CalMag is a calcium, magnesium, and iron supplement for cannabis plants. Cronk CalMag (2-0-0) delivers 3.2% calcium, 1.2% magnesium, and 0.1% chelated iron per dose. In coco coir, it's required at every feed (2-5 mL/gal) because coco's cation exchange capacity actively binds calcium out of solution. In soil with tap water, it's often optional. With RO water in any medium, it's non-negotiable. It always goes second in the mixing order, after silica and before Micro.

What Is CalMag?

CalMag is a supplemental nutrient formula that delivers calcium, magnesium, and iron to cannabis plants. It's not a base nutrient. It doesn't replace your Micro, Grow, or Bloom. It fills the specific gap that most water sources and growing media can't cover on their own.

Cronk CalMag carries an NPK of 2-0-0. The breakdown is: 3.2% calcium, 1.2% magnesium, 0.1% chelated iron (from Iron EDTA), and 2% nitrate nitrogen used as a carrier for the chelate chemistry. It's derived from calcium nitrate, magnesium nitrate, and iron EDTA. All tested by A&L Canada Laboratories (Report C24036-20002, Feb 2024) and confirmed clean on arsenic, cadmium, mercury, and lead.

These three elements work together. Calcium builds cell walls and moves nutrients into the plant. Magnesium is the central atom in every chlorophyll molecule, which means no magnesium means no photosynthesis. Chelated iron enables chlorophyll to form in the first place. Deficiency in any of the three shows up fast, and they all look similar enough that diagnosis takes some care.

What Is CalMag Used For?

CalMag solves a very specific problem: cannabis plants are calcium-hungry, and most water sources plus most growing media can't supply enough without help.

Here's what happens without it. Calcium deficiency in cannabis always shows up on new growth first. That's because calcium is immobile in the plant. Once it's locked into a leaf, it stays there. The plant can't pull it back and redirect it to new growth. So you see the damage on the youngest tissue: new leaves pale at the tips, brown spots appearing with yellow halos, tips curling up or under, growth coming in crinkled or distorted. As it worsens, new shoots come in smaller and the plant can't push canopy effectively.

Magnesium deficiency looks different. It shows up on older leaves first, because magnesium is mobile and the plant pulls it from older tissue when new growth needs it. The sign to look for is interveinal chlorosis: yellowing between the leaf veins while the veins themselves stay green. It can look like a general nitrogen issue, but nitrogen deficiency yellows the whole leaf uniformly and starts at the bottom. Interveinal yellowing on mid-canopy leaves is magnesium.

One important note before diagnosing either: check your pH first. Calcium locks out below 6.0 in coco. Magnesium locks out below 5.5 in any medium. Adding more CalMag to a pH lockout situation just raises your EC without fixing anything. Confirm pH, then assess whether you actually need more supplement.

Do You Need CalMag in Coco Coir?

Yes, every time. This is the one medium where CalMag is genuinely non-negotiable.

Coco coir is made from coconut husks. During processing, those fibers carry high concentrations of potassium and sodium bound to their cation exchange sites. When your nutrient solution hits fresh coco, those exchange sites trade: they grab your calcium and magnesium and release potassium back into solution. This is the cation exchange capacity (CEC) at work. Coco's CEC ranges from roughly 40 to 100 meq/100g depending on brand and processing method, which is genuinely high. The practical result is that coco will strip calcium out of your solution until its exchange sites are fully saturated.

This is also why pre-buffering matters. Before you plant into fresh coco, soak it with a 2 mL/gal CalMag solution at pH 5.8-6.0 and let it drain. That pre-soak saturates the exchange sites before your roots ever contact the medium. Without it, your first weeks of feeding are going toward feeding the coco, not the plant.

Even after buffering, you keep running CalMag at every feed through most of the grow. The feed chart dosing for coco runs 2 mL/gal in early veg, peaks at 5 mL/gal in early flower, then tapers off before flush. See the full dosing table below.

CalMag Dosage by Grow Medium

The right CalMag dose depends entirely on your medium and water source. Here's the full breakdown from the Cronk feed charts:

For coco specifically, the stage-by-stage dosing from the Classic 3-Part Intermediate program (drain-to-waste coco) looks like this:

The dose peaks in early flower because that's when the plant is making new growth most rapidly, building bud sites, and stretching hard. That's peak calcium demand. It tapers and stops before Ripen because you're winding the plant down, not building new tissue.

