Cal-Mag Supplement for Plants: When Cannabis Needs It and How Much to Use

Last updated September 27, 2026 by Shariful Alam Pavel

Scientifically reviewed by Dr. Lukman Syed Rony, PhD, plant microbiologist and Lab Director at Innovate Phytoceuticals. Reviewed September 2026.

Most growers reach for Cal-Mag the moment a leaf yellows, and that is often the wrong first move. Botanicare Cal-Mag Plus is a 2-0-0 plant supplement that adds calcium, magnesium and iron to a complete nutrient program. It does not replace a base fertilizer. It works best in three situations: reverse osmosis water, coco coir, and fast-growing fruiting crops. In hard tap water or rich soil it is often unnecessary, and adding it anyway can push feed strength too high. The current label rate is 5 ml per gallon for roots and 10–15 ml per gallon for a foliar spray. Before you dose, read the leaves: where the symptom starts tells you which nutrient is missing.

Most of the pages that rank for Cal-Mag are product listings from the brands and shops that sell it, so they rarely say when you don’t need it. The honest answer is narrower. Cal-Mag is a calcium and magnesium top-up for a base feed. Cannabis needs it when the water or medium brings little of either mineral: reverse osmosis, distilled or rainwater, and coco coir. With hard tap water it often needs none. At its label rate of 5 mL per gallon, Botanicare Cal-Mag Plus adds about 49 ppm of calcium. Hard tap water already carries hardness equal to 48–72 ppm of calcium. So test your water before you buy a bottle.

How we sourced this guide: Deficiency symptoms and dose targets come from peer-reviewed cannabis studies and North Carolina State University researchers. Product figures come from each maker’s label, product page or safety data sheet, checked on 27 September 2026. We did not test these products. UprightCBD sells no fertilizer and earns nothing from any product named here.

Cal-Mag for cannabis: key facts

• At the label rate of 5 mL per gallon, Botanicare Cal-Mag Plus adds about 49 mg/L of calcium and 18 mg/L of magnesium, by our calculation from its label and data sheet.
• The US Geological Survey classes water above 120 mg/L of hardness, measured as calcium carbonate, as hard.
• In a 2023 study of vegetative cannabis, 35–70 mg/L of magnesium in the feed was optimal, and 140 mg/L caused toxicity symptoms (Morad and Bernstein, Plants).
• Calcium deficiency showed first on new growth and magnesium deficiency on older, lower leaves in North Carolina State University research on hemp (Cockson et al., 2019).
• Coco coir releases potassium and sodium and binds calcium and magnesium in their place, according to coco substrate maker CANNA.
• Calcium reaches new growth through transpiration, so high humidity can starve young leaves of calcium even when the feed has enough (Cannabis Business Times, 2026).

What is a Cal-Mag supplement for plants?

A Cal-Mag supplement is a fertilizer additive that supplies calcium and magnesium, usually with a little nitrogen and iron, on top of a base nutrient feed. The products compared here contain no phosphorus or potassium, so none is a complete fertilizer.

“Cal-Mag” is a category, not one product. Cal-Mag Plus is Botanicare’s version, and other makers sell CALiMAGic, Bush Doctor Cal-Mag, PuurCalMag and Sensi Cal-Mag Xtra. Most labels read 1-0-0, 2-0-0 or 4-0-0 for nitrogen, phosphorus and potassium, with calcium and magnesium listed separately. Plant Cal-Mag is also different from the calcium-magnesium tablets sold as human dietary supplements, which share the name.

Cal-Mag deficiency signs in cannabis

Calcium deficiency shows first on new growth, as stunted, distorted young leaves and dying tips. Magnesium deficiency shows first on old, lower leaves, as yellowing between green veins. Nitrogen, potassium and iron shortages can look similar.

The clearest descriptions come from Cockson and colleagues at North Carolina State University (Applied Sciences, 2019). They starved hemp plants of one nutrient at a time in sand culture and recorded how symptoms developed. Symptoms appeared within 5 to 7 weeks. Leaf calcium fell from 3.73% of dry weight in fully fed plants to 0.39% in calcium-starved ones, and magnesium from 0.61% to 0.12%.

Calcium shortages can also show as “marginal necrosis, tip burn, distorted new leaves,” in young leaves and “developing floral structures.” NC State researchers Veazie, Cockson and Whipker described this in Cannabis Business Times in 2026. They point to water testing, substrate monitoring and tissue analysis as the way to keep calcium available. Nitrogen, potassium and iron shortages need their own fixes, not Cal-Mag.

When cannabis needs Cal-Mag, by water and grow medium

Cannabis needs Cal-Mag when its water or medium supplies little calcium and magnesium: reverse osmosis, distilled or rainwater, and coco coir. In soil watered with hard tap water, it often needs none.

