Last week we went to a biochar workshop that opened our eyes to the full awesomeness of the stuff!


We were lucky enough to have one of Australia’s leading biochar experts, John Sanderson (on the left in the photo above), delivering the workshop in Castlemaine (and lucky that local group Growing Abundance organised it for us). He was joined by his fellow Earth Systems colleague Lann Falconer.

John made the point that biochar as we know it now is not a new soil amendment, and in fact has been used for thousands of years by many cultures around the world. The most famous are the ‘Tera Preta’ soils discovered in South America, but similar types of soils have been found throughout Africa and other places, where biochar is actively used as a soil amendment to this day.

Despite some good early work on biochar in Australia, much of the activity has shifted to Europe and California, which have become the hubs for biochar research, partly due to difficulties in attracting research funding and finding markets in Australia. Considering Australia’s arid environment and propensity to drought, we reckon there’s a missed opportunity in researching biochar as a way of mitigating against the effects of drought in horticulture!

While biochar is similar to charcoal, it’s a bit different, being produced at higher temps of 350°C–750°C. This is the perfect range; if you make it at a higher temperature you lose some of the properties that help make it so useful in the soil. If produced at lower temperatures it will still contain volatiles that may have a detrimental effect in the soil.

One of the amazing properties of biochar is its incredibly large surface area, from 500–600 square metres per gram. If you look at biochar under a microscope, it’s full of holes. The holes are full of holes, which are full of more holes (called micropores). In fact, there’s more holes than matter! This is what gives biochar its incredibly large surface area, and what makes it ideal for both holding water and providing the perfect habitat for soil microrganisms—it’s basically like an empty block of flats for microbes! Biochar is so good at holding on to moisture that it could be a game-changer for Australian soils. Just 4% in sandy soil doubles the amount of water that the soil can hold.


Biochar has an inherent greenhouse gas sequestration effect if it’s made from biomass that would otherwise rot, go to the tip, or be burnt. When biomass is turned into biochar, approximately 50% of the carbon is burned off, but the other 50% is turned into a highly stable form of carbon, which will last for at least 1,000 years in the soil. For every ton of biochar, this represents 3.6 tons of CO2 sequestered (and for a long time). John reckons that if we could fill up three open-cut mines with biochar, we’d be about half way back to when we took the coal out of the ground!

Biochar also decreases nitrous oxide (a potent greenhouse gas) emissions but the mechanism is not yet understood, and in fact there are a couple of situations where biochar application has made emissions worse, e.g., with the use of nitrogen fertilisers in dryland agriculture.

Before the workshop we knew that biochar makes soil much more fertile, and now we understand why—because of its awesome ability to hold nutrients and prevent them from being leached away.

biochar-mixing-superchar-480x269Why is biochar awesome?

When used in gardening, biochar has positive effects in several ways:

  • Physical—because of the vascular nature of the source materal.
  • Hydrological—dramatically increases the water-holding capacity of the soil because of the huge surface area and micropores.
  • Chemical—the inoculation process puts the chemicals (nutrients) into the biochar before you use it. Biochar is made of carbon, which is the chemical backbone of all life. Biochar can carry oxidised groups such as phenolic groups (charge of -1) and coboxyls (charge of –2), which are anions and like to bond to cations such as calcium (Ca, charge +2), magnesium (Mg, charge +2), potassium (K, charge +1) and ammonium nitrogen (NH4, charge +1). These cations can be released to plants in exchange for hydrogen ions (H, charge +1), and thus act as a store of these cations needed by plants, reducing the loss of nutrients from leaching and thus reducing the need for added fertiliser.
  • Biological—4%–6% of biochar is biological carbon (under international biochar standards, up to 20% can be labile carbon). Microbes can eat this carbon, and biochar is also the perfect microbial habitat. Biochar can also encourage enzymatic processes, and it also attracts fungal inoculation (this can happen even if biochar is out of the soil).


This is an industrial process that further erodes the biochar until more of the walls are gone and it becomes like coral. The process involves using oxygen, steam and high temperatures, and can increase the surface area even more—up to 2,000 square metres per gram. It only works if the biochar was originally made out of something very hard like coconut shells; if it’s made out of something soft, activation makes the biochar fall apart.

biochar-milo-tin-480x269How to make biochar

You can make biochar by applying heat to any plant material in a low-air environment. First the water boils out, then if you were doing it in air the material would burn, but because you’re doing it with less air present the volatile components are emitted. This smoke is acidic, and it’s important to make sure it’s burnt in the process of making your biochar. When it’s made industrially, this smoke can be condensed and collected, and then it’s called either wood vinegar or smoke water. Though it’s acidic, it’s also very nutritious and has various uses.

The technical term for a sealed container for making biochar is a “retort”, and at the workshop John demonstrated a simple method using a Milo tin with holes in the bottom and a gas burner, which made biochar in about 20 minutes and really demystified the process!

You’ll often find biochar at the bottom of your wood fire, which is the wood that didn’t burn because air couldn’t get into all of it.

Whatever system you set up, the aim is to heat the stuff as little as possible, but with enough air to burn the volatiles, or else you generate massive amounts of horrible smoke. The most important thing is to NOT make smoke, there should only be a heat haze.

What can biochar be made from?


