Quote:
Originally Posted by bethmarie
The RO I use runs on the acid side to begin with. When I add fertilizer to it, it becomes acid enough to damage roots. Which it did, on my plants. I mix in about 20% tap now and check the pH if adding fertilizer. I often have to adjust the pH back up to the 6ish range, even with tap added in when I fertilize.
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The initial acidity of RO water is somewhat misleading. Almost completely due to dissolved carbon dioxide from the air, it doesn't have much tendency to resist (buffer) pH change when other compounds like fertilizer are added, and the initial apparent acidity of the RO water doesn't really affect the final pH of the fertilizer solution much. RO water used for watering by itself (not recommended for more than an occasional flush) really has almost no affect on the pH of the media except by leaching out and washing away anything it can.
It is a good practice to check and adjust the pH when adding fertilizer, and some fertilizers are much more acidic than others. One that is formulated for use with rain water or RO water shouldn't end up too acidic to use if the concentration isn't extreme. But if the concentration is in the low range usually recommended for orchids and you flush out excess salts once in a while it won't change the pH of the media much. Many organic materials in orchid media have a large capacity to buffer pH to their own natural mildly acidic state. If your fertilizer is an unusually acidic formula or meant for use with hard (alkaline) water, or you use it at high concentrations or have salts building up in the media, or your media is entirely inorganic, then pH is a much bigger concern.
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