grill. If there’s any lumps remaining once you’re done, you can use those as biochar later!

Activated Charcoal: Not Just For Medical Use

Commercial pyrolysis reactor
Commercial pyrolysis reactors are pricey but very effective. Source: Sustainable Sanitation

Activated charcoal is occasionally called activated carbon. This type of charcoal is processed in a way which increases its porosity and surface area. As the pores open, there’s more surface area to grab on to water and other things. This grabbing effect is referred to as adsorption.

While it is sometimes used as a form of biochar, activated charcoal’s more often used in other ways. We often find it used in water filters, as an example. Its adsorptive nature lends it well to being used for filtration and purification. Medically, it’s used to treat poisonings and overdoses.

To produce activated carbon, the material is first pyrolyzed. It’s then oxidized using extremely hot oxygen or steam. There are forms which are chemically produced as well.

While it can be used as a form of biochar, the difficulty is determining which process it’s made with. The chemically-treated forms are not useful as a garden amendment and are better suited for use as renewable energy. But the pyrolyzed and oxidized forms tend to be pricey.

Horticultural Charcoal: What Is It Good For?

Horticultural charcoal goes through the same process as activated charcoals do. It hasn’t endured the chemical treatment, but has been pyrolyzed and oxidized. What sets it aside from activated charcoal is size and purity.

Filtration charcoal must be of a certain purity to be effective as a filtration device. Horticultural charcoal is activated charcoal that didn’t quite meet the mark. It is often in irregular chunks, and it has generally been derived from hardwood. But its porosity is why it’s popular for gardening.

Many potting mix recipes call for a certain amount of horticultural charcoal. This is especially true of those which are geared towards orchids. It’s a good substitute for some of the wood chips in orchid bark. But there are other moisture-retentive and aerating materials out there.

Biochar: Is It Just Burned Garden Waste Or What?

Biochar plant
Commercial biochar facilities can produce massive amounts of char. Source: Green Energy

The term “biochar” has multiple meanings right now.

In its purest form, biochar is pyrolyzed wood. Temperatures ranging from 350-600 degrees Celsius are used to superheat it. This process is done in a low-oxygen environment. Lacking oxygen, the charcoal is less likely to burst into flame.

Over recent years, bloggers and garden afficianados have shifted its meaning. It’s become “any garden waste that you can burn into coal”. And that does a disservice to anyone who actually needs biochar, because not all garden wastes are equal.

The real value of biochar, or for that matter activated carbons or lump charcoal, is its porosity. When you’re burning horse manure, leaves, grasses, or other waste, you do not form the same surface. Wood biochar is quite valuable. Grass or manure biochar will probably be more ash than carbonized material.

Strictly speaking, activated charcoal is fired at a hotter range. Because of this, more of the material turns to ash. You’re wasting a lot more wood to get a useful product. But biochar is superheated until it smoulders. The water and residual oils are burned out of it leaving pure carbon behind. It’s this remaining carbon that we’re after when we make biochar.

Actual biochar has another benefit as well. Often, it’s made from the byproducts of an oil-rendering process. Once oils are extracted, the waste products are turned into biochar. Using these carbons in the garden returns them to the soil instead of releasing carbon dioxide into the air. This also prevents soil degradation.

Recently, the use of terra preta has become more common in gardens all over the world. Terra preta is a byproduct of farming in the Amazon, where charcoal, bones, broken pottery, compost, and manure are all burned together in indigenous slash and burn agriculture. There are synthetic terra preta (which translates from Portuguese to black soil in English) sources you can purchase on the market.

Terra preta has been used to boost soil fertility and soil health for centuries, and is still a viable form of biochar today. Terra preta also helps limit the escape of greenhouse gases into the atmosphere.

Benefits Of Garden Charcoals

Biochar spread on soil
Biochar spread on the soil’s surface, before digging it in. Source: CenUSA Bioenergy

So what benefits can biochar, or for that matter other charcoals, provide for your garden and for climate and agricultural systems?

True biochar garners carbon sequestration, keeping carbon dioxide and other carbons in the soil, inhibiting soil degradation. This prevents carbon dioxide from escaping into the atmosphere. As it can take hundreds of years for the carbons to break down, and this reduces greenhouse gas emissions.

The porosity of biochar is a real benefit. All those tiny pores make it absorb water, along with any dissolved nutrients. Rather than letting them run off with groundwater, biochar reduces nutrient leaching. This relates directly to how it improves soil structure and soil health.

Biochar’s porosity also puts it on par with perlite as an aeration and drainage helper. The large, chunky bits of char keep the soil well-aerated, improving soil structure. Excess water easily drains through the soil, although some of it will be absorbed by the char.

