What is fertilizer?

When you ask someone what fertilizer is, you’ll often get the ubiquitous answer “fertilizer is plant food”.  But that is not really the case, as we know that plants photosynthesize, and that light is food for plants.  Fertilizer really is just a multivitamin for plants – there are no calories in fertilizer.  And like a multivitamin, fertilizer does not have to be added all the time; just when it’s needed.

Fertilizers are comprised of macronutrients and micronutrients – named as such because of the relative quantities that they are needed by the plant.  Macronutrients include elements such as Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and Sulfur (S), and are consumed by the plant in the highest quantities[1].  Micronutrients include many elements, but the most important are Boron (B), Iron (Fe), Zinc (Zn), and Copper (Cu).  Micronutrients are typically used as cofactors for enzymes and for electron-transport during the light-harvesting reactions of photosynthesis.  Nutrients in general are also classified as mobile or immobile nutrients due to their ability to move (or not) throughout the plant.  You can diagnose certain deficiencies based on where they are appearing on the plant.

Epiphytic orchids have adapted to low nutrient environments – their only input is from the rain and any bits of tree debris from higher branches that falls on them.  Tree bark is typically not that nutritious.  Therefore, orchids generally do well even if they are deficient in nutrients, but even they have their limits.  Additionally, many orchids will induce flowering or other growth when an influx of nutrients happens upon them, as a sudden spike in nutrition is a sign of spring, or the beginning of the rainy season.

Certain elements are only absorbed at certain pHs, so it’s important to keep your potting media fresh.  Old media acidifies over time and turns sour for the plant, preventing the absorption of certain nutrients. 

Water Quality and Fertilizer

If you have naturally soft water (water low in dissolved solids), then you generally should not have problems with salt buildup.  Growers whose water comes from aquifers typically have a high amount of dissolved solids in their water.  Generally, water with less than 525 ppm (parts per million) of total dissolved solids is considered tolerable for most orchids[2].  However, water with less than 175 ppm of total dissolved solids is truly ideal, as it mimics rainwater[3].  Salt buildup is a problem that can also affect growers that overfertilize but is typically from watering with hard water.  In many areas, this is unavoidable, so some people treat their water.  You must be very careful – some water softeners add sodium to water, which kills plants nearly instantly.  They also add chlorine, which is falsely blamed for plant maladies.  You may have heard of leaving water out to remove the chlorine, which may hurt plants.  In fact, chlorine is an essential micronutrient that plants need to function, and is so abundant in nature, that we do not need to supplement it.  Additionally, the concentration of chlorine in the water is not harmful to plants.  If it’s safe for you to drink, it’s safe for the plants to drink!

Probably one of the best water sources that you can use from the tap is New York City tap water.  With a total dissolved solids rating of 77 ppm[4], it’s very pure, and very “soft” water.  It is also slightly acidic, which is desired by most orchids.

Fertilizer minerals are all considered chemically to be salts, and salts they are!  Therefore, it can be tricky to fertilize if you are working with “hard” water.  If possible, you can add weak acids to keep the salts from building up, or you can flush your orchids every two weeks/month.  It’s also recommended to use rain or distilled water if your water is too hard.

What does mobility mean for plant nutrients?

Plant nutrients are generally broken down into whether or not they are mobile in the plant once consumed.  When there are deficiencies, knowing where the deficiency is occurring can help with diagnostics.  Remember, plants will always prioritize new growth over rescuing old growth.  This is universal for all plants, and is a telltale for where the nutrients are being distributed.

For example, nitrogen (N), phosphorus (P), and potassium (K), are all mobile elements.  When there is a deficiency in a mobile nutrient, the plant will show symptoms in the old growth.  This is because the plant actively breaks down proteins and such in its old leaves and redistributes them in the newer growth.

For immobile elements such as calcium (Ca), when there is a deficiency, the plant will show symptoms in the new growth.  This is because the plant has run out of building blocks, and can’t free these nutrients from their older growths and can’t redistribute them.

So, what does each mineral do?


Nitrogen – Is a mobile element and is responsible for protein synthesis, which leads to growth[5] [6].

