Cone shaped auger for biochar pit?

This post is more musing about the possible rather than anything more than that. Just some things I have been noodling about the last few weeks and figured I'd throw it out there into the world and see what conversation they get started. I have neither the equipment or the land to put these ideas into practice, but anyway...

You often hear of making biochar in cone-shaped pits. Digging such a pit is either quite labor intensive by hand or a bit tricky with most equipment. I came across mention cone-shaped augers for skid steers, mostly for tree planting. These seem to get as large as 48" wide.  (Link to an old Farm Show entry about one of these).



Would something like this work for creating biochar pits (fairly) easily?

Digging one pit in this manner would be a time saver, for sure, but it raises another idea.

What if you dug an entire series of cone-shaped holes, and did a biochar burn in each one? After the burn and quench, perhaps the char gets inoculated in place with nutrients, and then the holes are simply filled in and capped off with soil and/or compost.

This isn't something you'd want to do on good farmland or in your garden. The ideal would be to mix the char into soil. But if you put that aside and just had the goal to get as much char into the ground for carbon sequestration, you could do this at something like scale.

You'd ideally want some marginal land that was near enough to a plentiful supply of biomass suitable for pit burning. You could then drill a bunch of cone pit holes spaced fairly close together to do the burns in.

A half acre is 21,780 square feet. If you made that into 10' x 10' squares and put one 48" pit in each one, you could put over 200 holes into that half acre. That's a lot of carbon sequestered in a fairly small area of land.

Anyway, I have no idea of any of this is useful, practical, etc. I get that it's not ideal in several ways...but it presents an interesting potential way to sequester a large amount of carbon in small area with a lot less digging.

Anyone got a skid steer?

I love it. The only hangup I would have is that so many of the places where I go around here are on rocky soils (on our section, which used to be a riverbed, your odds of hitting something at least the size of a softball are close to 100% and I have had to realign fences as they were going in because I struck layers of watermelon-grade boulders just below the surface). But anywhere that doesn't have this problem is a perfect site for one of these augers...I'm going to start sniffing around the farm equipment traders.

I like the idea, but if the plan is to burn directly in the drilled hole, there are additional things to consider.

That drill would create a 45-degree wall, perhaps less. I'm not sure you're going to get a proper burn that fully cooks the char unless you somehow add additional air or devise a means to stir the coals from bottom to top. You could do a burn above and dump finished coals into the hole I suppose.

Long-term wildfire / ground fire hazard: this is a significant worry. Are there tree roots or any signs of peat in the dig zone? In the right conditions, these act as fuses or repositories of fires that will flare up later. And bite you in the hindquarters. Be very damn careful of this.

In dry areas, this kind of sequestered charcoal could be an issue, but given the ability that charcoal has for sucking up moisture,  any play with moderate or better rainfall should be pretty safe.

Pits for biochar needn't be cone shaped.
A series of trenches would work and they might be easier to create, by hand or by machine.

The fire risk is in the direct burn in situ. Roots are fuses, especially evergreens, and they can run hot through a long winter. This is well known to wilderness firefighters.

Related example: I have done char burns in trenches at -30C with snow all around, heaped heavy snow 2 ft. on top and packed it down tight with a heavy shovel and my size 12 boots, and come back 3 days later to find it has melted through and is hot enough to make tea. Do not be fooled. My burn area has now been dug deep with all roots removed, and at least 1 ft. of pure sand below any coals.

I'll second Phil's concern about augers and rocks. There are places I can imagine this being useful, but my land isn't one of them. Our rocks are much larger than watermelons thanks to the last Glacier to visit our area.  

I'll also back up Douglass' concerns. I think small pits may be harder to be sure the char is thoroughly quenched and cool before walking away. When I tried it, I thought I'd used plenty of water, but the water all just seeped away out the sides and the char didn't cool and the dirt I covered it with wasn't deep enough. Since I also have a lot of trees, I've gone to container biochar making rather than taking the risk.

That said, I think there are lots of places where this might work. I would try, after quenching, to mix back in the soil that came out. I would try to find a safe source of manure (organically fed animals with no risk of residual long acting gick that broadleaf growies wouldn't like) to add to that mixing process. I think there are many soils that are desperate for organic matter that would benefit. Sandy soil might particularly benefit from this sort of treatment.

As soon as things were settled, I'd have a mix of seeds to add to the top. The plants may not thrive due to the disturbance, but any roots they put down will help the soil evolve to the next level. I could easily see doing something like this to the site of a future food forest, planting the trees in the center of several pits that will all be at least 5 ft away from the new tree to give the tree roots something good to aim for! Moisture held and moderated by the biochar!

I was looking at it and thinking that you could bore two cone shaped holes, say 4 feet apart.  Then it would be pretty easy to dig out the part in between them.  Of course, you all made excellent points about dry conditions, roots, and big rocks.  So that could only work under certain conditions.  I couldn't tell if digging between two cones was what William was already saying, but it might work for someone to burn there, and for others to dump it all in there after burning.

John S
PDX OR