If the biochar itself has been “charged”, it’s been soaked in compost tea or another organic material nutrient source. This lets the biochar absorb nutrients your plants will need. Once added to the garden, it’ll slowly release these nutrients. It works to increase soil fertility and soil health much like a slow-release fertilizer in this way.

The biochar can also act as a habitat for many soil-based microorganisms. Microbial life has been shown to be a real necessity in gardening. Beneficial microbes help plants to take up nutrients more readily. They also help defend against plant diseases. Providing housing for the microbes is a good idea, and it improves overall soil health too!

And, of course, it’s far better than putting waste products from oil-extraction into a dump. There is a certain pleasure in taking waste products out of circulation and repurposing them for good uses. Add this to sequestering carbon and reducing greenhouse gas emissions and you’ll find it’s a pretty good amendment when used in the right context.

Drawbacks Of Garden Charcoals

Medium particle biochar
This biochar has been crushed to a slightly-chunky particulate. Source: marcia-oc

Most biochar tends to have a high, alkaline pH. This can cause your plants stress, especially if they’re acid-lovers like lingonberries. Avoid adding biochar to your acid plant beds!

Charcoals other than true biochar may contain stuff you really shouldn’t be adding. For instance, charred horse manure may still harbor potential contaminants in uncharred portions. Burned leaves are too ashy, and offer no absorptive benefit. Cooking coal may have meat drippings and oils in it that need to be burned off before use. And, of course, there’s the ever-present risk of chemicals in the charcoal itself.

The adsorptive nature of biochar and other charcoals has a negative impact as well. If it’s not “charged” by saturating it with compost tea or fertilizers, it will grab onto nutrients. While it will eventually release them and improve soil fertility, you may have to fertilize more often for a while.

Many people recommend adding biochar or other charcoals in the fall. This allows you to add other materials like composted manure at the same time. The char will absorb some of the nutrients from the bed until it can’t hold more. After that point, it becomes a holding vessel in the soil and a benefit… but until it’s “full” or charged, it is a liability.

It’s very slow to break down. This can be both a blessing and a curse. As it doesn’t decompose as rapidly, it can be used in limited amounts to bulk up beds. But you also will want other organic material like compost, which breaks down and adds to the soil.

While it retains water well, so does coconut coir or vermiculite. Biochar isn’t meant to be a sole replacement for any of these other materials. There’s plenty of moisture-retention options out there, and too much biochar is a bad thing.

So You Want To Make Biochar…

Inner chamber of a biochar retort
The inner chamber of a home biochar retort, filled with wood chunks. Source: NoiseProfessor

Unfortunately, most of us don’t have access to the right equipment to make true biochar. To do the pyrolysis process correctly, you have to have a specific heat range in a low-oxygen environment. Specialized, sealed chambers are used to create most true biochars.

But you can make activated charcoal, if you can get the heat high enough. Biochar requires a range of 300-600 degrees Celsius. Activated charcoal can be done between 300-900 degrees Celsius, so if you get a bit too hot, it’s less of a concern. And while less oxygen is preferred to knock down the flame potential, it’s not required.

Basic Activated Charcoal

To make charcoal, you don’t need any fancy tools. All that’s required is a pit, your fuel, and time.

Begin by digging your pit. A pit that’s 2 feet deep and 3 feet long is a good size. The walls of the pit will minimize the amount of oxygen that can reach the bottom.

Build a fire in the bottom of the pit. This initial fire can be used to deal with all your leafy material. It’s also a great way to use up dried manures or other flammable starter fuels. The goal is to have enough wood in there to make a good, hot coal bed.

Once you’ve developed lots of hot coals, spread them evenly across the bottom of the pit. Add more fuel on top to cover the surface. You’re trying to slowly smother the flames in the initial starter material. As the upper stuff begins to develop ash, it’s time to add another layer.

This is where some skill comes into play. If you burn it too long, all you’ll have is ash. Don’t burn it long enough, and you’ll have lots of unburnt material instead of charcoal. You’ll need to practice this a few times until you gain experience at it.

When you’ve added all the material you have and the upper surface is starting to develop ash, it’s time to stop. That uppermost layer may not be completely coal yet, but you can reuse it to start your next pit of charcoal later. Use a garden hose to put out the fire and to stop the coals from burning.

Once you’ve stopped it from actively burning and reduced most of the smoke, you’re not done. The material inside will still be hot, and it will still need to be monitored. If you have a large metal sheet, you can lay it over the pit to reduce the oxygen more and prevent it from flaring up. Otherwise, crack a beer and relax with your garden hose until the heat dissipates.

As soon as the contents of the pit are completely cool, you can remove any unburned material for later use. Then, pull out your finished and cooled charcoal. Rinse off any ash, and it’s ready for use in the garden.

This video provides a great visual aid for how this biochar production process works!

