# Iron sensitivity?



## loneroc (Apr 4, 2019)

Howdy all, I occasionally use an iron supplement on my plants, including orchids, to treat cases of chlorosis. A while back I ran accross info (somewhere) indicating that some paphs are sensitive to iron. Is this true? Which species run the risk of iron toxicity? Thanks!
Steve H


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## Ray (Apr 5, 2019)

I cannot answer your question, but have two for you:

What makes you think you're dealing with an iron deficiency? Chlorosis can have many causes, including some that are not nutritionally-based.

Why not just use a decent fertilizer that contains all of the needed minors and eliminate nutrition as a potential cause? As far as iron is concerned, anywhere of 0.1-0.2% seems to be the norm.


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## Stone (Apr 9, 2019)

loneroc said:


> Howdy all, I occasionally use an iron supplement on my plants, including orchids, to treat cases of chlorosis. A while back I ran accross info (somewhere) indicating that some paphs are sensitive to iron. Is this true? Which species run the risk of iron toxicity? Thanks!
> Steve H


Xavier reported Fe sensitivity in P sanderiana (and maybe others) plants a while back. If you are going to see Fe sensitivity/toxicity, I would imagine you would probably see it first (if at all) in Paphs from limestone habitats.
The availability (solubility) of Fe is intimately correlated to pH. Including some form of calcium carbonate in the growing media should see off any Fe toxicity problems as it will help in neutralizing organic and other acids to some degree. However I think incorporating the limestone, shellgrit or similar into the mix is probably better than top dressing. You might also consider using FeSo4 rather than chelated Iron in your fertilizer. Having said that I use both together and have yet to notice a problem. Remember also that chlorosis in the NEW leaves can be caused by other metals besides Iron. Chlorosis in the OLDER leaves is more likely to be a lack of N or Mg. Good roots is your number one priority to counter deficiencies!


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## loneroc (Apr 9, 2019)

Thanks to you both. I should fertilize the orchids more consistently. I have a couple of hundred of them, scattered throughout a greenhouse, and under HID lights in my basement. Many are potted, others are slabbed, or growing on trees, or growing directly in the ground. (I'm starting to plant some species paphs on a couple decaying mossy logs I brought into the greenhouse from the woods.) The orchids tend to get the type of fertilizer given to their nearest non-orchidaceous neighbors.

Anyway, my specific concern with this post was a Paph. Prince Edward of York. I flowered it a couple years back. Wow. Spine tinglingly beautiful flowers, until a peeved cat chewed all the hanging petals off. I should have let him out when he asked. That'll show me. I won't use chelated iron on PEOY. I've given the Prince a toot of Epsom salts and urea. In a day it's become significantly greener. Is it just me or do paphs prefer some urea in their fertilizer? That's the subject of a different post.

Thanks for your thoughts.
Steve H in USDA zone 3, SW Wisconsin USA, frost free greenhouse


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## Ozpaph (Apr 9, 2019)

do orchids utilise urea as a source of nitrogen?


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## loneroc (Apr 9, 2019)

Orchids aren't supposed to use urea directly. Microorganisms need to break it down, yadda yadda etc. My own experience is that orchids seem to take it up fast, way too fast for the urea to have been broken down in the medium. But maybe bacteria work on the urea faster than I know. Sort of like yeast in dough. Steve H


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## Ray (Apr 10, 2019)

Orchids can, indeed, use nitrogen from all three chemical sources, but it's a matter of how and to what degree.

For nitrates and ammoniums, root absorption is the preferred route. Foliar absorption is also possible, but to a far lesser extent. The converse is true of urea - the preferred route is foliar, with root uptake less so.

Of course, there are some other factors to consider: the cutex on orchid leaves can be quite developed, and while intended to retard water loss, it is pretty effective at preventing absorption, too. Plus, as was pointed out, bacteria in the rhizosphere do break urea into ammonia pretty readily.


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## loneroc (Apr 10, 2019)

Ray said:


> Orchids can, indeed, use nitrogen from all three chemical sources, but it's a matter of how and to what degree.
> 
> For nitrates and ammoniums, root absorption is the preferred route. Foliar absorption is also possible, but to a far lesser extent. The converse is true of urea - the preferred route is foliar, with root uptake less so.
> 
> Of course, there are some other factors to consider: the cutex on orchid leaves can be quite developed, and while intended to retard water loss, it is pretty effective at preventing absorption, too. Plus, as was pointed out, bacteria in the rhizsphere do break urea into ammonia pretty readily.


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## Ozpaph (Apr 12, 2019)

thank-you


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