"Jeff & Vanessa" <[EMAIL PROTECTED]> spaketh thusly: >Although my education and experience with growing orchids >is limited, I thought that most orchids were more >acidophilic in nature. I have mentioned this notion a time or >two, only to receive surprised looks and comments. As I >understand it, acidity in nature is dictated by the amount of >rain an area receives, combined with the availability of >organic/humic matter. Input?
As a very broad generality, most plants like slightly acidic conditions. The pH scale goes from 0 to 14 (and actually beyond it, but let's ignore that for now). Neutral is 7.0, and stepping by one unit in either direction means a change in magnitude of the hydrogen ion concentration by an order of magnitude (10x as much, or 1/10th as much). As pH decreases, the hydrogen ion concentration increases, and becomes more acidic. So, pH 6 has 10x the H+ concentration of pH 7. These are vast simplifications that hold true only for dilute solutions; it all falls apart under serious scrutiny, and few people really understand pH. We'll ignore all that for now since there isn't a great deal of relevance with regards to horticulture for the most part. Anyway- rainwater has a pH of ~5.6 in areas unaffected by industrialization (which tends to lower it, from sulfuric and nitric acids). Although this sounds relatively acidic, seeing how neutral is 7.0, it doesn't mean much since there's hardly any buffering capacity. In other words, a bucket of rainwater at pH 5.6 can have its pH swing dramatically with a tiny drop of acid or base. (For those interested, the pH drops as a function of dissolved carbon dioxide forming carbonic acid. A few hundred PPM of CO2 in the air drives the pH down to 5.4-5.6.) Contrast this with the substrate in which orchids are grown, virtually all of which are acidic, with certain exceptions (mainly in the genus Paphiopedilum, but also on broader geographic terms, such as lithophytic orchids from calcareous areas including Central and South America, just to throw out some examples). Mild acidity is considered beneficial for several reasons, including nutrient availability and (presumably) reducing bacterial and fungal decomposition of the substrate (which are presumably reduced as a function of all kinds of stuff in bark such as tannins that are naturally antimicrobial). However, a strong reduction of pH will cause certain nutrients to be "unavailable" in that plants will not be able to remove them from their environment. See: http://ag.arizona.edu/pubs/garden/mg/soils/images/p19large.gif Note how at pH 5.6, nutrient availability is generally quite good. Above or below this range, the availability of certain nutrients can be reduced, causing deficiencies. In particular, around pH 4.0, many nutrients "pinch out." Interestingly, some orchids seem immune to this treatment, and may in fact require such abuse. Cypripedium acaule is a classic example, having virtually eluded culture until Scott Durkee at Vermont Ladyslipper found out that watering it with feed water acidified with vinegar did the trick. It has been reported that "weird" nutrient deficiencies crop up with some orchids grown in live sphagnum, probably due to similar issues. I suspect that these deficiencies may be a function of the substrate (type, age, degree of decay), feed water (pH, hardness, and other factors), and nutrient availability (fertilizer, etc.). These are complex issues with diffuse symptomology that may not manifest until such time that it is too late to do anything about it. I've said it before, but orchids are downright weird in their nutrient requirements. I have some experimental flasks missing certain mineral components (boron, calcium, molybdenum, etc.) going on three years now, and only a handful of them are dead or dying because of such deficiencies. It's really quite amazing. Tomatoes or other plants would have expired horribly within weeks, but the orchids have toughed it out- many with virtually no growth, but they're still alive. Cheers, -AJHicks Chandler, AZ _______________________________________________ the OrchidGuide Digest (OGD) [EMAIL PROTECTED] http://lists.orchidguide.com/mailman/listinfo/orchids
