Hi Nancy, guilty, I live in a rural area and I have no shortness of land, so doing this is super easy space wise. That and my neighbors can't really see what I'm up to. I'm burning in a spot where I had had bank run gravel dropped off for a previous
project. Because of that there's very little organic matter in this spot. With my 5' diameter pit the soil heats up significantly and I'm sure that some number of inches into the soil it gets hot
enough to sterilize that volume. I'm fairly sure it's less than 6 inches, but pure guessing. But if you use a smaller pit, say 2' diameter, and char smaller diameter branches, say no bigger than 1.5-2", then the charring session will not take as long or generate nearly as much total heat. I'm only just planning to experiment with that this spring once my snow melts, probably next weekend. I'll try to remember to take some pictures and notes. Have you seen the pictures of farmers in Asia making Biochar in trenches directly in the garden? They char their material, then bury it under the garden soil and have been doing this for a very long time. Much less heat involved, so you don't have to worry about burning up the soil organic carbon. People make metal cone kilns that sit on top of the soil, essentially functioning like a pit, but probably heating the soil significantly less. Also, integrating a fire pit into the landscape can be beautiful, fun and functional at converting prunings and dropped branches into a nice evening by the fire while making Biochar for your
compost and sheet mulches. The only concern to me is that of making sure that your community doesn't forbid firepits.
-Art Donnelly
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