  • Soft wood is fine, e.g. pine, cypress. The issue is volatiles—the wood vinegar is no good, but the biochar is good. Moisture content is not relevant, but the drier it is, the easier it is to light and to keep burning.
  • Sawdust is excellent.
  • Treated timber (ex-building) contaminated with creosote is OK. The creosote is burnt off in the thermal process and leaves no residue.
  • Garden clippings or prunings.
  • Any plant material.


  • Timber that’s been treated with chlorine-based chemicals should NOT be used because the heat process creates dioxins which form a toxic emission, e.g., railway sleepers have been treated with dieldrin which creates toxic dioxins.
  • If wood has been treated with copper or chromium for example, this should also not be used because it creates a contaminated biochar.
  • Rotten, light or spongy wood is not as good because it has lost its structure.
  • Wood that’s painted with lead paint.

Easy home method (a scaled-up version of the Milo tin!)

  • 205 litre drum
  • Tight lid
  • Holes in bottom: Start with small holes, because you can make them bigger. Recommended: 12 mm holes in a complete ring at 25mm spacing, and another 10 or so holes in the middle (this has worked well). If there are too many holes the biochar will burn and you’ll end up entirely with ash. Too few holes and you may overpressure the system, which will lead to the drum distorting (it’s unlikely to explode). Smoke goes out the bottom through the fire, so the volatiles are burned.

A typical fuel that works well is garden pruning waste about finger thickness. This burns about 25mm per hour, so wood with a diameter of 150mm will take about 6 hours to turn into biochar. If you use smaller fuel, the process is faster, but you have to weigh up the energy and time involved in reducing the size of the fuel with the energy and time involved in the burning process.

You need to keep stoking the fire under the drum until it’s all turned into biochar (you can’t tell until you open it, so notice the diameter of the wood as it goes in and use that to guide you). You don’t need a temperature gauge, but you can use one particularly if using large pieces of wood. Drill into the middle of the largest and put the probe in, and burn until it reaches 500°C—that should make sure it’s finished.

Once biochar is made it needs to be crushed, and a small mulcher is perfect for the job. The finer the better, about 3-5 mm is ideal, or slightly larger (10-20 mm) if you’re using it in potting mix.

Caution: the process can produce carbon monoxide and so should never be done in a closed environment.

Toplift Pyrolyser Method

This is a great technique for using fine material like sawdust. Use a drum that is packed in a uniform fashion (like making a cigarette), and light the material at the top. It’s a good technique if you have a fine source material, as it burns uniformly down from the top and no air gets in so there’s no ash, just charcoal.

Inoculating biochar

If biochar is applied to soil without being ‘charged’ or inoculated first, it will quickly suck water and nutrients out of the soil until all the pores are filled. This takes the water and nutrients away from plants and can cause shock for them.

Before use, biochar must be filled first with both water, nutrients and microbes. When charged biochar is applied to the soil, the water and nutrients become available for the plants and the biochar can start weaving its magic.

biochar-ingredients-480x269When inoculating, you’re aiming for complexity, so use a diversity of products from lots of different sources.

An easy way to inoculate the biochar is to keep it near your compost bin, and add a couple of handfulls of biochar to the bin every time you add scraps. The right ratio is about 1 cup of biochar to a small bucket of food scraps.

You can also add biochar to a worm farm, as worms LOVE biochar.  One of the tests performed to test any new soil amendment is called the Worm Avoidance Test and when biochar was tested in this way, rather than avoid the biochar, the worms flocked towards it.

What to use for inoculation

Mix up your choice of inoculants from the following list (or the ingredients listed on the slides in the photo), mix with your biochar and leave to sit for a week or two before applying it. When inoculating the biochar it should be completely covered by liquid to prevent mosquitos breeding in it.

  • Worm juice
  • Compost tea or compost extract
  • Comfrey tea—high in phosphorus and potassium. Make by soaking comfrey in water for 20 days, turn a few times. It stinks but is brilliant.
  • Wood vinegar—has lots of nutrients, but optional.
  • Water-soluble trace elements, e.g., Thrive.
  • Aged manures.
  • Compost.
  • Limebiochar—has a slightly alkalizing effect because it contains the positively charged minerals that were in the original plant matter, and usually has a pH of 8.0–8.5.

Biochar plus worm tea or a complex of ingredients is the equivalent of creating rocket fuel for soil! 


Using Biochar

When using biochar as a soil amendment it’s best to incorporate it into the soil rather than apply it to the surface, where it can actually increase the temperature of the soil because it’s pure black.

  • Add 10%-20% in wicking beds, though smaller amounts will still be useful. It can either be incorporated into a new bed (in which case it’s fine to use up to 40% biochar), or an existing bed.
  • Fruit trees: apply to root system, particularly useful in helping establish mycorrhizal fungi in the rootzone.
  • Broadscale: e.g. it’s being used in wheat crops in California (Greg Barber)
  • Supercharged biochar will also act as a slow-release fertiliser, and smaller amounts (e.g. 5%) are useful.
  • In potting mix, it makes a more porous mix which resists compression and is great for indoor plants.
  • It’s fine to use biochar from your fireplace.
  • Animal husbandry: about 80% of the biochar being produced in Europe is fed to animals. It’s great for their gut and means they belch and fart less.

Other uses

  • Really good as a spill clean-up medium (and then the biochar becomes a contaminated waste that must be disposed of responsibly).
  • Eating biochar: good for human gut health as well!