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Biochar (for soil amendment) originates as charcoal that is subsequently ‘charged’ with soil microbiology. Charcoal is the remaining carbon content of woody materials after ‘cooking’ these materials in low oxygen conditions – a process known as pyrolysis. Charging charcoal with soil microbiology involves mixing the charcoal (20 to 25% by volume) with materials destined for composting and co-composting the mix.

For details see this website.