RO water acidity question
 

So my boyfriend has a RO system that he uses for his reef tanks and I use it too to water my orchids. He brought to my attention that the water that the RO makes ends up being kind of acidic after it's ran through the system. the ph is about 5 when it's all said and done. Is this going to be damaging to my plants?

In my opinion it would probably be better if the plants were watered with something between 6.0 and 6.5. Strange that his ph level's at 5.0. I use an RO system too, and it's usually right around the 6.5 to 6.7 mark (or maybe my RO's the one that's screwy!). I think a ph of 5 is pretty low, especially if you pot with sphagnum, which is already acidic.

Something sounds strange. I think your pH sounds right and his far too low. If my memory serves me, the pH of pure water having reacted with the atmospheric pressure of carbon dioxide in the air is approximately 6.2. Has he changed the filters and RO membrane on his unit recently?

The equilibrium pH for an aqueous carbonic acid solution at normal atmospheric carbon dioxide concentrations is about 5.7. Differences from that are likely to be due to contamination or simply a meter calibration error.

In any case, it is such a weak acid that it won't harm your plants.

However, continuing use of pure water (without fertilizer addition) will literally draw nutrient ions from the plant, so weaken it, long-term.

Also, if you use RO - or any pure water supply, for that matter - DO NOT use ordinary fertilizer with it. Having no dissolved minerals to provide buffering capacity, the addition of the mostly acid-forming minerals in most fertilizers will cause the pH to plummet to dangerously-low and damaging levels. Either adjust the pH back up, or use a fertilizer intended for pure water so you don't need to.

Hi Ray,

I have been using pure bottled water for a year now, demineralised, and once in a while i add regular fertiliser to the mist reservoir, only very little maybe once am onth, i barely fertilise at all.

my plants are growing fine and flowering.

What do you recommend to use as a fertilizer for pure water then? im in europe

MSU for RO/Pure water is good. If I want to alternate with another fert, I use Dynagro, with a little Protekt which helps to buffer PH.

I water my orchids (these are grown under lights indoors and in vanda baskets or on mounts) with RO water about every other day. I also use MSU for RO weekly, after soaking the plants.
I have had good results with this, but remember, the methods that one uses are dependent upon the specifics of how one grows as well as the orchids one grows.
This includes temperature variances, humidity levels, amount, intensity and timing of lighting, type of potting and potting mix, type and purity of water, even temperature of the water, etc. etc. etc.
MANY variables to be concerned with in comparison of one growers results with another!

Correct me if i'm wrong but RO water is deminiralised water so measuring the pH is pointless.
As soon as it comes in contact with your potting medium the ions of the potting medium are dissolved in the water. That will determine the pH.
Or when you dissolve a fertilizer in the water, measure the pH after adding the fertilizer.

I used to use RO water, but I would add a little tap water so that there were some minerals.

This issue of RO water leaching nutrients from plants if appropriate fertilizer hasn't been added has me wondering: I spray my tolumnia daily with RO water, but only fertilize once a week (with a good RO fertilizer). Should I be adding weak fertilizer more often, do you think, to avoid removing more nutrients than I'm adding, week-to-week?