For hydro, the dosing is on a separate recirculating schedule. In DWC and RDWC, CalMag starts at 1 mL/gal in V1, runs at 4-6 mL/gal through veg, steps up to 6-8 mL/gal during early and mid flower, then tapers the same way. Change your reservoir every 7-10 days and top up with fresh water in between. Full schedule on the Cronk feed charts page.

When You Need CalMag (and When You Don't)

CalMag is one of the most over-dosed supplements in the grow room. Some growers run it at 5+ mL/gal across every medium because they assume more is always better. It isn't. Here's how to think about it.

You almost certainly need CalMag if: you're growing in coco coir, you're using RO or distilled water in any medium, you're running LED lights (which run cooler and can affect transpiration and calcium transport [VERIFY]), or you're already seeing brown spotting on new growth or interveinal yellowing on older leaves.

You may not need CalMag if: you're growing in soil with tap water that has natural calcium hardness, your tap EC is already above 0.3-0.4 mS/cm from dissolved minerals, or your base nutrients already contain enough calcium for the stage you're in. Cronk Micro (5-0-1) does contain 5% calcium, which counts toward your plant's total calcium supply. In soil with mineral-rich tap water, between Micro and the natural water hardness, some growers get through veg without CalMag supplementation.

That said, CalMag is inexpensive insurance. If you're uncertain, running 2 mL/gal in soil with tap water costs you almost nothing and removes one variable from your diagnostic work.

The Correct Mixing Order

This is where growers lose calcium and magnesium availability without realizing it. The order you add nutrients to water is chemistry, not preference.

The problem: calcium and phosphorus in high concentration react to form calcium phosphate, which precipitates out of solution. Your mix goes cloudy. Those locked-up minerals never reach your roots. If you add Bloom before CalMag, or add CalMag and Bloom to the same concentrated solution without water, you're throwing nutrients away.

Here's the correct mixing order for Cronk Classic nutrients:

1. Start with water. Fill your container first. Never pre-mix nutrients without water.
2. Armadillo Armour first (if using). Add and wait 2-5 minutes. Monosilicic acid reacts with calcium if other nutrients are added too quickly. Let it integrate.
3. CalMag second. Add at your dose and stir. This is the rule regardless of whether you're using Armadillo Armour. CalMag always goes before base nutrients.
4. Micro next. Always before Grow or Bloom. Adding Bloom before Micro causes calcium-phosphate precipitation because Bloom has phosphorus and you haven't diluted the calcium adequately yet.
5. Grow and/or Bloom. At your stage-appropriate ratio.
6. Additives. Bud Booster, Sticky Bandit, any other extras.
7. Adjust pH. Target 5.8-6.2 for coco and hydro. 6.0-6.5 for soil.
8. Monkey Juice last. After pH is locked in. The beneficial bacteria are pH-sensitive and can't go in before you've finalized pH.

Need the full breakdown on why each step is in this order? See the full Cronk mixing order guide.

Common Mistakes to Avoid

Skipping the pre-buffer in coco. Before planting, soak fresh coco with a 2 mL/gal CalMag solution at pH 5.8-6.0 and let it drain fully. One good soak is enough. Skipping this step means your first 1-2 weeks of feeding are saturating the coco's exchange sites instead of feeding your seedling.

Overdosing. Running CalMag consistently above 6-7 mL/gal raises your EC and can cause calcium and magnesium to compete with each other and with potassium uptake. Follow the feed chart. If you think you need more, check your runoff EC and pH before increasing the dose.

Treating lockout like deficiency. Brown spots on new growth and interveinal yellowing can be pH lockout just as easily as a true deficiency. If your pH has drifted above 6.5 in coco, calcium becomes unavailable no matter how much you feed. Fix the pH first, wait 3-5 days, then reassess whether you need more CalMag.

Dropping CalMag too early in flower. Some growers cut it when flower starts, assuming it's only a veg supplement. Look at the feed chart. It runs through F7 because flowering plants are still building new tissue and stretching hard in the first half of flower. Cutting it early leads to spotting on bract tissue and loose bud structure.

Confusing CalMag with PuurCalMag. If you're on the PuurOrganics line, you use PuurCalMag (derived from calcium carbonate and magnesium chloride), not Cronk CalMag 2-0-0. The synthetic CalMag is not compatible with PuurOrganics base nutrients.

Frequently Asked Questions

What is CalMag?

