Andy,
Orchids need all sorts of minerals to grow and bloom well, and vandaceous plants tend to be pretty heavy feeders, too when they're in active growth. Check out the free info page at my website, and you can learn a lot more, if you wish.
My suggestion is to buy a fertilizer designed specifically for orchids, having complete trace elements and minor ingredients, but avoid a high-nitrogen blend like Mir-Acid (even though it was actually invented specifically for orchids, a long time ago), as newer info suggests that high nitrogen fertilizers actually reduce- or stop plants from blooming altogether.
"Natural" fertilizers like fish emulsion, kelp extract and the like sometimes have the disadvantage of smelling bad and often do not contain all of the minor and trace elements the plants need. The decent orchid fertilizers on the market from GreenCare, Peters, Dyna-Gro, and the like are made from beneficiated minerals mined from the earth. It's hard to get much more "natural" than that.
Besides, those so-called "natural" things are really just chemicals anyway, and they have to we ionized in water for the plants to absorb them...
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