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pH and TDS

This is a discussion on pH and TDS within the General Orchid Culture forums, part of the Orchid Culture category; Great info, thanks, Julie. Looking forward to your mix runoff results too, Gin. question: Is ...

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  1. #11
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    Great info, thanks, Julie. Looking forward to your mix runoff results too, Gin.
    question: Is there a correlation between pH and TDS? I mean, could high pH be indicative of higher TDS, or of some components of TDS such as sodium? I see how dissolved nutrients would affect pH, but would TDS?

    Tmai

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    Hi Tami,

    Without digging around in some chemistry books, I'm going to hazard an answer...

    pH is influenced by the hydrogen ion activity relative to hydroxide ion activity they're oppositely charged. When it's hydrogen ion activity is high, a solution is acidic (low pH). When hydroxide ion activity is high, a solution is basic or alkaloid (high pH).

    I don't think those ions are large enough to be measurable as solids in a TDS reading. Other charged particles (anions and cations), though, are large enough and can influence TDS. But pH can effect when certain chemicals dissolve or precipitate out of a liquid, so there is an effect.

    Whether or not larger charged particles can influence pH, I'm not sure, but I don't think so.

    Julie

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    Hi Julie,

    Thanks for yet another great article. Anyway, you didn't say what the best orchid pH should be. Is it between 6.5-7.5????

    To try and answer Tmai's question (though not totally) pure water has an equal amount of Hydrogen ions (the ions that make stuff acidic) and hydroxide ions (the ions that make stuff basic). So, once fertilizers start getting added, like Uncasteeb mentioned, the pH can go wild real easy. It will probably go up or down. It all depends on the pH of your fertilizer. If you are adding a fertilizer with a pH of 8 to pure water, then the overall pH will be 8(I think). And vise versa.
    Hope I helped!

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    Most orchids like to grow in a pH of 5.0-6.0, but each genus and species has its likes based on its natural environment. Many Paphs grow on limestone cliffs, and like their pH at the more basic end of the range.

    Note that potting media also effects pH: bark and sphag are slightly acidic, rock wool is slightly basic, and Perlite is neutral. That's why I add some dolomite lime periodically to some of my Paphs. I grow in sphag (acidic) and these guys want more basic. As in alkaloid...there's nothing basic about pH and fert!

    Julie

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    Ummmmm.... not sure I'm glad I asked. However, how would (or would you?) adjust pH if you were to add fert to pure water? Being that you add fert (going with the weekly/weakly camp) in between plain water flushing, would it even matter?

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    Sorry, Tami! I got sort of focused on the technical answer and not the practical one.

    I know dolomite lime not only raises pH, but it also adds Ca and Mag trace elements, so I use that with some of my limestone cliff Paphs. Some ferts are intentionally acidic, like Miracid. But I don't use them with orchids, so I couldn't guide you.

    I'm only stepping up to measuring pH and TDS myself, which is why I was compiling all the factoids. I'm no pro at it and welcome those more experienced to chime in.

    Julie

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