Charcoal in the planting mix oh my! Guys this is realy a great idea in a whole bunch of ways, but won't absorb salt from solution. Primitive man used slash and burn to not only clear the land but improve the soil. When we put "biochar" int the soil it does wonders. It provides many benefits, check out the biochar oven on Y tube, can't remember the links for it and the paper on Biochar, but it was truly impressive. Folks in NorCal are using it in their grows and are into it big.