I think the main criterion would be to have sufficient nitrogen - the first number should be at least as high as the other two numbers, higher being better. Ask 10 orchid growers about fertilizer and you're likely to get 12 opinions. I truly don't think that orchids particularly care. They need so little of anything. I also doubt that the "urea free" thing is important... if orchids were not able to utilize organic nitrogen in nature they would not have survived, because that's what they get (rotting detritus, the occasional bit of bird poop, etc) Maybe the orchid-oriented fertilizers are more efficient, but I doubt that "efficiency" is particularly important. If you have a small collection, and some fertilizer lying around, use what you have (maybe at half or 1/4 strength) Actually the one thing that supposedly is formulated for orchids that is pretty useless is the so-called bloom-booster type. It's just a nitrogen-deficient formula, that doesn't boost blooming. If you want less nitrogen, just cut back the fertilizer or use none (like for some of the orchid types that don't want it in winter) Personally, I only use time-release for orchids that grow fast (since fertilizer is used by the plant for growing new tissue, green plants make their own "food", carbs, by photosynthesis) This would include things like Cymbidiums, or Catasetinae during the growth phase. The downside of time-release is that you don't know how much the plant is getting. So save it for the "hungry" ones.
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