FERTILIZING
No plant will live on air and water alone. Most orchid potting media provide support only, and have little or no food value. When the plants are in bark they need a high nitrogen food (30-10-10 ETC.). Plants in most other media need a balanced food (18-18-18 ETC.). Use a water soluble fertilizer at the dilutions recommended on the label. Plants in lower light need less fertilizer or the tips of leaves start to shrivel. Flush the pots with plain water occasionally to prevent deadly salt buildup.
Many orchids aren't heavy feeders, so feeding every couple of weeks is enough. They have an indefinite life span, so fertilizer isn't critical as it is with an annual. One months missed fertilizer won't stop blooming or mortally wound a plant that can live a century or more, given good care. Proper light is more important for good blooming than a rigorous fertilizer regimen.

sources:orchids.org
photo courtesy of aka fortex