Leave a guy alone on a Saturday- Biochar!

I have been interested in making biochar for a while, and have been discussing the positives and negatives with various people, some on here and some Godless heathens who are not.

Here is the thread that Bryant Redhawk was discussing the uses, it motivated me to make it finally. So, it is Bryant's fault.

I was abandoned by my adult supervision today, and decided to dig a pit and make some char with my trusted juvenile assistant. My primary motivation is to improve the compaction of the soil, which is basically clay with clay on top. Other benefits desired would be to improve the CEC, provide dormitories for food web critters, and retain moisture. The wood used died of natural causes, there was a beetle-kill area of pine trees, and the fire risk is significant, so I also cleared out a three downed trees and about 1/2 acre at the same time. Hugels would be fine but the ground is too hard to dig down and I don't have another source of fill dirt without buying it. Since this is in a future pasture I really just desire the area cleared, as there are boss species moving in and more getting planted this spring.

I went with the pit method. As mentioned it is clay soil, and I figured it would make an easy containment vessel and keep the heat in. The pit was about 4' around and 3' deep. It took a couple hours to get it dug. Wood collection was rapid, there is so much deadfall I probably had a couple cords of 3" diameter or less wood collected during the burn. I thought about using some logs but there is so much deadfall this seems like a no-brainer. There was about 20' of clearance around the pit, and the leaves were cleared. We had rain a couple days ago and things were not damp but not overly dry, so good conditions. We had no significant wind. Be safe my friends!

Fire was started in the bottom, the idea was to bake the pit before the addition of the main wood. I put in some ashes from a recent bonfire to insulate as much as possible. We burned it HOT with dry cedar sticks and twigs for about 10 minutes and the rim was visibly baked like pottery! After that we had a decent bed of coals in the bottom and the heat was impressive. We loaded it up so the fuel was flush with the top and every 10 minutes or so tried to collapse the coals into the basin. The priority was to keep the fire on the surface, and limit the oxygen available to the coal bed. Hopefully the midburn picture will show the glowing coals under the flame layer.

We kept it burning hot for the next couple hours, adding wood on the surface to try to cover the whole pit.

After we ran out of wood and I was tired of literally running around gathering more, we used leaves and small twigs to cover the pit completely for the next hour. I kept adding leaves on top any time the wood started to get exposed.

Then we covered it with the dirt, making it about 8-10" deep in the ceramic vat. My trusted assistant helped me tamp it down to remove air pockets.

Tomorrow I will dig it up and see what the yield is. Let me know what you want to see and I will try to post it. I'm interested in the ash quantity and if I can get the char without much ash, but I can make observations as people want...

Final product: the pit was full to the brim with charcoal, so yield was about 1 cubic meter. Ash collected in the base of the pit and I did not collect the bottom 4-5" of material. Most of it looked good but I don't need that much ash in my compost. Ash from the top burn/leaves also set aside, as this was high in ash.

The pit was basically a brick, the clay baked about 1.5" into the surrounding soil. This made removing the material much easier than anticipated. The quality of the burn was "good". There was some incomplete burn on the perimeter of the top, which I removed and will put in the next attempt. Still, this was definitely <10% of the material, maybe 3-5%.  

The material is very very light. I think this is a product of the wood used, the pine was dry and fairly spongy. I think this stuff is going to take on the inoculation very well! Since the size of the wood was so small there was little material to break up, almost nothing was over marble-sized after a few encouragements from the shovel.  

It all went into a compost pile about 5 times as large. I watered it down and mixed it until everything looked saturated, and there was some water beginning to come out the bottom of the compost pile, and will check on it and probably rewater it tomorrow. The water content was very low in the char, and I purposely did not saturate it in the pit, because I want to glaze the pit a few more cycles before getting it saturated with water. I did add water into the pit about 5 hours after the burn, through the top clay, and then covered it back up. It was pretty awesome because the clay soil sealed it so well that we had a steam vent coming out of the top for a while, like a pressure cooker.

Lessons learned:
1) deep pit is better. I don't think it needs to be parabolic at all. The material is so light that getting it out even of a deep pit is no big deal. I dug an additional 5" out, and may go down further.
2) top burn with leaves was the most effective use of time. The center coals were unbelievably hot, and I could have used larger wood in the pit if needed as long as the top burn was retained. The leaves were much more effective at removing oxygen than even small twigs, and retained heat in the pit exceptionally.
3) I would never do this with any significant wind. We had minimal wind, but the heat coming off made it uncomfortable within 10'.

When burning in a pit has anyone encountered a danger with igniting tree roots which continue burning underground and potentially starting an escaped fire?  In the woods where I live there are many tree roots buried at various levels no matter how far I dig down.  I've usually done my burn piles above ground and end up with a cone like a volcano. I can get char to form if I douse out the fire early and even then the pile might continue burning in the middle another 24 hours or more if not stirred.  It has re-ignited on occasion.  It's hard to understand how some have put out the big pit fires with very little water.

Denise Cares wrote:When burning in a pit has anyone encountered a danger with igniting tree roots which continue burning underground and potentially starting an escaped fire?  In the woods where I live there are many tree roots buried at various levels no matter how far I dig down.

Good point. I think there is a real hazard there. Personally, I would dig out an area twice the size of the finished pit and remove all organic matter/roots. I did the same with my burn pile area for safety.

Denise, my first thought was, will using these holes with tree roots damage the trees?

Anne Miller wrote:Denise, my first thought was, will using these holes with tree roots damage the trees?

Anne, as I understand from what I've read in various places, a fire above ground can catch fire to underground roots and spread the fire underground. So I gathered that a pit fire would also pose the same danger.  I think it's possible to damage or even kill a tree if enouth roots are burned up.  The most immediate danger would be an escaped fire however that would arise seemingly out of "nowhere".

I feel it has to do with how big the roots are.  Large roots the size of a finger could very well catch fire and might burn underground until the roots are exhausted and very well damage the tree.

Hair like roots might not be affected/catch fire.

It would be great if someone who has experience would speak up.

I've seen a lot of people making biochar in a trench.  They aren't very deep.  I haven't really seen deep pit biochar burning until now.  I have never heard of this problem listening to many people using the shallow trench method. My guess is that it doesn't happen a lot with the trench method, but I'm not completely sure.
John S
PDX OR

A few links with info:       https://cottagelife.com/outdoors/qa-heres-how-to-prevent-a-root-fire/
https://www.cbc.ca/news/canada/british-columbia/here-s-how-wildfires-can-burn-underground-for-months-or-even-years-1.5111276
https://www.thenewstribune.com/news/nation-world/national/article247823145.html
https://www.sfgate.com/bayarea/article/Mysterious-underground-fire-Yosemite-Stanislaus-14428686.php
https://sectionhiker.com/root-fires-and-leave-no-trace/

Thanks Denise, root fires are a real!