Many years ago I became disenchanted with bark as a potting medium. I wanted something that would hold up better, so I tried things like Turface, Aliflor and Perlite. They worked, sort of, but none gave me the overall results I was looking for. On a trip to Florida I stopped by a nursery that had all their plants in charcoal. They were overgrown had lots of weeds, and had obviously not been re-potted in several years but when I looked between the weeds I found that the orchids were growing well in spite of the obvious neglect.
I tried charcoal alone and found that in the Northeast it didn't retain enough water unless I watered at least once a day, so I tried mixtures of charcoal and sponge rock (extra coarse Perlite) and found that a mix of two parts sponge rock to one part charcoal gave me good water retention with good drainage. I eventually added a little chopped sphagnum moss to the mix and settled on five parts sponge rock, four parts charcoal and one part sphagnum moss. For me it was the holy grail.
In response to government regulations the town I lived in started adding KOH to raise the pH to ca 8.0-8.3. They never seemed to get it right, however, usually ending up around 9.3. I was concerned about this and decided to see what effect charcoal has on the pH. It raised it considerably, but my mix had a smaller effect because the sphagnum moss was quite acidic. I added a little vinegar to my water and this have me the desired pH. My plants grew well, and but I really have no proof that this was because of the pH control. It made me fell better at least.
A year ago I moved; new water supply (lower pH but 40 ppm of sodium!); lights instead of a greenhouse; RO water instead of municipal water. I saw the thread on charcoal and decided to do a few tests. I found that the conductivity increased by ca 150 5s in one hour when I added 20 grams of charcoal to 200 ml RO water, or to RO water plus 30 ppm Calcium (as the nitrate) or to a fertilizer (Peters 15-16-17) solution containing ca 150 ppm nitrogen. The pH of the RO water jumped to 8.4, that of the calcium nitrate solution to 8.0 (from 5.6), and the fertilizer solution to 7.0 (from 5.3). I suspect that the charcoal has a significant amount of residual potash which is extracted and raises the pH. Rembember in the old days charcoal was used a a source of lye in soap making.
In any case, my plants continue to thrive. I suspect that the lye is extracted in the first few waterings and cause no long term problem. This probably is not true in flasking media. My only problem is that with the large amount of air movement in my plant room, combined with the long days (16 hours) I am using cause the plants dry out too quickly. I am experimenting with coconut chip (with some sponge rock and some charcoal). So far no problems but too soon to be sure.
Anyway, far from adsorbing things, the horticultural charcoal I use seems to be a net contributor to the potting medium...at least initially.
Martin _______________________________________________ the OrchidGuide Digest (OGD) [EMAIL PROTECTED] http://lists.orchidguide.com/mailman/listinfo/orchids