YouTube video

Making Biochar At Home With A Retort

If activated carbon isn’t quite enough for you, there is at least one home method you can use. It’s a double-chamber burner that you’ll have to build from scratch. Sometimes these burners will be referred to as a biochar kiln or biochar retort. You’ll get good quality biochar out of it, but there are drawbacks.

Depending on the material you use, you may have to replace it regularly. At that point, you’ll want to consider how viable it is for you to make the biochar. It takes some practice to make good quality biochar, too. And you’ll have to find a reliable source of feedstock for the biochar – wood chips don’t work. You’ll need larger material than that.

But if none of that is deterring you, I highly encourage you to watch the following video on making biochar. It walks you through the process of how this double-chambered system works. For those who have orchards or lots of shade trees and access to plenty of wood to burn, it’s fantastic.

YouTube video

Producing Potassium Carbonate From Ashes

Any time you’re making fires, you’re going to have ash. If you’ve got good quality inputs, that ash can be useful for you!

Wood ash is a source of potash. This term is used to describe a variety of components which remain after all the wood has burned away. Commonly found in wood ash is potassium carbonate, sodium carbonate, potassium chloride, sodium chloride, silica, and calcium carbonate.

Now, if the pH of your soil is acidic, you can work these wood ashes into the soil directly. Over time, it will slightly raise the pH level towards the alkaline scale. But it will take a long time to have an effect, and you’re adding a lot of silicates to the soil while doing it.

There is an old-fashioned way to render this potash into something useful for your garden – and in fact, potash got its name from “pot ash”. Pot ash is essentially potassium carbonate fertilizer derived from ashes.

Save up a large batch of your wood ashes. Then, soak the ashes in water for a week. Stir it daily during that time to be sure that all the ashes have contact with the water.

On the day after your week of stirring has ended, go out and siphon off the top layer of the water. This water now has lots of soluble sodium and potassium salts in it. Place it into a pot over a heat source and bring it to a boil.

Boil this down until you start to see insoluble materials in it. This will look like little flecks of material forming in your solution. Once you’ve reached that point, continue to boil until you’ve reduced the water level by half. Stop boiling, and wait for all the particulate to settle. Then, pour off the liquid into another container.

As the liquid cools, the sodium carbonates will separate, forming a layer on the bottom. The potassium carbonate will remain dissolved as a solution. You can then drain off the liquid again to separate the sodium from the potassium.

This dissolved potassium carbonate can be mixed with water and used as a fertilizer. You’ll get very little fertilizer for all your efforts, but it’s a great way to extract all nutrients from ash!

Be sure to use it right after it’s cooled. This ensures your potassium carbonate doesn’t have time to oxidize. Over time, exposure to air will render it less useful in a garden setting.

Wrapping Up: Is Biochar Right For You?

Like most amendments, biochar is a good component, but it should never be the majority of your soil. It will improve moisture retention in your garden, and can hold on to some nutrients until later. It can raise the pH of the soil in which it’s placed for a short period of time. And it’s a great way of returning carbon to the soil rather than to allow the byproduct, carbon dioxide to rise into the air.

Because of these things, I do think biochar has earned its place in the garden. As long as you realize it is not a cure-all, nor a good choice in all situations, it’s very useful. It’s an excellent way to get rid of wood you’re not composting, and it’s worth keeping around if you need it.

Best of all, you now know what is biochar and what is merely charcoal, and what goes into making it. This may save you a lot of money on amendments later!

Frequently Asked Questions

Q: What is the purpose of biochar?

A: Biochar adds porosity to the soil, helping it retain water and nutrients.

Q: How do you make biochar?

A: There are 3 ways you can do this: make activated charcoal, use a retort, or simply use ashes leftover from fires. Any of these work, but each is best applied to certain situations.

Q: Are biochar and charcoal the same?

A: Not exactly. Charcoal is more of an umbrella that encompasses biochar and much more than that.

Q: What is biochar made out of?

A: There are lots of different compositions biochar can have depending on the process used to make it.

Q: What are the negatives of biochar?

A: All the good things about biochar are related to the disadvantageous aspects. For instance, too much water and nutrient retention means not enough available for your plants. Additionally the source of the biochar could contain chemicals.

Q: Are coffee grounds biochar?

A: They are not, but you can mix them with biochar.

Q: Can I put biochar on top of soil?

A: Top dressing is actually a great way to apply biochar in the garden. Add a thin layer around plants and water it in. This will help with carbon sequestration and reduce greenhouse gas emissions in the process.

Q: Can BBQ charcoal be used as biochar?

A: It can, but it’s not the best source of biochar, as it can contain chemicals that help you start your grill, and it can contain meat drippings which can contaminate your garden. If you’re using biochar to prevent greenhouse gases from escaping into the atmosphere, use one of the other viable forms listed above.

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