People spend a lot of time worrying about what fertilizer to use on their orchids. Manufacturers make so many different blends that it can be difficult to know which is “the right one.” Generally, just about any fertilizer may be used on 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 the 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 of orchids 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 microorganisms consume a large amount of nitrogen as they work, and will eventually leave the plants nitrogen-deficient. This made it 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.
This leads us to the question about the use of “bloom-booster” formulas. These 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. More recent studies at Michigan State 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.
Nutrient Availability
The nutrients needed by a plant can be found 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. Chemical analyses of rainfall and plant tissues can be used as guidelines, but it’s only through trial and error that we learn what the plant needs.
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
It doesn’t matter if minerals occur in the soil or are in fertilizers, they 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. 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
- Soluble and so 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 the water supply. Most people use tap water so most general-purpose formulas are designed with a generic array of dissolved solids in mind, and provide a good pH when used out of the box. If these same general purpose formulas 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. 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 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.
The Role Fertilizer Elements Have with Plants
Nitrogen (N)
A major component of proteins, hormones, chlorophyll, vitamins and enzymes that are essential for plant life. Nitrogen metabolism is a major factor in stem and leaf growth (vegetative growth). Too much nitrogen can delay or prevent flowering, while deficiencies can cause yellowing of the leaves and stunted growth.
Phosphorus (P)
It is necessary for photosynthesis, protein formation and almost all aspects of growth and metabolism. It is also essential for flowering. Phosphorus deficiency – sometimes associated with purple leaves – results in slow growth, poor flower production or premature loss of flowers.
Potassium (K)
This element is necessary for the formation of sugars, starches, carbohydrates, along with protein synthesis and cell division in plants. It helps to control water absorption and loss, improves the physical sturdiness and cold hardiness of plants, and enhances flower color. Too little potassium can result in mottled, spotted or curled leaves or a burned look to the leaves.
Sulfur (S)
It is a structural component of amino acids, proteins, vitamins and enzymes and is essential to produce chlorophyll. A deficiency usually shows up as light green leaves.
Magnesium (Mg)
This is a critical structural component of the chlorophyll molecule and is necessary for functioning of plant enzymes to produce carbohydrates, sugars and fats. Magnesium-deficient plants show yellowing between veins of older leaves, and they may appear limp. Some feel that regular supplementation of magnesium in fertilizers is important.
Calcium (Ca)
Helps with the functioning of enzymes, is part of the structure of cell walls, helps control the water content of cells, and is necessary for cell growth and division. Some plants must have calcium to take up nitrogen and other minerals. Calcium, once deposited in plant tissue, cannot move to other plant tissues, so it must be supplied regularly. Without a sufficient supply of calcium, your plants may display stunted or stopped growth. Other possible symptoms include distorted new growth, black spots on leaves, or yellow leaf margins. Recent studies indicate that calcium apparently plays a much bigger role in plant health than previously thought.
Iron (Fe)
It is necessary for enzyme functionality and is important for the synthesis of chlorophyll. It is also essential for young, actively growing tissues. Iron deficiencies are indicated by the pale color of young leaves followed by yellowing, and large veins. An adequate supply of soluble iron in the plant nutrient also inhibits the formation of phenol compounds, which can kill roots.
Manganese (Mn)
This element is involved in enzyme activity for photosynthesis, respiration and nitrogen metabolism. In young leaves, a deficiency may be indicated by a network of green veins on a light green background similar to that seen in an iron deficiency. Dark spotting may occur near the veins. In extreme cases, the light green parts become nearly white, and leaf loss may occur.
Boron (B)
Boron is used in cell wall formation, for membrane integrity within cells, for calcium uptake and may aid in the transfer of nutritional sugars between plant parts. It affects a variety of plant functions, including flowering, pollen germination, seed development, cell division, water balance and the movement of hormones. Boron must be available throughout the life of the plant as, like calcium, it is fixed in the plant once absorbed. Deficiencies can lead to very stunted or irregular growth, with leaves that are thick, curled and brittle. Roots can become discolored, cracked and covered with brown spots.
Zinc (Zn)
Is a component of enzymes or as an important aid in the functioning of them, especially auxins, the plant growth hormones. It is essential to carbohydrate metabolism and protein synthesis. Deficient plants have mottled leaves with irregular chlorotic areas. Zinc deficiency leads to iron deficiency causing similar symptoms.
Copper (Cu)
This is concentrated in roots of plants and plays a part in nitrogen metabolism. It is a component of several enzymes and may be part of the enzyme systems that use carbohydrates and proteins. Deficiencies can result in the die back of the tips of new growths.
Molybdenum (Mo)
Is a structural component of the enzyme that reduces nitrates to ammonia. Without it, the synthesis of proteins is blocked and plant growth ceases. Seeds may not form completely, and nitrogen deficiency may occur if plants are lacking molybdenum. Symptoms may include pale green leaves with rolled or cupped margins.
Chlorine (Cl)
Chlorine is involved in osmosis, the ionic balance necessary for plants to take up mineral elements and in photosynthesis. Deficiency symptoms include wilting, stubby roots, chlorosis (yellowing) and bronzing. Flower scent may also be decreased.
Nickel (Ni)
This element is required for iron absorption. Plants grown without additional nickel will gradually reach a deficient level at about the time they mature and begin reproductive growth. If nickel is deficient, plants may fail to produce viable seeds.
Sodium (Na)
Is involved in osmotic (water movement) and ionic balance in plants (much as it is in people).
Cobalt (Co)
Required for nitrogen fixation and a deficiency can result in nitrogen deficiency symptoms.
Silicon (Si)
This is a component of cell walls. Plants with supplies of soluble silicon produce stronger, tougher cell walls making them more heat and drought tolerant. There is also some evidence that silicon plays a role in the prevention of fungal infections in the case of tissue damage.
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 specific lighting conditions. A grower in Florida has more light flux than a grower in Pennsylvania, who has more than someone in Canada. The food requirements decrease as you move north. That analogy may be applied elsewhere as well, for example to HPS versus fluorescent lighting.
This may suggest general trends, but it doesn’t provide a quantitative answer. Many professional growers base their nutrient concentrations on the amount of nitrogen provided to the plants over a finite time to harvest. Orchid growers need to include the frequency of feeding in the 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 too busy to cater to individual plants and use an average instead. Others find that increasing the concentration is beneficial, but make sure to irrigate with fresh water periodically to flush residual minerals from the medium.
You can determine the amount of fertilizer to use based upon the formula of the blend you have and these simple calculators.
Ray Barkalow has been growing orchids for over 45 years, and owns First Rays, which offers horticultural products to the hobby grower. He may be contacted at raybark@firstrays.com and you can visit his website at FirstRays.com.
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