Water type and P/H for my Orchids

[Nexogen]

Originally Posted by Ray

You'll note that both of those links refer to soil pH.

In soil, the pH can effect the cation exchange properties of the clays and fine organics - causing them to trap or release the various nutrient ions, hence their availability to the plants.

As orchid media have little-to-no CEC, if it's in solution, it's available.

Cation Exchange Capacity refers to the media’s ability to hold nutrients having a positive charge. Soilless media have low nutrient-holding capacities, because of this; nutrients for plant growth should be supplied constantly. Theoretically Hydroponics(?!) or S/H is the best method for growing orchids.

[naoki]

Nexogen, that's what Ray is saying. Those links you provided recommend certain pH for soil based culture.

In soil with high CEC, pH influences the balance of nutrient availability (e.g. N:P:K:Ca:Mg ratio). So people think pH around 6.5 is best (based on soil based argument). Basically, this is the effect of pH on retention of nutrients in the soil. But this doesn't quite apply to epiphytes.

The other aspect of pH is its effect on nutrient uptake at the epidermis cells of roots. pH influences which ions are easier to absorb. I believe that ease of absorption of NO3 vs NH4 is one example.

I haven't seen scientific papers discussing the effect of pH on nutrient uptake in epiphytic orchids. But Ray's point is that the optimum pH is different from the results based on soil-based experiments (where pH influence both nutrient retention and uptake).

[Nexogen]

Yes, I know.
To maximize plant growth, it is essential to achieve a pH which will optimize the availability of all essential elements. Optimum (5.2 – 5.5)pH. Determining the pH of soilless growing media is not easy, most soilless media are acid. Nutrient availability is determined by the pH of a growing media.
The type of media, weather, plants, age of roots all effect pH. When you are growing S/H adjusting pH is simple.

[Ray]

Another "confuser" in this discussion is the effect of the plants themselves on the pH, and as Naoki mentioned, the nutrient ions play a role.

When a plant absorbs ions, it has to emit an equal net charge of ions to compensate. Absorb a nitrate (NO3-), and either OH- or HC03- is emitted, increasing the pH in the rhizosphere. Absorb an ammonium ion (NH4+), and H+ is emitted to compensate, lowering the pH.

That was a total surprise to me when I started testing the pH of the reservoir in S/H pots years ago, where even though the solution pH was adjusted to the same level before application, within 24 hours it would vary between formulas, and time of day the measurement was taken.

[The Orchidist]

I must say this is becoming a very informative thread I have created and learning lots ...

[naoki]

It's interesting to hear that pH changed depending on the time of the day. By any chance, did you see that pH goes low at night for CAM, but pH goes low at day for C3? In inorganic media, I wonder pH fluctuation could be a good indicator of root activity.

As Ray pointed out, influence of roots on pH is not negligible. People used to think that break-down (decomposition) of media (e.g. sphag moss) causes the lower pH over time. But it appears that the roots of the plants are the main cause of lower pH in older media. Therefore, the pH of synthetic media (which doesn't decay) can go down over time. Here is an experiment to show this:
The Acidification of Sphagnum Moss Substrate during Phalaenopsis Cultivation

[Ray]

At the time I was testing, I don't know that I was even aware of stuff like CAM and C3....

May have to resurrect that.

[bethmarie]

I've read this thread with interest. And a mild headache. I have to leave cation exchange and C3 alone for now. Maybe forever.

I am curious how RO water can have a high pH? I thought the process of reverse osmosis would inevitably produce fairly acidic water.

[Ray]

Ar RO system removes a percentage of the dissolved solids. That percentage can be between 92-99% in a new membrane, depending upon the brand and design, and can deteriorate with time.

If the feed water contaminants are alkaline to begin with, and especially if they are high in concentration, the tiny bit getting through can shift the water to a higher pH.

Personally, I have never seen it go anywhere but acidic, due to the absorption of CO2 from the air forming carbonic acid, but anything is possible.