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Thread: Fertilizing a mounted orchid

  1. #11

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    I'm guessing 97.7ppm is approximate TDS (from a TDS meter), which is different from nitrogen concentration. TDS is an important factor, too, but I was talking in terms of N conc. (most agricultural/horticulural people use it as the measurement of fertilizer strengths). Ray's website has an approximate calculater (you give N-P-K, and tsp/gallon, and tells you "approximate" ppmN). So you are using it around 50ppmN every other water, which seems to be reasonable.

    If you are using tap water, there should be enough Ca (unless your water has extremely low TDS), and miracle-gro will be ok. No, ceramic won't leech sufficient Ca.

    If it isn't drying out within a day, maybe you have high humidity. Or maybe you are using a giant mound of sphagnum. So it will probably grow well.

    If you can, you should timing the water so that you water just before inside of the sphag becomes almost dry. But they can acclimate to your schedule. With mount, it is difficult to suffocate the roots, but if you keep it too moist in the pot, you can easily suffocate the roots.
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  2. #12

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    I don't have a TDS meter, but my guess is that the water can't be that soft, because Boston water isn't treated beyond ozone and UV treatment, and I imagine the reservoir already has some hardness just by being a sedimented lake. But I'm not positive how that works.

    As far as sphagnum dryness is concerned: I'm not sure what you mean by totally dry, it certainly dries out over the day but still has a characteristic dampness that's gone if you leave it over a couple of days, in which case it becomes dry enough that it's hard to rehydrate. But it might be gettin high humidity. I don't think I have an especially large amount of sphagnum, but what I do have seemed to like being humidified last night, as some of it turned bright green again (much of the sphagnum is still living because I took it live out of a sundew pot).

    Speaking of humidifying, my humidifier is shockingly effective, to the point that I woke up this morning and the room had reduced visibility! I imagine the humidity got well above 80%, so this seems like it will work until I set up a terrarium.

  3. #13

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    Oh, so you measured the weight of 1/4 tsp of fertilizer to get 97.7ppm?

    Yes, you probably don't have to worry about the 3 missing essential elements in MiracleGro: Ca, Mg, and S. If you do, you usually can get the water analysis from your water supplier. e.g., here is one from Wellesley, MA: http://www.wellesleyma.gov/Pages/FOV...B3/waterreport

    As a reference, if you use MSU at 30ppmN, you'll get about 30, 5, and 0.2 ppm of Ca, Mg and S, respectively. Wellesley water provides sufficient amount.

    You are right, there are different level of driness, and it is difficult to describe. There is a technical definition of too dry for the plant to get any moisture (based on something called water potential). But I (or most orchid growers) call it dry, when it doesn't feel moist when you touch it. When it is moist, your finger feels it to be cooler than air temp.

    But, I guess that it isn't behaving like the normal sphag since it isn't dead yet.

    Be careful with the room humidity (for your health and house).
    Last edited by naoki; 07-30-2017 at 04:35 PM.
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  4. #14

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    Thanks for the info about the water. With regards to ppm: I didn't weigh the fertilizer, but that might not be a bad idea. However, I'd have to find a way to convert to moles if I did so.

    With regards to the moss: it does feel cool to the touch after at least 18 hours, so I guess it must be a bit moist, which is what I thought. I'm definitely not using the standard stuff, I just had a bunch of sphagnum naturally growing in a sundew pot so I used it. Honestly, I'm not even 100% positive it's sphagnum; I just assumed it was since it liked being soggy, got drought-stressed super easily, and holds a huge amount of water. It also looks texturally like sphagnum. But either way, it's either a local species or, more likely, it grew from spores in the substrate, which is peat moss heavy anyway.

    With regards to the humidifier: I have it on a somewhat lower, but apparently still nearly as effective for plant health, setting. I also only have it on overnight. It feels nice for me--my house is rather dry, and some people already humidity when they visit, though perhaps not as heavily as I am. My assumption is that it won't cause rot as long as it doesn't condense on the floor, which happened at the super high setting but is not happening now. I think my orchid has already perked up, though, although I might be imagining it and it would be impossible to tell if the cause is humidity or fertilizer (or both, or neither).

  5. #15

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    ppm is weight based, and not the number of molecules. So you put 100mg of fertilizer in 1l of water, it is approximately 100ppm TDS. Strictly, you should use (1 Kg - 100mg) of water.
    Then with 30-10-10 fertilizer, 30% (w/w) is nitrogen, you'll have 30ppmN (and 10ppm of P2O5 and K2O).
    Last edited by naoki; 08-01-2017 at 08:51 AM.
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