I may have them mixed up. Which ever is which, the ashes go on when bloom starts.
Biochar is good, terra preta is better. Charcoal makes a great habitat for the beneficial soil microorganisms. You usually just work the biochar into the soil, the terra preta you compost first and it acts like biochar on steroids.
They tend to downplay the living aspect of terra preta these days. You make the charcoal for that at a lower temperature... out of hardwood only, and mix it with your compost. There are inoculations you can buy, or just use leaf mold from a local forest, but once it gets going it grows on it's own (although very slowly). There are deposits 6 to 9 feet deep. Natives with stone tools didn't till it in that deep it grew down from a garden plot in the 2,000 to 2,500 years since that native culture was making it.
Here is something else on soil microorganisms that is pretty important. I buy spore from Paul Stamets anyway, I'm going to get some of the variety for that bed the bees like so much.