Photo by Cody Gantz
A burn pit is simply a conical hole in the ground. The conical shape pulls air up from the narrow point in the bottom of the hole, evacuating oxygen from the feedstock. Free access to air at the top allows the fire to burn the gases being released from the pyrolyzing feedstock below. This helps keep down smoke and noxious gases by burning them off, but it’s less effective than making biochar in a TLUD.
1. Test Your Soil Drainage
If you’d like to build a permanent burn pit for making biochar, you’ll first need to determine how well your soil drains water; if you have clay soil that holds water, you may end up with a stagnant swamp over time. To test your drainage, perform a perc test: Dig a small 1-foot-deep hole in the ground and fill it with water from the hose. After it drains, fill it again. This second filling should have completely drained out within an hour. If it does, then your soil percolates well and your fire pit won’t fill up with rainwater. If it doesn’t drain within an hour, consider building a cooker instead.
If you want to make biochar in a pit, you need soil that drains relatively well. Photo by Cody Gantz
Make a Burn Pile on Bare Ground
Rather than dig a pit, you could, instead, build the burn pile on the bare ground, which is easier than digging into the earth. However, the biochar won’t be as high in quality because the feedstock will be exposed to a lot of air. Not only that, but just as you would for a fire pit, you need your local fire department’s permission for open burns on bare ground. Sparks can float away and set nearby brush on fire. Open burns can produced a lot of smoke. If you must burn on open bare ground, follow the safety rules mentioned previously and keep your piles small - just 3 or 4 feet square, made of very dry wood stacked in layers set at 90-degree angles to each other, no more than 3 feet high. Keep it small and douse the wood when it’s reduced to red coals.
2. Dig the Pit
The burn pit should be conical, with the point facing downward. Dig the fire pit so it’s about 3 feet in diameter, with sides that slope toward the deepest point in the center, about 3 feet deep. Remember, you don’t have to dig it all at once! Take a few days if you need to.
Lay a generous, sturdy tarp on the ground. Shovel out the pit soil and dump it onto the tarp. Remove any large rocks from the excavated soil, and, while the tarp is still light enough that two people can move it, transfer the dirt to a place away from the fire pit. (You can consider using the soil as the basis for a new raised bed in your garden.) Then bring the tarp back for the next load.
Alternatively, you can do what Hawaiian biochar makers do and use the excavated soil to build up walls around the lip of the pit. After the burn, shovel the soil back onto the biochar in the pit. Add some compost and mix it all together. The end result is a circular area of soil and biochar mix ready for planting. This allows you to conduct several burns exactly where the biochar is needed by simply rotating burn pits around the garden area.
Building up walls of dirt around the pit allows you to then easily convert the pit into a biochar-rich growing area. Photo by Cody Gantz
3. Load Your Feedstock
Put your main load of feedstock, which will become the bulk of the biochar you produce, into the bottom of the pit. Do not stack it tightly, as this will slow down the burn too much. Rather, tumble it in loosely or stack it in layers — one layer in one direction, the next layer above it perpendicular to the previous layer, and so on — until the pile is about 8 inches below the rim of the pit. Top the feedstock layer with a 1-foot-thick layer of very small, very dry, and densely packed kindling.
Layer the feedstock by type in the burn pit with the heaviest wood (a, b) at the bottom and lightest (c, d) at the top. Photos by Cody Gantz
4. Light the Fire
Light the top layer of kindling with a match or a propane torch. The torch is easier and lights the entire kindling layer quickly and evenly across its surface. As the kindling catches fire, it will draw air from the feedstock in the pit below it.
Photo by Cody Gantz
As the feedstock layer’s top pieces start to burn, they’ll produce smoke, but the kindling fire on top should consume most of the smoke so that you don’t see it, thus keeping the carbon within its charred remains instead of sending it into the atmosphere.
5. Keep It Roasting
As the kindling is consumed, add more kindling on top. This will keep air moving up through the feedstock layer, providing fire that consumes the smoke, and encouraging the feedstock layer below to catch fire. The process will proceed fairly slowly at first, but shortly, the entire contents of the pit will be blazing. The flame will be mostly yellow as the gases are consumed, and very little if any smoke will be produced. That’s what you’re aiming for here, as the setup is constructed to burn off the gases. If the kindling burns off before the feedstock turns into biochar, then add more kindling. The purpose of the kindling is to keep a good fire going on top of the feedstock to combust the smoke and volatiles coming off the feedstock.
As the fire in the pit slows down over the next 45 minutes to an hour, use the steel-tined rake to push unburned remnants in toward the center of the pit. Soon the feedstock in the pit will have collapsed into a thick bed of red-hot coals. It’s at this point, before the feedstock burns to ash, that you want to stop the fire.
Add more kindling as the top layer burns down to keep air moving up through the feedstock layers. Photo by Cody Gantz
6. Douse the Burn Pile
This is a critical point in the process. You want to douse the fire while the feedstock is red coal but before it turns to white ash. It’s better to douse too early than too late. You can always burn the feedstock a little more later, but once the feedstock is ashes, it’s gone.
With the hose, thoroughly wet the red coal bed. Rake the wet coals to reveal any hot spots and spray with more water until you’re sure that the coals are no longer smoldering and are entirely cold.
Douse the pile to kill the burn before the organic matter goes to ash. Photo by Cody Gantz
When Is It Done?
Properly made biochar is light and rigid but easily crushed. It’s fine-grained and not resistant to being ground to powder. It feels dry, not greasy. It rinses off your hands easily with water alone. And if you drop a piece onto a hard surface, it should make a slightly metallic ringing sound.
7. Let It Dry Out
After you’ve made sure the coals are out, let them sit for a day then check to make sure the fire hasn’t started up again. Then rake the coals again, douse with more water, and allow them to sit for another day. At this point, when you pick up the coals in your hands, they should be black and cold to the touch. The biochar should crumble easily in your hands. Wet biochar can go straight into the compost pile or other system for activation, such as a container of worm castings, as long as you’re sure it’s fully extinguished and not smoldering.
If you have any white ash, it means you let the fire burn the feedstock too far. Next time, get the water on the coals as soon as the feedstock collapses into red-hot coals.
When finished, biochar should be black, cold, dry, light, and rigid but easily crushed. Photo by Cody Gantz
Excerpted from Gardening with Biochar by Jeff Cox. Photography by (c) Cody Gantz. Used with permission from Storey Publishing