Fertilizer dosage/frequency based on potting media
 

As the title states, I have some questions on fertilizer use depending on the potting media. My plants are all either in traditional bark mix or sphagnum moss. I have been using a liquid fertilizer from orchidlimited/orchidweb called Green Jungle. I like it and my plants like it, but it’s too expensive so I’m switching to MSU formula. My question is regarding dosage. Currently I feed my plants in sphagnum every 3rd feeding while I feed my plants in bark at each watering with a flush every couple weeks.

My thought with the MSU formula would be to feed at 1/4 tsp/gallon with a frequency of at every watering for the bark mix and every other watering for the sphagnum. This is ~50 ppm N. I’m not really sure how much I was feeding with the green jungle fertilizer. It just said 1% nitrogen on the bottle, but that doesn’t sound right.

Basically I’m just needing some advice so I make sure I don’t burn up my orchids.

Edit: don’t know if it matters, but many of my plants are young. I have Cattleya, Angraecum, Vanda (seedling), Bulbophyllum, Paphiopedilum, Psychopsis, Beallara Marfitch, and several Laelia purpurata

At 1/4 teaspoon per gallon, you won't be doing any harm to the orchids, that's a good dilution. As for frequency, you may be working too hard trying to treat different media differently... I would not be the one to advise on that... at my house everybody gets the same, I use 1/2 teaspoon per gallon MSU every 2 weeks, though the plants would probably be happier with a bit more frequency.

Taking a cue from Mother Nature, I like the "frequent and sparing" approach.

Potting medium plays no role in the formula or frequency of feeding.

Over the course of 5+ decades of growing, my feeding has evolved from "I have no idea what I'm doing" to trying to maintain 75-125 ppm N solution applied weekly.

If you divide 8 by the %N in the formula, the result is teaspoons/gallon for the middle of that range, giving you room for round up or down for measuring convenience.

K-Lite: 8/12.9=0.62, so I use 1/2 tsp/gal.
MSURO: 8/13.2=0.606 - also 1/2
MSUWW: 8/19=0.42 - so 1/3
Green Jungle: 8/1=8 tsp/gal. (You're paying a lot for water...)

Thanks, Ray. So I should be good at 1/2 tsp/gal weekly. So the media doesn’t matter? I was worried that feeding weekly in sphagnum may be too much for those plants that only get watered every 6-7 days. Would you recommend a flush ever so often? Maybe once per month?

If those plants in sphagnum only need watering every 6-7 days your temperatures might be lower than ideal, or they might be overpotted.

I have some (near) flowering size Cattleya and Laelia purpurata that arrived in sphagnum. I let them nearly dry out which is usually every 6 days. They appear to be thriving and are not in need of repotting so I plan on leaving them be until next year.
My temps get up to the 90s during the day and down to 65 at night. I keep humidity at 65%. This grower just really has sphagnum dialed in for younger plants so I’m going with it until they get larger and need repotting. I’ll be asking repotting questions then 🤣

If you have some advice I’m definitely open to it. I’m here to learn!

In similar temperatures I need to water seedling Cattleya alliance plants, including Laelia purpurata, in sphagnum in plastic pots, every 2-4 days.

Lower humidity?

I’m not talking about seedlings. These are two of the larger ones. Looks like on average I water them every 6 days. The smaller ones every 2-3 days.

Sorry....I don't think in terms of "sphagnum" any more, as it decomposes too quickly here.

I flood the daylights out of my plants at every watering, which can be 3-4 times a week in summer. That's tough to do with sphagnum.