cure your rain water with fertilizer - continuous feeding
Working in the chemical industries I usually do not comment on water quality issues because the interactions between pH, buffer capacity & dissolved solids is difficult to explain in a way that people can make useful decisions around. When the total dissolved solids are low, then the buffer capacity of the water will be low, and pH is a trivial issue. Pure distilled, DI or RO water can have wild pH readings, but because the buffer capacity is so low, the instant it touches your potting media, the pH will adjust to something civil, and the orchid's own roots will be able to buffer the pH to an ideal range easily. Yes, when healthy, orchid roots excrete buffers to moderate the pH in their immediate root zone. So with very pure water, don't bother even testing the pH, the plant will be able to take care of it.
. Rain water - in theory should be pure, but pollution, especially the various nitrous oxides and sufur oxides dissolve into the water from the air and can buffer the water to a highly acidic pH. Depending on how heavy the smog, the amount of dissolved oxides can become more than the plants roots can buffer out - hence 'Acid Rain'. My suggestion would be to dose this water with a roughly 150 to 250 ppm level of nitrogen, of your favorite fertilizer that you would normally use for continuous feeding of your orchids, l use MSU-Orchid Special RO formula, but Dyna-Grow and others make useful products also. The MSU RO formula in particular has both Calcium, Magnesium and some acetates which make for excellent buffer capacity. The fertilizer will buffer the water into an acceptable pH range for rain water in most areas except maybe the smoggiest days in Los Angeles.
. What about leachates dissolved into your rain water from leaves, pollen, seeds and other debris. Generally most organic leachates, tend to run in the moderately acidic, but acceptable range for most orchids. These do have a medium to high buffer capacity. Oak trees contain tannins, which will buffer the water to pHs that are acceptable to your Cyp pubescens, Goodyeara and Platanthera ranges, down around 4.5 to 5.5, not bad unless your are watering Calciphiles like bellatulum. But if you have a little oystershell or lime in your potting mix for your bellatulum, that should take care of the pH issue. The issue would be concentration. If there is a faily small amount of debris in your rain barrel, I wouldn't worry about it, maybe add a little MSU and go ahead and use it. If the rain barrel is half full of oak leaves - well then, you might have a significant concentration - here I would just dump it out and start over.
So all in all, my summary would be to just add a little fertilizer to every watering, and throw out your pH meter. Your orchids will grow better and you will be less worried. This may sound a bit flip, but I'm thinking if the rain isn't killing your lawn or your trees, it can't be 'that' bad. A little fertilizer is an excellent buffer, it will adjust your water to an acceptable range for your plants. There is no need to make orchid growing overly complicated. There are many benefits to continuous feeding and water conditioning is one benefit.
