Wood (beetle-kill in BAS's case) that has been pyrolyzed — burned in the absence of oxygen — so that it turns into high-carbon charcoal with a porous structure that can hold nutrients and microbes. Raw biochar will steal nutrients from soil like a sponge; it must be pre-inoculated before use.
Jeremy adds 2–10% by volume; 2 gallons in a 70-gallon mix is about 3%.
Discussed in: Ep 003