Biochar is really interesting stuff, I've been reading a lot about it lately and will be including it in my next batch of living soil. It has a very high CEC- cation exchange capacity. Other people can better tell you about it's uses, just be sure you account for it's high PH in your soil mixes and it's good to "charge" it either by soaking it in something like fish hydrolysate for a few days or just by adding it to your soil before you "cook" it. It's supposed to house a tremendous population of rhizobacteria.