Secondary Macronutrients: What Are They? Why Should We Care?

Secondary Nutrients: What Are They? Why Should We Care?

Hi Everybody! Welcome to this installment of Dr. NPK! This post is dedicated to the unsung heroes (if you’re a football fan like me, this is the “O-Line” tribute for cannabis science): the secondary elements! So much 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 calcium (Ca), magnesium (Mg), sulfur (S), and iron (Fe). I want to make one important distinction: many people categorize iron as a micronutrient and thus would not be considered a secondary macronutrient; I make a personal choice to incorporate iron into my list of secondary macronutrients for two reasons: 1) it is typically used at a higher treat rate than many other micronutrients, and 2) iron deficiencies often present the same symptoms as those of secondary macronutrient deficiencies.

Calcium: Builds Strong Bones and……….Bud?

By far, calcium is the most commonly discussed secondary nutrient. Water-soluble calcium is usually found in the highest concentration in fertilizer formulations of the secondaries. Calcium is usually listed as “water soluble calcium” on the back of fertilizer bottles; for some organic products, it is a suspension in water (usually derived from dolomite, a calcium/magnesium carbonate). The most commonly used source for fully water-soluble formulas is calcium nitrate. Elite CalMag (COMING SOON!) and Elite Base Nutrient A both contain high concentrations of fully water-soluble calcium.

Calcium plays an important role in the cannabis plant: when you think of calcium, you should think of “cell wall.” Calcium ions (Ca2+ if you really want to get into it) helps to develop the cell wall. Establishment of the cell wall is important to 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.

One final note: ever add in your total nutrient regiment and notice some white solid show up out of nowhere, quickly? Don’t worry, that’s likely not biological (don’t you wish that was the news you always got?): it’s likely a sign of nutrient lockout. You are likely 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.

Magnesium: Keepin’ it Green

Following behind calcium is magnesium on the secondary nutrient totem pole. Water-soluble magnesium typically runs in smaller concentrations (1–3%); sources of magnesium in liquid nutrients include magnesium nitrate, magnesium sulfate (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 mean customer one time ask me “where are your chlorophyll pills?” To which I replied, “go out and eat a plant.” Yes, I was written up and probably should not have been so snappy (hindsight is 20/20). But, 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, too little magnesium = less chlorophyll = crappy photosynthesis.

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

Silly Sulfur, S is for Kids

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 like fart. Regardless the smell, plants absolutely need sulfur. Sulfur is provided to the plant in a variety of ways: elemental sulfur (as a milky suspension), the sulfate (SO42-) ion (such as in magnesium sulfate) which is also the most plant-available form, or bisulfite/sulfite (SO32- and HSO3-, respectively) in sodium bisulfite. High concentrations of water-soluble sulfur are found in Elite Base Nutrient B. Our source is via magnesium sulfate, which is the most plant-available form of sulfur available on the market. Oh yeah, and it doesn’t smell like fart. That’s good.

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

Sulfur is important in the plant for a variety of reasons. Although it is important in catalytic processes and chlorophyll production, one of its hallmark attributes is via amino acid production. Amino acids are the building blocks of proteins, and these proteins play a pivotal role in the strength of your plant as well as your yields. Sulfur is also important in essential oil production (http://www.sulphurinstitute.org/fertilizer). Thus, too little sulfur = less amino acids = lower yield + plant strength. No one lines droopy plants!

Pumping Iron

Although needed in significantly smaller concentrations, iron is 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; 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 has 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 = no chlorophyll = no green = no green, in the bud sense).

Closing Thoughts

Although used in lower quantities, secondary macronutrients play a pivotal role in the plant growth. You don’t pay attention to them, and 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!


Who is Dr. NPK? With a PhD in Chemistry from UC Irvine, Dr. NPK was instrumental in developing the Elite Nutrient line.  Learn more about him here.