People spend a lot of time worrying about what fertilizer to use on their orchids, and manufacturers make so many different blends that it’s difficult to know which is “the right one.” Generally, just about any fertilizer may be used on your orchids, within certain guidelines. To make it really simple, select a formula that contains a wide array of minor and trace elements. There are some that feel that those minor ingredients are the most important components of the formula. If your water supply does not already contain them, use a fertilizer formula that contains calcium and magnesium, as well.
When orchid collecting began, it was noted that the majority grew on the bark of trees. Naturally, that led to the idea of growing orchids using bark as a potting medium, and that has been the standard for many, many years. Unfortunately, wet crumbled bark in a pot will slowly decompose, courtesy of various microorganisms. The little critters also consume a large amount of nitrogen as they work, and would end up leaving the plants nitrogen-deficient, so it became necessary to compensate for that in the formula. The problem is that feeding your plants too much nitrogen can lead to the delaying, or outright stopping of blooming, which defeats the goal of the orchid grower. The key, therefore, is to provide a moderate amount of fertilizer so that we don’t overdo the nitrogen.
That leads us to the question about the use of “bloom-booster” formulas. Those are the blends with augmented levels of phosphorus in the formulation. They are commonly used for a number of weeks prior to the start of inflorescence growth, as a way to “build up” the plant for blooming. Are they necessary? My own experience doesn’t say so, and when asking that of others, you’ll get the full spectrum of responses, but it sure can’t hurt. More recent studies at Michigan Sate University suggest that blooming is less an issue of boosting phosphorus than that of not overdosing nitrogen, so maybe the effectiveness of the bloom booster formulas is related to the ratios of the two, and not so much the phosphorus level itself.
One can surmise the nutrients needed by a plant by determining the mineral content of the plants themselves, or the mineral content of the solutions they see in nature, but neither gives us the “correct” formula, as plants take up nutrition both passively and actively, in some cases storing greater amounts of nutrients than they really need. Folks may use chemical analyses of rainfall and plant tissues as guidelines, but it’s only through trial and error that we understand what the plant need.
Choosing a fertilizer that contains the correct nutrients in the proper concentrations, however, is only part of the story. A critical aspect that is often overlooked is the availability of those nutrients to the plant.
Minerals – whether naturally occurring in the soil or in fertilizers – are only absorbed by plants if they are in the form of ions in solution. The size and reactivity of those ions determines how readily they can be taken out of solution and absorbed by the plants, and the pH of the solution is probably the most significant factor in controlling the ionization of the minerals. Greatly simplified, depending upon the pH, a mineral can be insoluble and unavailable to the plant, soluble, but in a form that is difficult for the plant to readily absorb, soluble and in a form that the plant can absorb with ease, or so soluble and concentrated that it can be toxic. Without going into solubility details of the specific ions, research has shown that a pH of around 5.5-6.5 is ideal for the vast majority of orchids.
Remember that the chemistry of your nutrient solution is determined by both the fertilizer and your water supply. Figuring that most people will use tapwater, most general-purpose formulas are designed with a generic array of dissolved solids in mind, so will provide a good pH when used out of the box. If those are used in pure water – reverse osmosis, distilled, deionized, or collected rainwater – it is likely that the pH will be extremely acidic and not suitable for the plants. In that case, the addition of a neutralizer is necessary, whether that be aquarium “pH-Up,” Dyna-Gro ProTekt, or some other means. Recognizing the importance of pH in the overall equation of plant nutrition, the blend developed for Michigan State University’s study was designed to provide the proper pH when used with pure water.
What Do Fertilizer Components Do?
There are approximately 20 elements necessary or beneficial for plant growth and blooming. Some are derived from air and water – Carbon (C), hydrogen (H), and oxygen (O) – while others are mostly absorbed from the nutrient solutions we provide. Six of the elements that should be supplied in your fertilizer – the “macronutrients” – are used heavily by plants: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). The remaining essential elements, the micronutrients, are required in small amounts only: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), sodium (Na), zinc (Zn), molybdenum (Mo), and nickel (Ni). Additionally, it appears that both silicon (Si) and cobalt (Co) may play a beneficial role in plant health.
How Much Fertilizer should be Used?
Like pretty much all other factors of orchids growing, there’s no set answer, and “it depends.”
As a general rule, fast growers in bright conditions require more food than do slow growers in heavy shade. Similarly, those trends can apply to your specific lighting conditions. A grower in Florida has more light flux than we do here in Pennsylvania, and we have more than someone in Canada, so the food requirements decrease as you move north. That analogy may be applied elsewhere as well, for example to HPS versus fluorescent lighting.
While that may suggest general trends, it doesn’t provide the quantitative answer we need. .
Many professional growers base their nutrient concentrations on the amount of nitrogen provided to the plants over a finite time to harvest. For orchid growers, we have to include the frequency of feeding in our estimates, with 250 ppm N being common for bi-weekly feeding., 100 ppm N if you feed weekly, etc. At First Rays, we shoot for roughly 30-50 ppm N, and feed at that rate at every watering. We settled in on that level because of our varied collection – vandas may like more and phrags less, but we’re way too busy to cater to the individual, so came up with an average. Others find that increasing the concentration is beneficial, but irrigate with fresh water periodically to flush residual minerals from the medium.