CalMag is a calcium, magnesium, and iron supplement for cannabis plants. Cronk CalMag has an NPK of 2-0-0 and contains 3.2% calcium, 1.2% magnesium, and 0.1% chelated iron. It's used alongside base nutrients to prevent deficiency in coco coir, RO water grows, and any situation where water or medium doesn't supply enough calcium and magnesium on its own.

What is CalMag used for?

CalMag supplements the calcium and magnesium that cannabis plants need for cell wall integrity, chlorophyll production, and overall growth. It's most critical in coco coir (because coco's CEC binds calcium out of solution) and with RO or distilled water (because those water sources have no mineral content). It also supplies chelated iron, which helps with chlorophyll synthesis in new growth.

Do I need CalMag in coco?

Yes, every time. Coco's cation exchange capacity pulls calcium out of your nutrient solution and holds it, so plants can't access it even when it's present in your feed. Running CalMag at 2-5 mL/gal in coco, at every watering, is standard practice. Pre-buffering fresh coco with a 2 mL/gal CalMag soak before planting also helps saturate those exchange sites in advance.

How much CalMag should I use in coco?

In coco coir, run Cronk CalMag at 2 mL/gal during early veg, stepping up to 4 mL/gal at pre-flower and 5 mL/gal during early flower. Drop back to 2 mL/gal for late flower and stop for Ripen and Flush. These numbers come directly from the Cronk Classic 3-Part Intermediate feed chart for drain-to-waste coco.

When should I add CalMag in the mixing order?

CalMag goes second in the mixing order: after Armadillo Armour (silica, if you're using it) and before Micro, Grow, or Bloom. Adding phosphorus-containing nutrients before CalMag in a concentrated solution causes calcium and phosphorus to bind together, precipitating out of solution where roots can't access them.

Do I need CalMag in soil?

In soil with tap water, CalMag is often not needed. Soil buffers calcium naturally through its organic matter and mineral content, and most tap water supplies some baseline calcium hardness. With RO or soft water in soil, add CalMag at 4-5 mL/gal at every feed. If you're on tap with an EC above 0.3-0.4 mS/cm, check your runoff before adding CalMag at all. You may already have enough.

What does a CalMag deficiency look like?

Calcium deficiency shows on new growth first: brown spots or rusty speckling on young leaves, tips curling, distorted unfurling. Magnesium deficiency shows on older leaves: yellowing between the veins while the veins stay green. Before adding more CalMag, check your pH. Both deficiencies can be caused by pH lockout (calcium locks out below 6.0 in coco, magnesium below 5.5), which won't respond to more CalMag at all.

Can I use CalMag with PuurOrganics?

No. Cronk CalMag 2-0-0 is not compatible with the PuurOrganics line. Use PuurCalMag instead. PuurCalMag is derived from calcium carbonate and magnesium chloride chelated with acetic acid, formulated to work within the organic system. Never mix CalMag 2-0-0 into a PuurOrganics feed.

Key Takeaways

• CalMag is a 2-0-0 supplement delivering calcium (3.2%), magnesium (1.2%), and chelated iron (0.1%). It fills the gap that most water sources and growing media can't cover on their own.
• In coco coir, CalMag is required at every feed. Coco's cation exchange capacity binds calcium out of solution before roots can absorb it, making deficiency almost inevitable without supplementation.
• Dose for coco runs 2 mL/gal in early veg, peaks at 5 mL/gal in early flower, then tapers and stops before flush. In soil with tap water, it's often optional. With RO water in any medium, it's essential.
• Mixing order is chemistry: CalMag goes second in the order, always before Micro, Grow, and Bloom. Adding calcium and phosphorus together in a concentrated solution causes precipitation and locks both out.
• Before adding more CalMag, confirm your pH. Calcium locks out below 6.0 in coco, magnesium below 5.5 in any medium. Lockout symptoms are identical to true deficiency and don't respond to more CalMag.
• PuurOrganics growers use PuurCalMag, not CalMag 2-0-0. The two are not interchangeable.

Related Articles

• Cronk Feed Charts: Full week-by-week dosing schedules for Classic, Autoflower, and PuurOrganics lines across all grow media.
• Additives and Boosters Guide: Full breakdown of all Cronk additives, when to use each one, and how they fit into your feed program.
• Cronk Nutrient Lines Overview: Classic 3-Part, Autoflower (Bonnie and Clyde), and PuurOrganics. Which line fits your grow.

Shop Cronk CalMag. Available in 500 mL, 1 L, 4 L, and bulk sizes on special order. Free shipping on orders over $125.

Questions about your grow? Reach the support team at growers.cronknutrients.com/contact or email [email protected].