JamGrower,
Welcome to the forum glad to hear from you! Yes it'd be a big mistake for someone to buy charcoal used for barbecues thinking they could break it up and spread it on their garden, and of course charcoal strait from a fire hasn't been washed or treated like quality horticultural biochar has. As for the nutrient leaching potential of untreated charcoal I'd be somewhat inclined to believe it to be possible, (I've had it bleach the color out of my clothes before so perhaps there could be a similar interaction taking place in the soil). In the particular instance of sempervivum (a plant which prefers a lean soil) that might not be the end of the world though, especially when adding other fertilizers to the substrate or using commercially produced potting mixes which contain slow release fertilizers. The question that follows would be WHICH sort of nutrient(s) would it be leaching from the medium? An answer to that would illustrate it's value or lack thereof in a succulent soil mixture. I do not use it in my bonsai soil mixes. What I can say about charcoal (untreated biochar) is this; plants roots grab hold of it's semi porous surface which likely has moisture retention/drainage properties similar to those of pumice or lava rock, it helps to loosen the soil and provides what is initially a bacteria free pseudo organic amendment to the substrate. What I suspect is that it (and ash) makes more readily and more cleanly available those minerals and nutrients found in the more microbial rich composts I've learned to avoid, but I have no proof of this theory, just a hunch. After years of mixing ash and fire-coals into my succulent substrates at a ratio of 1/9, I can verify that it doesn't kill them and that my mix outperforms potting soil straight from the bag, though the ash and coal may have less to do with that performance than my other selected amendments. I wish I could see what was happening in the soil at a molecular level, but all of my work has instead been trial and error, perhaps I can set up a test where I leave the ash and charcoal out of the mix to see if the plants prefer it that way. I'd love to use the biochar that has been treated and washed but it's brutally expensive and difficult to come by in large enough quantities to suit my applications, the reason I say charcoal instead of biochar is because I primarily use the raw waste from my friends wood furnace, not exactly the good stuff but there's certainly enough of it to go around.
I've never tried LECA but I hear good things, might be worth trying someday, I have a source for pumice right up the street which is quite affordable so I've never had to find a substitute but I do like to try new ingredients.
Cheers,
-Sol
