Vanessa wrote: >Although my education and experience with growing orchids is limited, I >thought that most orchids were more acidophilic in nature. [...] > 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?
Two questions here. First what do orchids like and second, what makes a rooting medium acid or alkali. Let's take those in reverse order. A rooting medium has two relevant properties: its buffering capacity (which means how much "acidity" it can hold) and its pH, which is a measure of how acid or alkaline a thing is. Low pH is acid - say a figure of 6 - high is alkaline - say, 8 - and neutral is at a measurement of seven. Soils seldom go above 8.5 but frequently fall to 3 in old, rain washed media: white sands in the high latitudes, the faded brick-coloured soils that you find in the tropics. Acidity and alkalinity are measures of ion balance, where an ion is an atom or group of atoms which have either gained or lost an electron, so acquiring a net charge. The buffering capacity of a medium is the measure of how much charge it can hold. Thus one may have a neutral soil with a low buffering capacity that is turned extremely acid by a single event, or one may have a soil that withstand centuries of acid rain without changing its pH at all. One crucial feature of soil pH is that some essential nutrients become insoluble at a given pH. Iron, for example, is really only mobile in acid soils. However, this mobility means that some acid soils with low exchange capacities have lost most of their essential minerals. Iron, for example, is washed out of tropical soils to form "laterite", a concrete-like red crust a few feet down. Now, to orchids. Epiphytes are primarily affected by the thing on which they are rooted - perhaps a tree. The tree and organisms in the forest struggle mightily to attain a neutral status in whatever soil they happen to be rooted, and the litter that the orchid "sees" reflects this. A typical litter patch will have a pH between 6.3 and 6.7, chiefly as a result of organic acids released by decay. A cup of black tea has much the same chemical characteristics. Why soil type affects epiphytes is much more to do with how the soil affects tree cover than how it affects epiphytes on those trees. Really good soil produces fine stands of trees, but the orchids on these are confined to the fast-growing upper branched, and are usually small in size and repetitive in terms of the species involved. Further, such soil attracts humans, who cut the lot down. Bad soil and awkward topography, by contrast, create straggling open and slow growing forest, which is orchid heaven. Branches stay in the sun for a long time. Leaf cover is not too dense. The phenotype of an given tree species tend to be mixed - that is, how it turns out from the same set of genes tend to vary - and so there are many kinds of niche. Many species can occupy these niches. The fertile soil forest have huge numbers of tree species, but all of them tend to grow into a single form, despite their genetic diversity, thus reversing this position in respect of niches. Lithophytes (rock dwellers) and terrestrials are, of course, directly exposed to soil type. Some like acid soils because they have specialised in finding nutrients in ways that coarser species are not. They can co-exist with (for example) grasses without being smothered because the grass is dwarfed by the lack of nutrients. Cymbidium hartinahianum is an example of this group. Other - a much larger group - survive alkaline environments, where the nutrients exist but are locked away in chemical forms that other plants cannot exploit. A subset of the Paphiopedilums live this way, often as semi- or absolute lithophytes stuck to limestone with a minimum of litter around their roots. It is questionable whether they actually need this alkalinity, or whether they simply can survive it when others cannot. My own take on this is that in difficult situations, they need to co-exist with fungi that break down the minerals in exchange for sugars and other products of photosynthesis provided by the plant. In easy conditions - in your glasshouse, perhaps - they do not need the fungus. However, they then become extremely vulnerable to any drift from neutral conditions. Therefore, potting them in media which require them to keep their fungus and which buffer the pH in a predictable way removes this vulnerability. My approach is to put limestone at the bottom of the pot and neutral compost on the top, and let them root where they want. I hope helpful: certainly, too long. _____________________________________ Oliver Sparrow Tel: UK (0)20 7736 9716 www.chforum.org www.treknepal.org www.datafreeze.com
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