This subject is often debated among the orchid growing community, is it really necessary to buy a specialist orchid fertilizer as opposed to a general plant fertilizer. Well the answer is simply no but when choosing any fertilizer for orchids you must realize that they have certain specific needs.
The first quite major thing to check out is that your fertilizer contains none of the compound urea. Orchids do not like urea in fact it can be very harmful to them so avoid it at all costs.
Fertilizer in general provides our plants with the necessary elements for plant growth in many different formulations. You will recognize fertilizers with numbers in them like 10:10:10 or 20:10:20 for example this simply refers to the ratio of the three main nutrients nitrogen, phosphorus and potassium, written as N P K. Therefore fertilizer with 20:10:10 NPK would contain 20% nitrogen, 10% phosphorus and 10% potassium. But what is the significance of the NPK ratios?
Nitrogen is important for strong growth of the leaves and stems and gives plants strong lush green growth. Phosphorus is used mostly for root and flower development and Potassium enhances the strength of the plant and is used in chlorophyll production, necessary for good orchid health. A good plant fertilizer will also contain many more of the minor nutrients and minerals needed for good healthy plant growth.
The main difference between a specialist orchid fertilizer and a general plant fertilizer is that a specialist one is specifically tailored to the orchids needs. The ratio of NPK is designed to suit the orchids needs not just the general needs of plants. That being said how much of a benefit this actually gives is subject to debate. It is often accepted that a well balanced, good quality general fertilizer applied correctly in the right amount will give excellent results. The main benefit of the specialist fertilizer is that it makes things that little bit easier with no thought required.