Fertilizer mixes (N-P-K amounts)

[Graham]

I've read that Fertilizer uses a compound mixture of Nitrogen (N), Phosphorous (P) and Potassium (K) and is displayed, on containers, in relation to this N-P-K ratio...

For example: 30-10-10 = a mixture containing 30% (N), 10% (P), and 10% (K)

I found a formula for creating your own Fertilizer mixture at OrchidUSA.com and there's a few bit I don't understand... It's at number 6.7 at the bottom of this web page:

Comprehensive Culture Guide—Fertilizing orchids

Here are my queries:

1. While I've read that 125ppm* is a desired concentration level for Nitrogen (in 1 gallon of water), I can't seem to find any reference to the ppm amount of either Phosphorous or Potassium

* Jan Szyren, guest=speaker at an Atlanta Orchid Society meeting (she was quoted saying 125ppm is the goal for orchids, in an online newsletter).

2. How do you know what % amount of each substance the different species of orchid require? I've seen different values on different website, e.g. 10-6-6; 30-10-10.

3. Is the N-P-K ratio same or different for the 7 most common varieties of orchid: Cattleya, Dendrobium, Oncidium, Paphiopedilum, Phalaenopsis, Phragmepedium, and Vanda?

Graham.

[Ross]

Check out this information under "feeding your plants" from our Orchid Board member Ray. First Rays' Free Info

[Graham]

Thanks Ross.

[Ray]

That info at orchidsusa.com is full of misinformation, starting with the formulas.

The macronutrients - N, P, & K - are expressed as elemental nitrogen, but as oxides of the other two. Therefore, while a 30-10-10 does contain 30% nitrogen, it contains the equivalent of 10% P2O5, and 10% K2O, meaning that in terms of the elemental ingredients, it's 30% N, 4.4% P, and 8.3% K.

Be that as it may, I would go so far to state that every individual plant (not just general types) has its own nutritional needs, just as individual people do. However, as their metabolism is far simpler than ours, the range is probably quite a bit smaller, meaning that as long as a sufficient amount of a relatively few chemicals are available, all orchid plants can do well on more-or-less the same fertilizer.

One of the ways you determine the needs is to take a healthy plant and burn it, then do a chemical analysis of the residues. However, that doesn't paint the entire picture, as while those residues may contain equal amounts of "X" and "Y", you don't know if the plant only absorbs "Y" at a slower rate (meaning it needs more in the fertilizer), or if it really doesn't need that much, but because your fertilizer was so loaded with it, osmotic pressure shifted the balance.

So starting with the tissue analysis, you then do a lot of controlled studies, varying the nutrients and seeing the results.

I don't know about you, but I don't have the wherewithal to do such a study, so instead, I rely on the knowledge and experience of others (some of whom have done those studies) to help me choose a good commercial formula.

People also worry about what formula to use based upon the medium. That is less of an issue than it was once thought to be, as well.

For example, it's a well known "fact" that bark-based media need more nitrogen. In reality, that's simply not true, especially with fresh medium. While it is true that wood-decaying microorganisms do consume nitrogen, in fresh bark, the population is so small, that it's not a factor. (Most bark is steamed to extract the phytotoxic oils and resins, so any natural microflora and -fauna are killed.) Now then, let that bark hang around so long that the population has grown and the decomposition is well along, and it's probably more of a factor. However, you'll also be putting the plant at risk of suffocation and root death, so you'll take it out of that and put it into fresh medium anyway, negating the issue.

As a final thought: nutrition is actually one of the least significant factors that needs to be precisely controlled in orchid growing. Temperature, humidity, light intensity, water, and gas exchange at the roots are far more important, so you can "just feed" the plants regularly. It is only when you have those well controlled will you be able to see changes based upon subtle changes in nutrition chemistry.

[Graham]

Ray, you've raised a number of issues...I've had a look at your website and you seem to be a good mentor for those wishing to grow orchids.

Originally Posted by Ray

That info at orchidsusa.com is full of misinformation, starting with the formulas.

What makes their information, their formulas, incorrect? Won't they be basing it on results from orchids they've grown?

They appear to have placed themselves in the role of educators, but it can't be good if, as you say, they're handing out poor-quality advice.

[Ray]

I was referring specifically to their description of what the formula means...it's just plain wrong, as I explained. Reread what I wrote above - a 30-10-10 is not 30%N, 10%P, 10%K.

Folks often teach more than they know, including me sometimes, but I try to think about that before hand and qualify my comments.

There's also the frequent issue of folks proliferating incorrect info, and I try to clear that sort of stuff up - hence my "orchid iconoclast" moniker.

[Ross]

I have to say (as much as I hate to ) Ray, more often than not is correct. I have only once (that I recall) corrected him, and that was a really minor issue on lighting. I trust what he puts forth as pretty-much supported by the facts. If one chooses, they can do their own science and math, but why bother? I have never questioned Ray's science, background, experience, or other when it comes to fertilizer. It is to the point and helpful.

[Ray]

You shootin' for some sort of discount, Ross?

[Graham]

Ray, I have carefully re-read your original post

The macronutrients - N, P, & K - are expressed as elemental nitrogen, but as oxides of the other two. Therefore, while a 30-10-10 does contain 30% nitrogen, it contains the equivalent of 10% P2O5, and 10% K2O, meaning that in terms of the elemental ingredients, it's 30% N, 4.4% P, and 8.3% K.

Okay, taking into account my beginner-level knowledge-base...

...what you're saying is, the Nitrogen (N) is the main nutritional element of the fertilizer.

You say the Phosphorous (P) and Potassium (K) elements are oxides... by that, I take it they're of a lesser concentration than their original elementary form... I've seen a number of different definitions for the term Oxide...What do you define an oxide to be?

So, N is the major element in fertilizer and both P & K are still considered Macronutrients but of a lower status than Nitrogen.

Now, Ray, given your 30-10-10 example, the N element is straight-forward; but, how do you reach the P value of 4.4% and the K value of 8.3%?

[Ray]

You over-interpreted.

1. Nowhere did I say that nitrogen was "the main" nutritional element. All three are critical to the plant, as are the minor and trace elements.
2. I also did not say the P & K are oxides, I said they are reported as the equivalent of oxides. That is a throwback to the days when the only way you could test for certain elements was to burn them, then weigh the oxide, but now, even though there are better methods, it is the "standard" anyway.
3. You're kidding, right? An oxide is an oxide - a chemical that consists of oxygen and another element, often a metal.
4. "Macro" does not refer to importance, it merely refers the the amount present in the formula.
5. The way I came up with the 4.4%P and 8.3%K was by calculating how much of the element is in the oxide.