  • Interesting note: Excess nitrogen causes extra growth and a more robust plant.  While there is some truth to excess nitrogen causing vegetative growth, the growers who claim that nitrogen only affects foliar growth is erroneous.  Excess nitrogen causes growth of all parts of the plant, including flower, provided that the right flowering signals have been met for the plant[7].
  • Nitrogen deficiency typically manifests as canary-yellowing and lack of robust growth in the plant.  Sulfur deficiencies look identical to nitrogen deficiencies; for more information, see Sulfur.
  • The three major forms of nitrogen are nitrate, ammonium, and urea.  The table below demonstrates the differences amongst them.


Phosphorus – Is a mobile element, usually in the form of phosphate, and is essential to form new DNA.  New cells need new DNA, and flowers do too to create the pollinia and ovaries.  Therefore, Phosphorus is essential for flower production.

  • Phosphorus deficiency typically manifests as faded purple streaking along the edges of the leaves, and a loss of glossiness of the leaves.

Potassium – Is a mobile element and is a neutral salt when found in the form potassium chloride (KCl).  Potassium salts aid in water regulation.

  • Interesting note: The “salt burn” experienced by orchids is really just potassium excess, which causes the leaf tips to turn black.  Be careful!  Certain fungi also create the same black tips.  If you fertilize, and the new growth is free from black tips, then indeed it was a nitrogen deficiency.  Otherwise, you will need to treat for fungal/pathogens.
  • Interesting note: Tropical orchids are primarily found in leaf litter, epiphytic on trees, or lithophytic on cliffs or rocks. None of these habitats have large amounts of potassium relative to calcium and magnesium, and potassium is particularly rare in limestone lithophytic environments[9].  This is why, of all the macronutrients, you should use the least amount of potassium when fertilizing, if possible, and explains why fertilizer burn predominantly looks like just potassium burn.  Additionally, because orchids have evolved in such potassium-poor environments, they have evolved ways to hyperaccumulate potassium when it does come around, which can contribute to toxicity when potassium is in abundance in your fertilizer.

Sulfur – Is a mostly mobile element which is used for the two essential amino acids, cysteine and methionine.  Sulfur is critical for pathogen defense in plants[10] [11].

  • Interesting note: Because sulfur is used for protein synthesis, deficiencies are often confused with nitrogen deficiencies.  Remember to fertilize with sulfur AND nitrogen if you suspect a deficiency in one or the other.
  • Interesting note: While you can use wettable sulfur powder as a powerful fungicide, be aware that it will acidify your media if not properly buffered with some lime (Calcium carbonate)[12].  In many sulfur-containing fertilizers, the buffering has been done for you, and sulfur exists in those fertilizers typically as copper sulfate, calcium sulfate, or ammonium sulfate.

Calcium – Is an immobile element that is essential in forming healthy plant cell walls and resisting pathogens[13].  Calcium is also used in many enzymes, allowing for a healthy plant metabolism [14].

  • Interesting note: Calcium is relatively insoluble, and is responsible for “soap scum” and water hardness residue in your bathrooms.
  • Calcium burns from deficiency and excess look fairly similar.  Additionally, calcium burns look like damage from black rot or water molds.  Know the difference: calcium deficiencies always occur in brand new growth, are pitch black, and start at the tip and work their way down, often without a yellow border between black and green.
  • Calcium and magnesium are both in the same column in the periodic table, have the same charge, and therefore, compete with each other in the plant.  Because of this, ALWAYS balance calcium with magnesium!  Dolomitic lime is a reliable source of both elements in relatively good proportions.
  • Calcium deficiencies will manifest in the NEW growth, and the old growth will be fine.  This is because the plant cannot harvest and mobilize calcium from old growths in the event of a deficiency.

Magnesium – Is a mobile element that is essential for photosynthesis.  It is the element that fills in the center of the porphyrin ring of chlorophyll.  Without magnesium, there can be no photosynthesis, and therefore, virtually no life (unless you are a bacterium).