The US Geological Survey defines hardness as “the amount of dissolved calcium and magnesium in the water.” It classes 0–60 mg/L as soft, 61–120 as moderately hard, 121–180 as hard and anything above 180 as very hard, all measured as calcium carbonate. Multiplying by 0.4 converts that to calcium, so hard water at 121–180 mg/L carries the equivalent of about 48–72 mg/L of calcium. Real tap water splits that load between calcium and magnesium, but the total is the point. Garden writer Robert Pavlis reports his own tap water at 114 ppm calcium and 34 ppm magnesium, more than twice the calcium a label dose of Botanicare Cal-Mag Plus adds.

Coco coir is the other common reason. Coco substrate maker CANNA explains that untreated coco holds sodium and potassium and releases them during cultivation. In its words, “calcium (Ca2+) and magnesium (Mg2+) bind to the coco substrate in their place.” Buffered coco has been pre-treated with calcium fertiliser to prevent that swap. Soil is the case where Cal-Mag is least often needed. Pavlis argues that most soils other than sand hold enough of both. University of Minnesota Extension notes that magnesium shortages in Minnesota home gardens are most likely “on sandy, low pH soils.”

How much Cal-Mag per gallon?

5 mL (1 teaspoon) per gallon is the standard label rate for most Cal-Mag supplements. At that rate, Botanicare Cal-Mag Plus adds about 49 ppm of calcium and 18 ppm of magnesium, a top-up rather than a full supply.

Calculated by UprightCBD from each label’s guaranteed analysis and the product’s published density. Botanicare’s 2022 safety data sheet gives 1.15 g/mL, and CALiMAGic’s 2012 data sheet a specific gravity of 1.1. 1 mg/L equals 1 ppm in water. We found no published density for the other products, so their doses cannot be converted.

Magnesium has a ceiling as well as a floor. Morad and Bernstein grew vegetative cannabis at five magnesium levels from 2 to 140 mg/L (Plants, 2023). They found 35–70 mg/L optimal and wrote that “35 mg L−1 Mg can be applied in commercial production.” At 140 mg/L, toxicity symptoms appeared on the old leaves, and more magnesium “impaired uptake of Ca and K.” Magnesium from the base feed, the water and a Cal-Mag top-up all add together, so a bigger dose is not automatically safer.

Water decides the dose more than the bottle does. As a starting point, we suggest the full label rate for reverse osmosis, distilled or soft water, and half the rate for moderately hard water. For hard water, we suggest none unless symptoms and a test point to a shortage. That is UprightCBD’s reading of the arithmetic above, not a maker instruction.

How to use Cal-Mag, step by step

Using Cal-Mag takes five steps. The decisive one is testing your water first, because hard tap water can already carry as much calcium as a full label dose.

1. Test your water. Find its hardness in your water utility’s annual quality report or with a hardness test kit, and compare it with the USGS classes above.
2. Dilute Cal-Mag in plain water. Never mix it straight into other concentrates. General Hydroponics warns that mixing with concentrates high in phosphates or sulfates “will trigger chemical precipitation.” Follow your nutrient line’s mixing order.
3. Dose for your water. Start at the label rate for reverse osmosis or soft water and lower it as hardness rises.
4. Adjust the pH. Botanicare’s range is 6.2–7.0 and FoxFarm’s is 5.6–6.8. FoxFarm’s label adds: “A pH outside this range can impact the availability of calcium and magnesium.”
5. Watch where symptoms appear. New growth points to calcium and old leaves to magnesium. If symptoms persist at the label rate, check humidity and pH before adding more.

Coco growers usually start before planting. General Hydroponics’ label, as listed by HTG Supply, tells coco growers to apply 1 teaspoon per gallon “to wet the media and to nutrient solution for the first two weeks.” Makers disagree on flowering. Botanicare’s feed sheet runs 3–5 mL per gallon through most of flowering and stops for the final two weeks. FoxFarm says to “begin use just before bud development” and to use it “not more than once per week.” Follow the label of the product you bought.

When Cal-Mag is not the answer

Cal-Mag is not the answer when calcium cannot reach new growth or the root zone is out of range. High humidity, off-range pH, excess magnesium and already-hard water all cause or mimic deficiency on their own.

High humidity and still air

Calcium moves into new tissue “almost exclusively by transpiration,” write Veazie, Cockson and Whipker. When “high humidity, low vapor-pressure deficit, or restricted air movement” slow transpiration, calcium delivery to young leaves “can be restricted even when sufficient Ca fertility is available.” More Cal-Mag does not fix that. Better airflow and lower humidity do.

pH outside the maker’s range

FoxFarm’s label ties calcium and magnesium availability to a pH of 5.6–6.8. Measure pH after every addition, because each product shifts it.

Too much magnesium

In the 2023 cannabis study, surplus magnesium cut calcium and potassium uptake. University of Minnesota Extension gives the same warning for Epsom salt, noting that extra magnesium “can actually prevent adequate calcium from getting into your plants.”

Water that already carries it

Hard tap water can supply as much calcium as a label dose. Adding a full dose on top raises the feed’s salt load without fixing a shortage.

LED lights

Growers often blame LED grow lights for Cal-Mag problems. We found no controlled study showing that LED lighting itself raises calcium or magnesium needs. The documented route to calcium deficiency runs through transpiration, so check humidity and airflow first.

