Orchids are a member of the family Orchidale. Orchids originally grew on the ground, but over time they began to grow on the bark of trees, including dead and dying trees. Orchids that grow on tree bark can do so because of a symbiotic relationship with various fungi. Through photosynthesis, the orchid produces food that feeds the fungi. The fungi help convert nutrients into forms usable by the orchid. How you fertilize your orchid will vary, depending on what you are using as a growing medium.
All fertilizers, including those for orchids, are made up primarily of three nutrients, sometimes referred to as the N-P-K content. N stands for nitrogen; P is phosphorus; and K is potassium. If all of the ingredients are roughly equivalent, such as in a 10-10-10 fertilizer, the fertilizer is considered balanced. The numbers are percentages of each component in the fertilizer---so a 10-10-10 fertilizer has 10 percent potassium, 10 percent nitrogen and 10 percent phosphorus.
In general, nitrogen is primarily responsible for leafy green, or vegetative, growth. In general, high-nitrogen fertilizers applied from after the orchid drops its flower spike until fall will help your orchid develop green leaves. The leaves, in turn, help the plant produce nutrients through photosynthesis. However, continuing to give your orchid a high-nitrogen fertilizer, like 30-10-10, past fall may encourage green leafy growth at the expense of flowering the following season.
Potassium is responsible for flower and fruit development. However, potassium works in consort with phosphorus to help your orchid develop and produce flowers that are both larger in size and in quantity. Switching to a lower-nitrogen but higher-potassium fertilizer, like a 10-20-20, in the fall will help your orchids develop and produce more flowers.
Phosphorus is the element in fertilizer that is most responsible for flower production. Because it works closely with potassium, advice on potassium fertilization techniques is generally the same for phosphorus fertilization. Fall application of more of these two nutrients, with less nitrogen, will improve your orchid flower production.
Unique Orchid Considerations
Orchids are relatively low-maintenance. Light is more important to flowering than fertilizer. However, some differences in fertilization techniques are important to keep in mind. If you are growing orchids in bark, which is how many commercial orchids are sold, use a higher-nitrogen fertilizer for leafy growth development. Bark has less available nitrogen than many potting soils. However, if you are growing your orchid in soil, a balanced fertilizer, like an 18-18-18, is better for vegetative growth. In both cases, switching to a higher-phosphorus and potassium fertilizer in fall will help your orchid develop flowers.