Fertilizer and Potting questions

[MojoShoujo]

I'm just starting to think about repotting my oncidium that finished blooming recently, and running into some questions.
If you mix a 20-20-20 fertilizer at half strength, does it become a 10-10-10?
When I talked to some local growers about repotting into a bark/moss mix, both of the ones I talked to advised against it because the acidity of the moss would make the bark break down faster. They advised a mix of bark and pumice, or moss and pumice. Pumice is really hard to find in my area though, and I want to see if I can use things I already have. Would perlite work?
I also have some "Structure" from Earl-May. I add it to my cactus mix to improve drainage, it's pretty finely crushed hard ceramic and doesn't float up like perlite. Would that be too fine or too rough on the roots?

[Roberta]

All three numbers the same means "balanced fertilizer" with the same percentages of the major constituents Nitrogen (N), Phosphorous (P) and Potassium (K). I'd suggest that you use the fertilizer at half the strength suggested on the bottle (so instead of 1 teaspoon per gallon use 1/2 teaspoon per gallon) At that point the nutrients are in the parts-per-million range (not percentage), the ratios stay the same.

For potting mix, perlite (coarse - 1/4" to 1/2") works the same as pumice - it just serves to keep the mix "open". Don't use the fine perlite that you would mix with potting soil (you may have to order on line or pick up at an orchid show, "generic" nurseries only carry the fine stuff) What you want is an inorganic material with good porosity to fill the role... anything that meets that criterion is fine.

I don't think the problem with sphagnum in the mix is acidity, but rather that it holds too much moisture. Orchids tend to hate wet (and therefore rather airless) medium. The goal (and there are many ways to achieve it) is to have a medium that holds enough moisture to expose the roots to "humid air", with lots of air space. Some people use completely inorganic media... all dependent on your particular growing conditions. Lots of advice on this forum - just keep the goal in mind and your orchids will be happy,

[Ray]

Originally Posted by MojoShoujo

If you mix a 20-20-20 fertilizer at half strength, does it become a 10-10-10?

Yes. If you wanted to duplicate a certain concentration of the 10-10-10, you'd use half as much of the 20-20-20.

When I talked to some local growers about repotting into a bark/moss mix, both of the ones I talked to advised against it because the acidity of the moss would make the bark break down faster.

"They" don't know what they're talking about. Acidity has nothing to do with it. Decomposition of organic matter is done by microorganisms. The more moist the mix, the greater their population will be.

They advised a mix of bark and pumice, or moss and pumice. Pumice is really hard to find in my area though, and I want to see if I can use things I already have. Would perlite work?

Your choice of potting mix should be based upon the needs of the plant, your growing conditions, the type of pot you'll use, and how much you can-, or are willing to water. One size does not fit all - plants or growers.

I also have some "Structure" from Earl-May. I add it to my cactus mix to improve drainage, it's pretty finely crushed hard ceramic and doesn't float up like perlite. Would that be too fine or too rough on the roots?

As a very small percentage additions to potting mixes, I can't see it doing much harm, but I see no real benefit, either. Orchid roots need air flow around them, so a coarser mix is preferred.

[MojoShoujo]

Thanks! I repotted my phal today and am holding off on my oncdium a little longer.
My Phal was potted in 100% sphagnum, and PACKED in there. I swear, half a bale was in that 4" square pot. I didn't move up a pot size but I did replace the mix with a much looser one, Better-gro's mix of bark, moss, and perlite with most of the moss picked out in a clear vented pot. It seemed to be doing fine, no dead roots as far as I could tell, but it made me way too nervous knowing phals like airflow and can photosynthesize with their roots.
I'm not sure if the oncidium needs to be repotted, it's in a bark/pumice mix and I use the skewer method to keep an eye on the moisture. I'd like it to be in a mix with a bit of moss but I'm not sure if disturbing it would be worse.

[estación seca]

Tightly packed sphagnum moss can be used to grow orchids very well, but the grower has to learn how to water in this medium. It is different from watering in bark.