The Biochar Working Group has been formed within Transition Salt Spring to promote the production and use of biochar on Salt Spring Island. Many property owners on Salt Spring have burn piles in the spring or fall to manage wood waste—limbs and branches that have come down in windstorms or tree prunings. These burn piles typically produce large amounts of smoke, and are also a significant fire hazard. Moreover, they are a waste of material that could be transformed into biochar to improve garden and orchard soils. With simple-to-use open cavity kilns this wood waste can turned into a useful material with a much-reduced wildfire risk and impact on air quality. The Biochar Working Group is working to educate islanders on the making and use of biochar.
What is biochar?
Biochar is charcoal used for soil amendment or soil remediation purposes. Charcoal is wood that has been partially burned in an oxygen-reduced atmosphere in a process known as pyrolysis. Wood that has been subjected to pyrolysis is almost pure carbon, and when burned emits very little smoke.
Think of a campfire; when first lit, it tends to produce a lot of smoke and burn with a lot of flame. After the fire has burned for a period of time, if you add no more wood it burns down to a point where it is a bed of coals, producing a lot of heat and very little smoke. If the intention is to cook over the fire, this is a good time to do it—it’s producing a nice even heat and you don’t have to contend with a lot of smoke while you’re doing your cooking.
Now imagine that you could instantly smother your fire at that point, so that you could restart it again later when you wanted to cook. If you did, you would have charcoal, and this is what you are getting when you buy a bag of charcoal—wood that has partially burned to remove the smoky component, leaving close to pure carbon behind. The other components are what allow the various kinds of charcoal such as maple and mesquite to give their distinctive flavours to food when they’re used for grilling or barbecuing.
Why do we use it in the soil?
Putting charcoal into the soil has been found to be very beneficial for plant growth. There are a number of reasons for this: like all charcoal, biochar is a dense concentration of microscopic cavities, providing a habitat for microbes and fungi, which aids plant growth. As well, all those cavities permit charcoal to act as a sponge, substantially improving the soil’s ability to retain moisture. Biochar is a means of adding organic material to the soil in a form that is highly stable—once added to the soil it tends to remain in place for a very long time, perhaps centuries. (see ‘terra preta’) Charcoal has a neutral pH and because biochar typically contains a certain amount of ash, it helps to sweeten the soil, very helpful in places such as this where conifer forests tend to produce very acidic soils.
How do we make it?
Charcoal is made by burning wood in an oxygen-reduced atmosphere, and halting the burning entirely before combustion is complete. Charcoal made for cooking or other purposes is typically produced in a kiln, a vessel for burning wood in which the airflow can be carefully controlled, and ultimately cut off entirely. They can be made of bricks, stone or metal; at its simplest charcoal can be made in a pit in the ground, with dirt or clay put over top of the material to be burned, so called ‘pit charcoal’.
In recent years, highly sophisticated pyrolysis units have been developed that permit charcoal to be made with virtually no emissions—no smoke—and which conserve heat more efficiently than traditional kilns, with some even harnessing the excess heat to use for other purposes. They tend to be complex, expensive and more difficult to operate, and because the nature of the process requires that either biomass be transported to the kiln, or the kiln transported to the biomass, heavy expensive machinery requires a greater expenditure of energy than methods in which the pyrolysis can be carried out close to the source of the biomass. Nonetheless, the new pyrolysis systems offer exciting possibilities for the future.
The Biochar Working Group has developed a low-tech, yet sophisticated system for making biochar using a cavity kiln, sometimes referred to as a ‘Kon-Tiki Kiln’. Unlike most kilns, these are open at the top, allowing material to be easily and continuously added to the fire. The unit is completely closed on the bottom; all of the air feeding the fire comes in from the top, and air control is achieved—the partial smothering of the fire—by continuously adding new material so that the material below is burning in an oxygen-reduced atmosphere. Combustion is facilitated by the shape of the vessel; being made of metal and off the ground, the fire burns at a higher temperature than a similar fire on the ground. Furthermore, after they become hot the inwardly sloped sides of the kiln channel hot air upward and over the upper edge of the kiln and into the fire, pushing smoke back into the fire so that it can be more completely burned. When the supply of material to be burned is exhausted, the unit is close to full, or the operator is exhausted, then the fire can be extinguished by flooding the unit with water from below using a hose attached to the bottom. Afterwards the water is drained and the biochar can be shovelled out.
History – terra preta, how we learned about biochar
The use of charcoal as a soil amendment was pioneered in the Amazon basin, though it likely occurred in many other places in the world. When Europeans first arrived in South America they were surprised to find areas of very high fertility that were like islands in a sea of very poor soils. The soil in these areas was very dark, which they termed ‘terra preta’ in Portuguese, ‘dark soil’. It was found that not only was the soil more fertile, there was more of it—it was deeper—and over time they pieced together the story of how Amazonian farmers had made charcoal in pits, left it in place, added in soil and grew their crops on it. Centuries later, the soil in these areas is still very fertile, the charcoal within it acting as a permanent fertilizer.
Biochar as a means of carbon sequestration
Unlike wood decomposing in the ground, which over a period of decades is almost completely consumed by bacteria, fungus and insects, charcoal is highly stable, its carbon stays in the ground instead passing back into the atmosphere as CO2. This means that although a considerable amount of carbon is released into the atmosphere when biochar is being made, viewed in the long term, its net carbon emissions are lower than simply allowing wood to decompose, suggesting that biochar can be considered to be a means of carbon sequestration.
How can I learn more?
If you would like to learn more, contact the Biochar Working Group to organize a workshop or find out how you can make biochar on your own.