In my standards of practice here at the nursery, I do not consider Biochar and Horticultural Charcoal to be the same thing.
Horticultural Charcoal to me is just Charcoal produced from burning pure hardwoods without lighter fluid, chemicals and is considered safe for plants.
Biochar is charcoal that’s been enriched and “charged” with nutrients.
Here at the nursery, I’ve built a kiln that cooks discarded hardwood delivered by local arborists. After cooking and creating charcoal, I then place the finished product into a tub that filters water from my fish pond through the coals for a few days, enriching it with nutrients from fish waste and excess nutrients present in the pond. After a few days, the nutrients have bonded with the molecules present in the charcoal in a stable format that makes nutrients available to plants but also to beneficial microbes and mycorrhiza.
These are 1 gallon bags of this finished product. It can be used in small amounts in your potting mix for all plants as well as in your outdoor gardens.
Biochar is an ancient practice and is remarkable due to the molecular structure of the charcoal itself.
Here’s a few interesting links if you want to join me in the obsession!