While you probably do have some biochar there, your yield will be higher if you build a biochar reactor. A 55 gallon steel drum, a 30 gallon steel drum, some stove pipe, and you’re set. The key to making biochar is to heat it to 600 degrees in an oxygen free environment. This leaves a stripped carbon skeleton hungry for nitrogen compounds to bond with. If you want more details about the design I’m referring to here, just let me know!