Charcoal comes out of the kiln in different sizes, many of which are too big for amending garden soil. (There does not seem to be a too-small size, considering that the particles in terra preta had to be identified with a microscope.) A cornerstone of my biochar practice has been to reduce the charcoal down to 1/4” or less, according to instructions from Paul Taylor. There is a theoretical maximum surface-to-volume ratio at which the char still allows water, nutrients and microbes to fully permeate it, and 1/4” is the number Paul gave me.
The grinder that I designed and built in 2018 was effective at reducing char to this size with only a hand crank, well suited to life in an off-grid neighborhood with limited power.
Decking screws inserted in a lathed 2-1/4” dowel are backed out to rip the char, just clearing the 1/4’ gap. Works in both directions. Hopper is oak plywood; wheel came from a meat grinder.
I would have liked to find a method of grinding that didn’t require hours to build, but the solutions I saw online tended toward inefficiency, unsustainability and health threats. Mine yields a cubic yard in about three hours and propels the toxic dust into the bucket through the lid to which it’s attached.
The wooden parts gradually wore down. I was prepared to replace them this year, but two new factors had come into play. In 2021 we upgraded our solar system and could now consider an electric grinder. Secondly, we had found 1/4” biochar too large for our liking in the garden.
On a listserv to which I subscribe, someone recently claimed success at grinding biochar with an ordinary electric leaf shredder. These devices may not shred leaves very well, he said, but as a biochar grinder, the one he used was fast and efficient. I had to know!
With the loan of a 14-amp chipper/shredder from a friend, I was grinding char down to about 2mm or less about twice as fast as my wooden grinder. Prior experience with an 8-HP gas-powered shredder had taught me to aim for 50% moisture by testing the char with a hand-held probe (wetter material clogs the machine; dryer char makes too much dust).
At 50% moisture, this 14A Portland 1-1/2” capacity chipper/shredder from Harbor Freight can make finely ground biochar all day. Wheelbarrow contains 1/4” charcoal made with hand-powered grinder.
The moisture meter I use, General MMD4E, is the one Kelpie Wilson uses to test whether brush and wood is dry enough to pyrolyze without making too much smoke. It measures the electrical resistance between two sharp probes, which are designed to be inserted into wood, but which seem to work just as well with a pile of biochar.
Making biochar in the woods can be an alternative to chipping, incineration, and lop-and-scatter. It will probably generate more biochar than you can use in your garden. The remainder can be flung around for the benefit of the forest without grinding or inoculation, because it will gradually be pulverized and charged as it’s absorbed by the soil along with the other litter.