I haven't tested the pH of a fertilizer solution over time, but usually being in solution for a week or two does not seem to show any significant negative effects on the plants themselves.
At the moment, I am not aware of whether or not a fertilizer kept in solution for about a week or two will have significant pH shifts towards acidity.
The only component of the fertilizer that I can think of that can significantly change while in solution is the nitrogen portion of the fertilizer. Nitrogenous bacteria can turn the nitrogenous compounds present in the solution into Nitrogen gas over a period of about a week or two.
The potassium and the phosphorous portions of the fertilizer, I believe, tend to be more stable in solution compared to the nitrogen. A similar thing can be said of many of the trace elements.
Think of when you cycle a fish aquarium. Upon initially adding water to the aquarium, and there are no nitrogenous bacteria present, and when you start putting in nitrogenous organic material into the water, there will be a huge initial spike in ammonia. This eventually drops over some time when nitrogenous bacteria start to colonize and metabolize the nitrogenous organics in the water. By about 1 - 2 weeks, a large proportion of the nitrogenous organics in the water will have stabilized. There will eventually be 0 ppm ammonia, 0 ppm nitrites, and maybe, depending on how big of an organic load was used to cycle the aquarium, there could be some amount of nitrates (usually about 10 ppm - 20 ppm). Over some more time, the nitrates will then be converted into nitrogen gas.
The Nitrogen Cycle is about the only thing I can think of that you might have to consider. But as long as there is soluble nitrogen, the plants, for the most part, don't really care what form of nitrogen it is. Of course, certain forms of nitrogen are more bioavailable to the plants than others, but most of the forms present in the fertilizer itself are bioavailable to the plants from the get go (nitrate is considered bioavailable to plants). In the case of urea based nitrogen, I know that there is evidence that supports that Phalaenopsis can directly absorb urea as a nitrogen source when to some other orchids urea may not be as bioavailable (
Phalaenopsis can absorb urea directly through their roots | SpringerLink).
I will also say that most orchids tend to tolerate being grown in an environment that is slightly acidic to approaching slightly alkaline in pH (about pH 6.5 - 7.9). There are a few exceptions to this, there are a few orchids that tend to favor neutral to slightly alkaline conditions, (pH 7.0 - 8.5), and will suffer if the pH drops too low. Very few orchids tolerate acidic conditions of pH 4.5 - 5.0.
---------- Post added at 07:32 PM ---------- Previous post was at 05:59 PM ----------
You might also be interested in this article pertaining to Phalaenopsis...
Effects of Nitrogen and the Various Forms of Nitrogen on Phalaenopsis Orchid—A Review
After dissolving MSU fertilizer granules...How long will it last?
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