Orchids need very little fertilizer and too much will cause more damage than too little.
All fertilizers are a combination of Nitrogen, Phosphorous and the proportions used are referenced on the label. Use one where the numbers are balance 20-20-20 or 10-10-10. Use a water soluble type diluted at 1 teaspoon per gallon or less.
Several combinations are sold as 'Orchid Fertilizers'. There are two that should be avoided. High nitrogen 30-10-10 has a limited use only for older decaying bark mixtures. This is because the bark uses the excess nitrogen to decay. In other mediums the nitrogen is too much for the Orchid. Orchid Bloom Boost 8-30-10 should also be avoided. It does not give the plant enough food to develop naturally and does little to increase flowers.
Most commercial growers are going to a 17-3-17 mix with Calcium and Magnesium added.
Our personal favorite is not really a fertilizer but natural - organic worm manure. Humans have been growing plants with earth worms for thousands of years. There is no doubt that it works. The Worm Castings (worm manure) is a dry odorless product that is best used in pots that do not cause it to wash out.
Worm Tea is the liquid drippings from the worm bed. It is loaded with natural microbes that are symbiotic with plants. The brand we use has 76 micro-nutrients. These micro-elements are not common in other brands of Worm Tea, but are added into the worm feed by OurVitalEarth for our Worm Tea. It is a concentrate that dilutes 50-1 with water for use. It is re-usable with a shelf life of one year. The microbes enable the plants to use the nutrition immediately. Chemical fertilizers need to be broken down into forms the plants can use.
Other companies produce their 'Worm Tea' by soaking worm castings (worm excrement in dry form) in water. While this is still beneficial to plants it does not have the live microbes and will not give the full benefits to the plant.
The benefits of Worm Tea are amazing. We use it on all our Orchids once a month or more. We find about a 30% additional retention of water in the roots of Orchids. Vanda roots do not dry as fast when watered with Worm Tea. Growth especially on miniature orchids is noticeably stronger. And the biggest bonus is that it is a an organic insecticide without the harmful side effects of chemicals.
We do like slow release fertilizers for aggressive growing Orchids and when we are not able to apply fertilizer on a regular basis. Be careful there are two methods of slow release. Water released such as Florikan and Dynamite, which we recommend, and temperature released such as Osmocote, which can be a problem for orchids grown in areas where the temperature is consistently above 75 degrees. Temperature release makes the fertilizer available to the plant when temperatures are over 75 degrees. This can add too much fertilizer too fast especially for hotter areas of the country and indoor use. Water release is more consistent.
Worm Castings is a natural slow release fertilizer, that because of the microbes occurring naturally in the worm bed, is available to the plant faster. As an all natural fertilizer it is safer to use to avoid burning from excess fertilizers.
With all fertilizers keep the dosage light 1/4 to 1/2 the recommended amounts for tropical plants. Orchids do not need a lot of fertilizer but they need some.
We add Bracillus microbes (Companion brand) and rooting hormones (Essentials brand) to our use of Worm Tea as regular maintenance in our greenhouses.