  • Interesting note: When magnesium is in short supply, you will get interveinal chlorosis – in orchids, this manifests as splotchiness, which is sometimes confused with a virus attack!  This is because orchids, while highly evolved for pollinator success, have not really evolved beyond a primitive vascular system[15].
  • Interesting notes: Magnesium has an interesting relationship with light and electricity, and in physics, indeed, they are related!  When lit with fire, magnesium is the brightest burning substance known to man[16].  This reaction involves hitting magnesium with a free electron (seen as a spark), to force it to release energy as light and heat, but mostly light.  As all things are balanced in nature (something that eastern religions were keen to pick up on), the reverse is true.  When struck with a photon of light, magnesium in the right configuration will easily throw its electrons, which is used in chlorophyll to kick off photosynthesis (electrons are a physical manifestation of energy, more or less). 
  • Magnesium deficiency in orchids looks like mottling of the leaves[17], and may be confused with symptoms of a virus.  The color of the leaves may even look a little “watercolor-y”.  Under chilly conditions, some orchids may blush purple as well.  This is due to degradation of chlorophyll and the plant’s inability to regenerate it, thus revealing the other colors that the leaf is (similar to autumn leaves).


Micronutrients are called such because they are essential, but only needed by the plant in minute quantities.  There are many micronutrients including, but not limited to:   (I will highlight a few, but not all of them below.)

Iron/Copper – Iron and copper form the electron-receiving components of the photosynthetic machinery of the plant.  They are essentially used by the plant in an electrochemical way to capture the sun’s energy to make carbohydrates (sugars).

Boron – Essential for many enzymes as well as proper cell formation.  Lack of boron results in instant black death of all new growth.

[1] https://www.austintexas.gov/sites/default/files/files/Watershed/growgreen/2017LPT/day-four/GGLPT-Day-Four-2017-Plant-Nutrition-Richards.pdf

[2] https://staugorchidsociety.org/PDF/SolubleSaltsbySueBottom.pdf

[3] https://staugorchidsociety.org/PDF/SolubleSaltsbySueBottom.pdf

[4] https://www1.nyc.gov/assets/dep/downloads/pdf/water/drinking-water/drinking-water-supply-quality-report/2020-drinking-water-supply-quality-report.pdf

[5] https://www.canr.msu.edu/news/knowing_nutrient_mobility_is_helpful_in_diagnosing_plant_nutrient_deficienc

[6] University of Missouri https://ipm.missouri.edu/MEG/2011/6/Diagnosing-Nutrient-Deficiencies/

[7] Wang, Y. T., & Chang, Y. C. A. (2017). Effects of nitrogen and the various forms of nitrogen on Phalaenopsis orchid—A review. HortTechnology, 27(2), 144-149. https://doi.org/10.21273/HORTTECH03204-16

[8] https://staugorchidsociety.org/PDF/OrchidMyths-UreabySueBottom.pdf

[9] Rick Lockwood (March, 2013) “What do orchids eat?”, an AOS publication. https://staugorchidsociety.org/PDF/WhatDoOrchidsEatbyRickLockwood.pdf

[10] https://www.frontiersin.org/articles/10.3389/fpls.2014.00779/full

[11] Künstler, A., Gullner, G., Ádám, A. L., Kolozsváriné Nagy, J., & Király, L. (2020). The versatile roles of sulfur-containing biomolecules in plant defense—A road to disease resistance. Plants, 9(12), 1705. https://doi.org/10.3390/plants9121705

[12] Paul Vossen, University of California Cooperative Extension.  2604 Ventura Avenue., Santa Rosa, CA 95403 https://vric.ucdavis.edu/pdf/soil/ChangingpHinSoil.pdf

[13] White PJ, Broadley MR. Calcium in plants. Ann Bot. 2003 Oct;92(4):487-511. doi: 10.1093/aob/mcg164. Epub 2003 Aug 21. PMID: 12933363; PMCID: PMC4243668. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4243668/

[14] https://www.pthorticulture.com/en/training-center/role-of-calcium-in-plant-culture/

[15] They really do not have similar parallel vasculature, as is common in other monocots, and this is odd.  This is a Christopher Satch theory.

[16] https://rutchem.rutgers.edu/cldf-demos/1016-cldf-demo-burning-magnesium

[17] https://staugorchidsociety.org/PDF/CalciumAndMagnesiumbySueBottom.pdf