A reverse osmosis unit is basically a water purification system which works by forcing the chosen water supply(tap water) through a membrane, which filters out impurities like chemicals etc; the end result is clean pure inert water that typically has a pH range between 6.0- 8.0. One of the down sides is that RO units waste a lot of water in the purification process typically up to 4 gallon to one depending on the RO unit. I used to use rain water but I would always run out in the summer months, my large water storage tank would get dirty with sediment and be very difficult to keep clean. Tap water wasn’t a option for me as I like to know what my plants are recieving and give them a consistent electrical conductivity(EC) target of around 500-550 microsiemens at every watering. If I was using tap water and that tap water was 200 microsiemens then the feed content in my target EC should be 300-350 microsiemens thus not exceeding my 500-550 ms target. So by using RO you are giving more feed to your plants instead of tap water.
I have seen tap water at 625 microsiemens (400ppm), so if you are using tap water at this level and a feed to any recommendations then the TDS will more than likely be too high which can damage the roots.
When using RO or rain water with feed, special attention must be given to the pH of the finished mixture as both have no buffer, so the mix can crash resulting in a very acidic mix. Most orchid feeds designed for RO or Rain water usually result with an acceptable PH range, mainly at the lower level; but it really is wise to check the PH. When the RO membrane deteriated I found the PH gradually reduced below the standard 6.0 -8.0 range, until it was basically too acidic which drastically effected my end mixture PH; It was getting to 5.5 and just below.
So the pH will need to be monitored and adjusted if necessary with a pH up & down, a range of 5.5 -6.5 is ideal for the majority of orchids.
For more information on feeding orchids, see my other articles; for a really technical explanation of RO you can search the internet and find lots of long winded boring articles!
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