BY BILL TAYLOR
Hundreds of years ago, much of the Northeast was a center for producing charcoal for smelting iron, and pits in our forests attest to that history. These days, for farming and climate, biochar is often recommended as a solution to soil fertility and a way to sequester carbon. The principle is simple: when wood burns, first the heat releases volatile gases that burn, and then the carbon itself combines with the oxygen in the air at a much higher temperature to create carbon dioxide and ash. Making biochar or charcoal (mostly carbon, with some minerals present) requires excluding air once the volatile gas phase of the burn is complete. Unlike whole wood, char takes centuries or more to break down (yielding carbon dioxide) so it is a stable way to bring carbon dioxide levels down. It provides additional habitat for soil microbes compared to that provided by humus and soil particles, and the smaller the char pieces the more this happens. The reason it is advised to inoculate char with animal waste or composting material is that if it is added raw to the soil, it will take time for the char to fill with microbes and this takes them temporarily out of circulation in the soil food web which relies on them to help plants grow. This is an overly-simplified picture of what actually goes on. The real-world result is that after initial treatment, the biochar improves productivity in the garden or farm.
While there are all kinds of gizmos to make biochar, I am gearing this article to those of you who make a burn pile and light it and burn to ash. Before going there, though, I will give some background on these other systems, which are useful if going to various locations with a truck to transport them but do require more preparation of the feedstock (cutting up wood or chipping it), so the energy used doing the cutting or chipping may partly or totally cancel out the higher production of char per pound of wood. Starting with the simplest, there is the choice between the Oregon Kiln or a metal ring type (“ring of fire” kiln). Here is information from the Wilson Biochar Associates in Oregon (wilsonbiochar.com): “The Ring of Fire Biochar Kiln is a metal container for burning waste wood and brush for the purpose of making biochar. The kiln consists of an inner ring composed of three sheets of mild steel that are bolted together. An outer ring of lighter gauge steel bolts onto the brackets that hold the inner ring together. The purpose of the outer ring is to serve as a heat shield that holds in heat for better efficiency. The kiln is easily moved, as none of the individual pieces weighs more than 40 pounds.” The inverted trapezoidal-shaped, single-piece “Oregon Kiln” needs equipment to move. Either kiln costs about $1,000 to purchase. If you want to make your own kiln, this very long URL has plans.
The most ecological modern burners or “retorts” are much more costly and elaborate than anything discussed so far; they capture the heat to use in nearby buildings and some even make electricity; for information on these and much more visit the Biochar International website or in our region, New England Biochar.
On the farm or homestead wanting a simple and accessible method, I recommend two ways I have guided the burn to produce much biochar and little ash for little to no extra cost. The pit method takes more effort but does yield more and better quality char (easier to crush).
Our Experience Making Biochar
When in California, Jaye and I tried open pile burns. By burning very dry similar-sized brush and raking aside still-burning pieces from where coals were plentiful in the burn pile and dowsing those coals with water, we were able to get a fair amount of biochar. Mixed sizes and/or less dry brush yielded less char. The advantage of this lowest-tech method is that it is not much more work and is actually less time than burning to ash (just raking off the still-burning parts of the pile to allow watering the char before it can get to ash).
A higher yield can be obtained with a pit similar to those used in charcoal production. In our case, we do not cover it to starve it of oxygen and have it go for days, but instead, complete the burn in a few hours. In early January 2021, I got a burn permit and dug a 5′ diameter circular pit which tapered to make a cone shape over 3 feet deep. Piling the soil in a ring around the outside allowed a deeper and wider pit without actually going quite as deep. Rocks found during digging armor the sloping sides near the top to reduce soil falling in and act as a buffer between coals and topsoil. It is important to get roots out and make sure there are no large dead roots to help prevent the fire from creeping underground. Burn season opened January 15th and that day started with a foggy morning, a bit of snow around, and a go-ahead when I called the town offices.
Jaye, our friend Roz Aranow, and I brought down a hose (thankfully it was above freezing) and cleared the immediate surroundings of brush, and packed the pit loosely to a level a few inches above the rim of the pit. The starter fire needed to be above the rim and brush and with several pounds of very dry wood so it could get adequate air to start burning and light the less dry brush below. Although dry wood is recommended, so is the occasional spraying of a raging fire to tamp it down a bit, so we combined these ideas by throwing in green parts of the brush with some snow stuck to it to slow the fire down. I now think it’s better to add wood to the top only since many of the pieces we pushed into the coals did not char in the short time of our burn, although their being green did cool the sides and kept the burn more under control. When smoke against the gray sky was brownish that meant there were unburned gases and particulates. At other times there was almost no smoke and just a grayish-white color that indicated a cleaner burn. This was when we used more dead branches and fewer green ones. As the flames decreased, we raked the still-flaming wood to one side and watered the other side. Gradually the still-flaming pile got smaller and we could dowse more of the pit. We then soaked it well once most of the flames were gone, and stirred with a shovel. Over the next few days, I harvested about 120 or more gallons of char, crushed it, and mixed it with raw and partly composted dairy manure to inoculate it. About 10% of the pit volume was uncharred wood which we set aside and charred it in an April 2021 burn.
Hardwood brush may need to be drier; evergreens have more volatile oils and gases that allow burning even when green and wet. What we like about this burn method in addition to the biochar product is that it cuts the burn time in half compared with a burn-to-ashes above the ground burn. However, the pit method requires active feeding rather than the more passive open pile, but still, you can get exercise while spending less time overall. But if open piles with large-sized brush and wood are what you have, just the continuous raking aside of burning portions and dousing the coals beneath will still yield a good amount of biochar and also take less time than the burn-to-ash, time you can spend crushing and inoculating the char (that afternoon or another day) rather than watching and perhaps lamenting the carbon dioxide release. I like to be done before lunch (thoroughly dousing and stirring the char) rather than guarding a fire until evening.
You Can Do This!
For your garden, our climate, and to have a streamlined burn day, making biochar is very rewarding, whether from a traditional burn pile you may already have, or in a pit you can dig next to it. If you want to dig your pit right away, you may be surprised to find non-frozen ground under a wood or brush pile or snow or other insulating covering. (Although I did use a pickaxe to start the hole and pry out rocks). Once the biochar is made, to inoculate, one can use urine, add char to a compost pile, or mix it into animal bedding or manure. After a few months, it can then be mixed 10% more or less into a garden bed. Or, if you do not plan to use it, you can give it to others who will, or sell it, and know you have sequestered some carbon.
Please contact us if you are interested in attending our next burn at Floodgate Farm. It’s not possible to schedule it in advance but you may call 707-272-1688. We are in Sandisfield in the southern Berkshires.
Resources:
New England Biochar, newenglandbiochar.com/
Biochar International, biochar-international.org/