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” and “how much to use”.  Generally, just about any fertilizer may be used on your orchids, within certain guidelines. To make it really simple, select a formula that contains good levels of N, P, & K, plus a wide array of minor and trace elements. If your water supply does not already contain them, use a fertilizer formula that contains calcium and magnesium, as well.

Delving into it more, there are two ways to determine “what the plants want”, by tissue analysis and by the chemical analyses of the solutions they get in nature.  Analysis shows that about 95% of dry plant tissue is carbon (C), oxygen (O), hydrogen (H), and nitrogen (N).  The first three are supplied by the air and water, with the nitrogen coming from our nutrient solutions.  Potassium (K), phosphorus (P), calcium (Ca) and magnesium (Mg) make up the majority of the remaining few percent, with all of the other elements being present in truly trace amounts.

Looking at the nutrient solutions that literally rain down on epiphytes in the forest, we find that they typically contain no more than 10-20 ppm total dissolved solids (TDS) at the onset of a rain storm (it’s almost pure water after that), and that the analysis shows it’s nutritionally almost all nitrogen.

Both analyses suggest that nitrogen is the key fertilizer nutrient, and indeed that is true, but how do we reconcile that with the fact that too much nitrogen can suppress blooming?  Not to worry – the key is simply giving the plants a small amount of fertilizer, frequently, and not overdoing it. More on that in a moment, but that leads us to the question about the use of “bloom-booster” formulas – the blends with augmented levels of phosphorus in the formulation. In fact, no fertilizer “boosts” blooming. Since excessive nitrogen can suppress blooming, the added phosphorus merely dilutes the nitrogen, “allowing” the plants to bloom normally. If you’re not overdoing it in the first place, this is simply no issue.

How Much Fertilizer should be Used?
Like pretty much all other factors of orchid growing, there’s no set answer, and “it depends.”

As a general rule, in order for any plant to gain a pound of mass – a matter of weeks for corn, a year or so for a cattleya, a couple years for a phalaenopsis, or a lifetime for a tiny pleurothallid – it must absorb and process about 200 pounds (25 gallons) of water, but only 5 grams of N-P-K fertilizer!

As it is the most important nutrient, professional growers base their nutrient concentrations on the amount of nitrogen provided to the plants over a finite time to harvest, having selected a fertilizer formulation that gives the ratios they want for the other elements.  The same applies to hobby orchid growers, so we have to include the frequency of feeding in our estimates. For bi-weekly feeding, 150-200 ppm N is common, 75-100 ppm N if you feed weekly, etc.  Personally, growing my plants on a very warm deck in southeastern North Carolina, I use roughly 25 ppm N at every watering, two or three times a week.

Don’t let the “ppm’s” bother you.  A simple estimate is to divide “2” by the %N on the fertilizer label.  The result is the teaspoons per gallon for 25 ppm N.  For you “metricated” folks, 2.3/%N gives milliliters per liter for the same concentration. For example, for 25 ppm N using a 8-3-4 formula, we need 2/8=1/4 teaspoon per gallon. If we were using a 30-10-10, then 2/30=1/15 tsp/gal would result in the same nitrogen concentration.

Nutrient Availability
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, or soluble and in a form that the plant can absorb with ease. 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, but don’t be too concerned if your solutions are moderately outside of that range, as the plant and potting medium affect the pH as well.

Remember that the chemistry of your nutrient solution is determined by both the fertilizer and your water supply. Figuring that most people will use tap water, 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.  The use of formulas designed for pure water supplies can preclude the need for such adjustments.

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 solution – the “macronutrients” – incude 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 is rare, as plants actively absorb it, storing excess amounts in cell vacuoles.

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. A lack of 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. Unlike most other nutrient minerals, once incorporated in plant tissues, calcium cannot easily 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.

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.

In summary
As orchids are fairly slow growers in the plant world, the nutrient demand is low, but still an important part of our overall cultural regimen. Fertilizer will not improve blooming, will never fix a deficiency in another aspect of your culture, and “more” is never “better”. Feed regularly but sparingly, and give your plants the growing conditions they have evolved to expect in nature, and they will grow and bloom to their maximum, genetically-programmed capability.

Using Science & Logic to Improve Horticulture