We haven’t quite figured it out yet
JMG and I were both too optimistic. We both thought charcoal agriculture was ready to play a limited but real role in controlling global warming. Burn some high carbon biomass, turning it into charcoal that will stay stable for thousands of years; add it to soil, which builds tilth and structure; you have just sequestered some carbon and improved agriculture at the same time.
We know it can be done. Pre-Columbian Indians covered much of Brazil with terra preta (black earth) built up through “slash-and-char” agriculture over thousands of years. Terra preta is not just dead, well-structured soil. It hosts a complex ecology of living organisms that help maintain soil ions and PH — one of the most amazing agricultural environments ever created.
Unfortunately, it turns out we don’t know how to duplicate it yet. Applying pyrolysis to biomass and then mixing it with compost turns out not produce terra preta. According to Scientific American, modern attempts increase yields at first, but they drop after three or four harvests. So while it is worth more research, terra preta is not a solution for today.