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The benefits of biochar. © RED Garner, illustration by Kevin. D Brown.

“Biochar may represent the single most important initiative for humanity’s environmental future. The biochar approach provides a uniquely powerful solution, for it allows us to address food security, the fuel crisis, and the climate problem, and all in an immensely practical manner”. Prof. Tim Flannery 2007 Australian of the Year

Biochar is made by a process called pyrolysis, which means burning organic material with minimal oxygen. When woody biomass (dried, not green) is pyrolysed, all of the volatile components burn away and leave behind a carbon skeleton in a very stable, hard carbon form. This creates charcoal that has a pore structure which provides its near-magical potency in the garden.

In the soil, this hard structure serves as a kind of ‘coral reef’ for microbial life. Bacteria move in, paint the walls with their exudates and invite some fungi over for tea. Myceloid pathways are followed by hungry nematodes and microarthropods of all shapes and descriptions. The ‘reef’ begins to resemble a Star Wars cafe: all these busy soil creatures sharing grog and stories and, by the way, mentioning that the plant whose roots are just outside the front door is asking for some calcium or phosphorus, and that the plant would be happy to exchange those things for sugars and information.

Unlike the carbon that normally reaches the soil from the decay of plant and animal material (called ‘labile’ meaning unstable), this molecular structure (called ‘recalcitrant’) resists combination with oxygen to become carbon monoxide or carbon dioxide, or hydrogen to become methane. It just remains in the soil. And remains, and remains: thousands of years may come and go, but that carbon will still be there.

Biochar is charcoal but not the kind you’d buy at the shop to use on outdoor barbecues. The distinction is an important one, because to be biochar, and not charcoal, it should be suitable for putting into your garden, or even sprinkling onto your food. The charcoal you buy in packaged form may have come from treated wood, or have been treated with petrochemicals to make it smell sweet, light quickly and burn slowly. Real biochar is food grade.

It’s The Biology

At the end of the 19th century, Justis von Liebig got into a debate with Louis Pasteur: Liebig believed that plants needed only water, sunlight and food – things that can be described in the language of chemistry; Pasteur insisted that plants needed more than that – they need the microbes in the soil. Liebig attempted to confirm his thesis with experiments but, after many trials, had to concede that Pasteur was right. Liebig became a fan of biochar and its contribution to the soil food web, and when he died he had himself buried in a casket filled with biochar.

Pasteur’s insight has profound implications for agriculture, which has moved, in the short space of a century, from being based largely on biological processes (manures, grazing animals, the decaying mulch of annual crops) to chemistry and physics (fertilisers and machines). ‘Big pharma’-culture mines the soil of minerals and destroys its microbial ecosystem, setting in motion a chain of events that ends in desertification.

Machinery gets larger and stronger to try to till deeper, but only moves the compaction zone downward, souring soils and depriving plants of micronutrients. Nutrient-starved plants become nutrient-deficient foods, and thus we eat more, craving nutrient but adding bulk.

This is a microscopic cross-section of biochar, which illustrates the pore structure which provides its near-magical potency in the garden. Photo by Dr Jocelyn

How It Works

Now imagine that we follow the advice of Pasteur, and grow our food as if the biology mattered. We add biochar to the soil, a bit at a time, and watch it do its magic. One of the things we might notice is that it works as a sponge for water, nitrogen and many trace elements. As a microhabitat it becomes a cache of nutrients that can be exchanged with plants on demand. It buffers against both drought and flood. It scavenges unwanted chemical residues and entrains them in insoluble forms.

It should be no mystery that all of this was known to previous generations. In 1846 Richard Allen, in A Brief Compend of American Agriculture, (C.M. Saxton, 1847) wrote ‘Charcoal dust applied in the same way [as compost] has been found to increase the early growth from four to ten-fold’. One of the first written records of biochar is in the Nogyo Zensho, an agricultural encyclopaedia written in Japan in the late 1600s by the wandering samurai-turned-peasant Yasusada Miyazaki.

