If your orchid isn't in water culture does PH matter?

[Cymbaline]

Originally Posted by estación seca

Yes, nutrients are absorbed better at pH 6-7 than at 9. But.. Your RO is acidic because of dissolved carbon dioxide, carbonic acid. This is a very weak acid solution. When adding it to your containers the water and minerals already in the medium will probably raise the pH quickly.

If the pH were 5 because of a strong acid, like hydrochloric or sulfuric, it would be more of a problem. For your orchids I would use the RO without adjusting the pH.

MSU orchid fertilizers are formulated for tap/well water (which contains minerals) or for pure water. You could use a pure water MSU formulation with your RO.

Thank you! I do use that fertilizer currently. Not adjusting is sooo much easier. Thank you for your time answering this question

---------- Post added at 05:08 PM ---------- Previous post was at 05:06 PM ----------

Originally Posted by Ray

I think this deserves more explanation.

pH can affect nutrient uptake, but between 4.5 and 8, it is relatively unaffected. Folks often mistake the old “nutrient availability versus pH” chart to mean otherwise, but that relates to the cation exchange capacity of soil, so is not pertinent to orchids that don’t grow in soil. Basically, if it is in solution, the nutrient can be taken up.

Then there is alkalinity, referring to the resistance of pH change upon the addition of acid, which should not be confused with “alkaline pH”, that is, simply meaning that the pH is above 7. Alkalinity has a far greater impact on nutrient uptake than does pH. It is primarily due to the action of dissolved calcium-, magnesium-, and iron carbonates (and bicarbonates, to a lesser degree), and can preclude the uptake of other nutrients. I suspect your tap water has fairly high alkalinity, and that can build up in the substrate over time, making matters worse.

Another “fly in the ointment”, however, is that adjusting the pH of the solutions you apply may have very little effect on “correcting” things. The plant and the microbes living in them and the substrate have a very substantial effect on the pH in the rhizosphere, and for folks using pure, low-alkalinity water and the low levels of nutrient additives orchid require, pH just isn’t very important, within reason. 5.5-6.5 is the normal target, but even a full unit on either side is OK.

If, on the other hand, you adjust the pH of a high-alkalinity water source, you have done nothing to remove those carbonates, to the nutrient pitfalls I mentioned earlier still apply.

This is great information! Thanks for this. I appreciate all of the info from both of you. I absolutely love the knowledge this place has. Thanks for schooling me.