Six Cal-Mag supplements compared by label

Botanicare Cal-Mag Plus and General Hydroponics CALiMAGic are the only two here with a density we could find published, so their doses convert to ppm. FoxFarm Bush Doctor Cal-Mag and Cronk PuurCalMag are the organic-listed options.

Label analyses and rates from each maker’s product page, except CALiMAGic’s percentages and Emerald Harvest’s analysis, which come from HTG Supply listings. Checked 27 September 2026. Prices change too often to list.

For reverse osmosis water or coco, Botanicare Cal-Mag Plus and General Hydroponics CALiMAGic are the easiest to dose accurately, because their makers publish enough to convert a teaspoon into ppm. CALiMAGic carries more calcium per dose, and Cal-Mag Plus more nitrogen. Growers who want an organic-listed product can choose FoxFarm Bush Doctor Cal-Mag or Cronk PuurCalMag, and Cronk has the highest magnesium share. Emerald Harvest suits water that already has enough magnesium, since its calcium outweighs magnesium six to one. For the grow as a whole, see our indoor marijuana grow setup guide.

Epsom salt vs Cal-Mag

Epsom salt supplies magnesium only, about 9.9% by weight as magnesium sulfate heptahydrate, and no calcium. Use it only when a test shows magnesium is short, because extra magnesium can block calcium uptake.

University of Minnesota Extension says to “only use them if a soil test indicates that you have a magnesium deficiency,” and warns that Epsom salt sprays “can cause leaf scorch.” When water or medium lacks both minerals, a Cal-Mag product covers both in one dose.

Cal-Mag FAQs

How much Cal-Mag per gallon of water?

Most labels say 5 mL (1 teaspoon) per gallon. At that rate, Botanicare Cal-Mag Plus adds about 49 ppm of calcium and 18 ppm of magnesium. Use less with moderately hard water and usually none with hard water.

When should you use Cal-Mag in soil?

Only when your water is soft, reverse osmosis or rainwater, or when symptoms and a test show a shortage. Most non-sandy soils hold enough calcium and magnesium, and magnesium shortages are most likely on sandy, low-pH soils.

Do you need Cal-Mag with coco coir?

Usually yes, unless the coco is buffered. Untreated coco binds calcium and magnesium while releasing potassium and sodium, according to CANNA. General Hydroponics tells coco growers to add Cal-Mag for the first two weeks.

Can you use too much Cal-Mag?

Yes. In a 2023 study, 140 mg/L of magnesium caused toxicity symptoms in cannabis, and surplus magnesium reduced calcium and potassium uptake. Keep to the label rate and adjust for your water.

Should you use Cal-Mag during flowering?

Makers differ. Botanicare’s feed sheet runs Cal-Mag through most of flowering and stops for the final two weeks. FoxFarm starts its Cal-Mag just before bud development, no more than once a week.

Can you spray Cal-Mag on leaves?

Some labels allow it. Botanicare gives a foliar rate of 10–15 mL per gallon, two to three times its root-feeding rate. University of Minnesota Extension warns that Epsom salt sprays can scorch leaves.

Is plant Cal-Mag the same as Cal-Mag vitamins?

No. Plant Cal-Mag is a fertilizer additive for nutrient solutions. Calcium-magnesium tablets and drinks sold for people are dietary supplements with the same name.

Start with a water test

Before buying any Cal-Mag, find your water’s hardness. If you grow in coco or use reverse osmosis, distilled or rainwater, add Cal-Mag at the label rate and adjust pH afterward. If your tap water is hard and you grow in soil, skip it until new growth or old leaves show a pattern from the symptom table. When symptoms appear at a normal dose, check humidity, airflow and pH before adding more. For growing hemp indoors, our indoor hemp farming guide covers the rest of the grow. Home cultivation of cannabis is legal only in some states, so check your state’s rules first.

Sources: Cockson P, Landis H, Smith T, Hicks K, Whipker BE, “Characterization of Nutrient Disorders of Cannabis sativa,” Applied Sciences 9(20):4432 (2019); Morad D, Bernstein N, “Response of Medical Cannabis to Magnesium (Mg) Supply at the Vegetative Growth Phase,” Plants 12:2676 (2023); Veazie P, Cockson P, Whipker BE, “The Dangers of Calcium Deficiency in Cannabis Crops,” Cannabis Business Times (1 April 2026); US Geological Survey, “Hardness of Water”; CANNA, “How to use coco coir as a concept” (2018); University of Minnesota Extension, “Do common soil health home remedies work?”; Robert Pavlis, “Cal-Mag for Plants,” Garden Myths (updated 2024); product pages and labels for Botanicare Cal-Mag Plus, General Hydroponics CALiMAGic, FoxFarm Bush Doctor Cal-Mag, Cronk PuurCalMag and Advanced Nutrients Sensi Cal-Mag Xtra; Botanicare Cal-Mag Plus safety data sheet (2022) and feed sheet; General Hydroponics CALiMAGic data sheet (2012); HTG Supply listings for CALiMAGic and Emerald Harvest Cal-Mag. All checked 27 September 2026.