The comments on pH and acidity by AJ Hicks and Oliver Sparrow may prove useful to orchid newcomers like Virginia, but both commentaries ignored the principal pH factor for orchids, namely FERTILIZER. All commercial fertilizers I have tested are more or less acidic to compensate for the mild alkalinity of almost all tap and well water, usually due to the buffer BICARBONATE. Here in South Florida our tap water runs about 8.6, which frightens many newcomers, but is lowered into the optimal range by 1/2 TBL/Gal fertilizer.
What is the optimal range? Most authorities agree this should be in the slightly acidic range for the reasons and data referred to by Hicks [see http://ag.arizona.edu/pubs/garden/mg/soils/images/p19large.gif ]. As an experimentalist I recognize the prodigious amount of experimental data required to construct such a table and, at the most, it applies to one plant species and one substrate. It may contain arbitrary shortcuts since it differs significantly from another table in the same format [see http://www.merklesorchids.com/nutrient.html ]. We don't know how valid it is to extrapolate from these tables to orchids under our individual growing conditions, but I doubt that the poor availability of Phosphorous and Manganese shown in the Hicks table applies at neutrality, i.e., pH 7. From the second, alternate table shown above, all nutrients measured remain available over a broad range from pH 5.5 to 7.5, with an optimum from 6.5 to 7.5 The effect of the natural buffer in our tap water, bicarbonate, on the pH of applied fertilizer solutions is shown by 1/2 Tbl/Gal Excel Cal-Mag [15-5-15, 5%, Calcium, 2% Magnesium] producing a pH of 3.8 in distilled water, but 5.4 in Miami tap water. This would be raised slightly into the optimum range by interaction with, and dilution by, the medium. This also calls attention to the hazard of using the relatively acidic fertilizer, Excel Cal-Mag, with unbuffered water such as rain water, RO water, or relatively pure surface water delivered by some municipalities. If a newcomer avoids this unfavorable combination, all the technical aspects of optimal pH tend to cancel out and one is freed to agonize over other cultural considerations. Bert Pressman
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