Geoff Moxon explains Biochar and Terra Preta

Biochar is made by heating organic waste like wood and plant clippings to high heat in the absence of air. This is also known as pyrolysis. The volatile components escape as gases and can be burned to produce the energy to keep the pyrolysis going. There is also an energy benefit, 20 percent more energy is produced than the process requires, so this can be used for powering plant and machinery etc.