Interestingly enough I've used a kind of biochar in the past (and still do). I don't use regular bone meal. I use bone char. It is a slower source of phosphate than most fertilizers and doesn't leach out as much as most phosphate fertilizers do. When I tested my soil when the garden was first put in, it was very low in phosphates. I wanted to be able to put something on the garden that would last and not leach out to contaminate our local water sources. But I wanted the plants to have plenty of phosphorus. Bone char fills both needs.