Orchid plant food should be 20% or less nitrogen. The amount of the phosphorous and other nutrients is not so important.
Some growers like to use a slow release formula, however, this can sometimes burn roots and in course potting material, can be easily washed out during watering.
Therefore, a water soluble fertilizer is recommended. It is widely available, and easy to apply. It will need to be reapplied every 2-3 weeks.
Tips on Fertilizing:
Feed when orchids appear to be growing.
When they stop growing, stop fertilizing.
Very dark green leaves that are limp, have probably been over-fertilized.
Every few weeks, wash out excess fertilizer salts by flushing the plant growing medium with plain water.
It is better to under-fertilize than over-fertilize.
For More information about orchid fertilisers, check out the links below.