Secondary Macronutrients: What are they, and why should we care?

Hi everybody! Welcome to this installment of Dr. NPK! This post is dedicated to the unsung heroes: the secondary elements! If you’re a football fan like me, this is the “O-Line” tribute for cannabis science.

A lot of focus is placed on the primary macronutrients (nitrogen, phosphorous, and potassium, a.k.a. N-P-K), but these elements can’t do their job without the right supporting staff. I’m talking about the secondary macronutrients:

1. Calcium (Ca)
2. Magnesium (Mg)
3. Sulfur (S)
4. Iron (Fe)

I want to make one important distinction: many people categorize iron as a micronutrient and thus would not be consider it a secondary macronutrient; I make a personal choice to incorporate iron into my list of secondary macronutrients for two reasons:

1. Iron is typically used at a higher treat rate than many other micronutrients
2. Iron deficiencies often present the same symptoms as those of secondary macronutrient deficiencies.

By a mile, calcium is the most commonly discussed secondary nutrient. Calcium plays an important role in any plant, not just cannabis. When you think of calcium, you should think of the cell wall. Calcium ions (Technically Ca²⁺ if you really want to get into it) helps plants develop their cell walls. Cell walls prevent diseases as well as to ensure proper and even growth. Calcium deficiencies tend to manifest in the growth of younger leaves; calcium ions are immobile in the plant, so a deficiency in calcium will affect the younger leaves.

Water-soluble calcium is usually found in the highest concentrations in fertilizer formulations that include the secondaries. Calcium is typically listed as “water-soluble calcium” on the back of fertilizer bottles; for some organic products, it is a suspension in water what is usually derived from dolomite, a calcium/magnesium carbonate. The most commonly used source for fully water-soluble formulas is calcium nitrate. Elite CalMag and Elite Base Nutrient A both contain high concentrations of fully water-soluble calcium.

One final note: Have you ever added your total nutrient regiment into you tank and notice some white solid show up out of nowhere, quickly? Don’t worry, that’s likely not something biological (don’t you wish that was the news you always got?), it’s a sign of nutrient lockout. In all likelihood, you are forming one of a few insoluble salts: calcium phosphate, calcium sulfate, or maybe even calcium silicate. These salts are practically insoluble, but also harmless to the plant.

Following behind calcium on the secondary nutrient totem pole is magnesium. Water-soluble magnesium typically runs in smaller concentrations (Approximately 1-3%). Sources of magnesium in liquid nutrients include magnesium nitrate, magnesium sulfate (commonly called Epsom salt), or magnesium acetate. Elite CalMag and Elite Base Nutrient B contain high concentrations of water soluble magnesium.

I used to work at Rite Aid when I was in high school/college. I had a rude customer ask me one time where our chlorophyll pills were. I replied that perhaps he should go outside and eat a plant. Yes, I was formally disciplined and probably should not have been so snappy (hindsight is 20/20), but I promise you that this leads me to my next point: magnesium!

Magnesium is an important element in photosynthesis. Magnesium is believed to be a key contributor in converting sunlight to actual energy within the plant, likely due to its influence on chlorophyll (Yang et al, The Crop Journal 4 2016, 83–89). Thus, insufficient magnesium means fewer chlorophyll and therefore crappier photosynthesis.

Magnesium deficiencies tend to present themselves in older leaves as magnesium is a mobile nutrient. The plant will direct what little magnesium it has to new growth, leaving older leaves magnesium deficient.

Epsom salts may be great for tub soaking… but they’re also a great source of nutrients for cannabis!

Sulfur is an interesting element. The crux of my Ph.D. is based on this element. My wife isn’t a fan: sulfur compounds usually smell of rotten eggs (or for the less mature of you, farts). Regardless of the smell, plants absolutely need sulfur. Sulfur can be provided to the plant in a variety of ways:

1. Elemental sulfur (as a milky-yellow suspension)
2. The sulfate (SO₄^2-) ion (such as in magnesium sulfate) which is also the most plant-available form.
3. Bisulfite/sulfite (SO₃^2- and HSO^3-, respectively) in sodium bisulfite.

Sulfur is important in plants for a variety of reasons. Although it is important in catalytic processes and chlorophyll production, one of its hallmark attributes is its role in amino acid production. Amino acids are the building blocks of proteins, and proteins play a pivotal role in the strength of your plant as well as your yields. Sulfur is also important in essential oil production. Thus, insufficient sulfur means fewer amino acids and a reduced yield and droopy plants. No one likes droopy plants!

High concentrations of water-soluble sulfur are found in Elite Base Nutrient B, via magnesium sulfate, which is the most plant-available form of sulfur available on the market. Oh yeah, and it doesn’t smell like rotten eggs, so that’s a plus.

Although needed in significantly smaller concentrations, iron is still a vital element for cannabis growth. I will go into significantly more detail about iron in a later post, so a broad overview will suffice here. Iron is an ion that can be attacked by many other ions (Editor’s Note: “Attacked” is chemistry-speak for a redox reaction); to avoid this in your reservoir, iron should be supplied in chelated form; this protects the iron ion. The most commonly supplied form of plant-available iron is iron EDTA. Iron is found in Elite Base Nutrient A and Elite CalMag.

Iron serves a variety of functions in the plant, mostly involved with chlorophyll and chloroplasts. Iron, like magnesium, plays an important role in the development of chloroplasts, which are a key component of chlorophyll (Rout, G.; Sahoo, S. Reviews in Agricultural Science 2015, 3, 1–24). This is why iron deficiencies are categorized by the browning of leaves. No iron means no chlorophyll, which means no green, therefore you make no green, in the bud sense.

Although used in lower quantities, secondary macronutrients play a pivotal role in the plant growth. If you don’t pay attention to them, your plants will suffer. Elite Base Nutrient A and B provide all the secondary nutrients you may need. If you’re using RO water, Elite CalMag will provide the right supplement to your water to make sure you’re getting all those important secondaries!

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