Terra Preta

Many people have heard of ‘terra preta’, the ‘dark earths’ of the Amazon Basin. In a region where torrential rains and hot temperatures drive organic material from the soil and store it above ground in the living forest, the ancients learned to garden and build their monumental civilizations by building soil reefs of biochar. Biochar, however, is not terra preta. After a few hundred years it can maybe help to create terra preta, but terra preta is the product of a complex process of interaction between clay soils, microbes, worms and organic material that is not yet fully understood.

Looking at the density of biochar in terra preta soils, some farmers think that they need to have many tonnes per hectare if they are growing broadscale grains, vegetables, tree crops or pasture. But remember that the stuff lasts for thousands of years: if you put some in every time you add compost or mulch it will gradually accumulate.

Charging The Biochar

Albert Bates. Photo by Robyn Rosenfeldt

The first error many gardeners make when putting biochar to work is to take it straight from the kiln or the backyard fire pit and spread it on their top soil, or place it into the bottom of the hole they’ve just dug for a tree. Straight from the fire biochar is hydrophobic – it repels water – and its pores are empty. However, if gardeners put their biochar into the compost, or soak it in compost tea first, they will ‘charge’ it with microbial life – those little aerobic bacteria and fungi that do the good things for your plants. If you are willing to give it a urine bath, studies have shown that can improve plant yield by as much as thirty per cent.

Biochar blends are available from shops or online, but may be hard to find and expensive. It is not difficult to make, and anyone can do it if they remember four simple rules: moisten (before grinding), ‘micronise’ (grind it fine), mineralise (add rock dust or whatever else your garden may be short of), and ‘microbialise’ (add biologicals – effective microorganisms, compost tea or aerobic compost).

Improve The Soil

Many people find that adding biochar to their regular gardening practices not only improves soil productivity but also makes them feel good about cutting their personal carbon footprint. Taken to scale – if the world switched from chemical agriculture to biological agriculture – we could sequester as much as ten gigatonnes per year of carbon from the atmosphere, nearly twice the annual addition from fossil fuels.

In that way we could reduce greenhouse gas concentrations back to pre-industrial levels in this century, and possibly avert some of the worst of the climate chaos that the Anthropocene has in store. We could return to the comfortable Holocene, during which humans developed, and to which we are well suited.

Albert Bates is author of The Biochar Solution: Carbon Farming and Climate Change (New Society Publishers 2010) and several other books on history, environment and the future. He is director of the Ecovillage Training Center at The Farm in Summertown, Tennessee USA, and a faculty advisor to Gaia University, Colorado USA. He blogs at and tweets @peaksurfer.

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Photos by Robyn Rosenfeldt

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Making biochar in a drum, dish or earthen pit is a very simple way to start making charcoal and learn the process. From there it can become as elaborate or large as needed or wanted. The important part is to burn from the top down. There are many ways to make biochar and the internet is full of instructions on every method.

First you need some sort of kiln or container for your fire, these range from simple metal dishes, 44 gallon drums or a hole in the ground to much more complex engineered structures that can provide power as well.

  1. Fill your fire container/ hole with evenly sized biomass, (sticks, twigs, off cuts). (Image 1, right)
  2. Place some lighter, flammable fuel on the top and light it from the top. (Image 2)
  3. Once the fire has caught and fuel is burning, start excluding oxygen, either by placing a cover over the fire, covering the fire in soil or placing more biomass on top to keep the oxygen away from the lower levels. Leave to burn, keeping an eye on it of course. (Image 3)
  4. Once all the biomass has been burnt and turned into charcoal, quench the charcoal by tipping liquid over it. If your game, urine is the best as it fills the charcoal with valuable nutrients. (Image 4)
  5. Crush the charcoal to a fine dust while still wet. One way of doing this is in a small cement mixer with several river stones in the bottom.
  6. Mix through your compost to charge it with microbial life.
  7. Add into your soil and watch your crops power.


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