Preferred fertilizer for orchids?

[notsogreenthumb]

So, about 7 or 8 years ago (I've been growing orchids for over 45 years now), all of that led me to this growing regimen:

• Use a VERY open potting medium, so the roots always have excellent air flow around them.
• Water the daylights out of them: Frequent & Flooding. Water is the driving force behind plant growth.
• Use a very small amount of fertilizer at every watering. I mean really little. Divide 2 by the %N on the labels to get the teaspoons per gallon to use.
• Make sure that "fertigation" solution contains nitrogen, calcium, and magnesium; the other stuff can "tag along". (Your water supply may have enough calcium and magnesium in it, but as a user of RO water, I add it.). I prefer K-Lite, a 12-1-1-10Ca-3Mg derivative of the MSU RO formula.

All of my plants seem very happy with that, and are growing and blooming better than I've ever seen - figures it would take me almost 40 years to figure that out....

Ray, I think I have read one of your posts or in an article by you that was talking about watering at 125ppm of N once a week, but then I thought you said something about watering more frequently (daily or every other day) at a lower concentration like 20 or 25 ppm of N. so is your recommendation above dependent upon how frequently you water? And then how frequently do you flush? I assume it's something like once every XX number of waterings with fertilizer?

Just to clarify Ray...if my %N number is 13 the number of teaspoons per gallon is .15 correct?
Just bought some MSU formula fertilizer but haven't fed any Orchids yet.

Mike, I'm wondering the same thing and see my post above. So the easiest way to get that would be to use 1/8th tsp since that's .125 tsp.

[raybark]

When the MSU fertilizer was first released, the developers recommended 125 ppm N for feeding. Figuring they knew what they were talking about, that's the level I used. Later, after doing some research, especially that related to the analyses of the "through fall" and "trunk flow" that orchids to see in nature, and the frequency with which they are fed in nature (whenever it rains), I dropped my concentration to 25 ppm N, and started applying it every time I watered. In later conversations with the PhD that led the development team, he told me that there was no scientific basis for the 125 ppm, but they "tried it, and it worked," so that's what they recommended.

As far as the concentration is concerned all you have to do is a simple mathematical fraction:

2÷13 = 1/6.5, so 1/6 teaspoon/gallon is fine, as is 1/8. Rounding up or down a bit isn't going to hurt anything. By the way, for those of you civilized folks on the metric system, 2.3/%N on the label gives you the ml/L for 25 ppm N.

Something I would recommend to everyone however, is to make a concentrated stock solution, and then use a bit of that solution to let down to your final fertilizer concentration. Many fertilizers, the original MSU for example, are very heterogeneous. Each ingredient is a large flake, small flake, powder or prill, so taking a fraction of a teaspoon or ml and expecting it to be true to the fertilizer formula is questionable. Add to that the fact that powder bulks densities can vary greatly, and it is folly to try to measure them by volume, hoping it equates to mass.

Instead, I recommend that folks make up a concentrate with a larger, known weight of powder. If you're looking for 25 ppm N, for example, that means you want 25mg N per kg (liter) of water in the final solution. As the powder is 13% N, that means you're shooting for 25/0.13=192 mg of fertilizer powder per liter (or 192 x 3.785=728 mg per gallon). But that's kind of tough to measure, so we should make life easier and make a concentrate in which that much is contained in a smaller volume - for example, let's say we want that in 10 ml. 192 mg/10ml = 19200 mg/L, so of you put 20 grams in a liter, that's close enough, and 10 ml of that solution, diluted to 1 liter, gives you your ~ 25 ppm N solution for feeding.

For Imperial measurers, 0.2 pounds of that 13% N fertilizer, when used to make up a gallon of concentrate, allows you to use one ounce to make up that same gallon of 25 ppm N final solution.

[notsogreenthumb]

Ray, what a fantastic post with lots of practical applicable information, especially the bit about making a concentrated solution to use to create the regular solution. Thank you for the explanation with reasons and methods. Now we can mix up the correct amount of fertilizer for our plants.

[raybark]

Steve, I believe it has been stated already, but fertilizer is probably the least important aspect of orchid culture.

Feed lightly and regularly, and dedicate your time to making sure ALL of the other aspects of culture are correct.