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Fertilizer Basics

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 does not contain excess nitrogen, but does contain 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.  Of interesting note is the fact that the majority of nutrients are absorbed via only the green tips of the roots and the undersides of leaves.

Why do we want to avoid high-nitrogen fertilizers (even though they're often specifically called "orchid food")? To answer that, let's look at some history: 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 was 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 seemed necessary to compensate for that in the formula. Unfortunately, more recent research has shown 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.

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 give me an answer, 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.

Nutrient Availability

One can surmise the nutrients needed by a plant by determining the mineral content of the plants themselves.  Typically, healthy plants are incinerated under controlled conditions and the mineral content in the ash is determined.  That information is the basis for the formulations in modern, complete fertilizers.

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 absorbable by plants if they are in the form of ions.  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.

Below is a brief synopsis of the roles the elements from fertilizers play in the life of your plants:

Nitrogen (N) is a major component of proteins, hormones, chlorophyll, vitamins and enzymes 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) is necessary for photosynthesis, protein formation and almost all aspects of growth and metabolism. It is essential for flowering. Phosphorus deficiency - sometimes associated with purple leaves - results in slow growth, poor flower production or premature loss of flowers.

Potassium (K) is necessary for the formation of sugars, starches, carbohydrates, for protein synthesis and cell division in plants. It helps to control water absorption and loss, improves the physical sturdiness and cold hardiness of your 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) is a structural component of amino acids, proteins, vitamins and enzymes and is essential to produce chlorophyll, so a deficiency usually shows up as light green leaves.

Magnesium (Mg) 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) plays a role in 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 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) is necessary for enzyme functionality and is important for the synthesis of chlorophyll. It is 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) 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) 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. Boron 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) 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) 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 be decreased.

Nickel (Ni) 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) is required for nitrogen fixation, so a deficiency could result in nitrogen deficiency symptoms.

Silicon (Si) is found as 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 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, with 100 to 250 ppm N being common.  At First Rays, we shoot for roughly 100-150 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.

You can determine the amount of fertilizer to use based upon the formula of the blend you have and these simple calculators.

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