# Cal-Mag: what it is, when you need it and when you don't

> Cal-Mag is a calcium and magnesium supplement, usually calcium nitrate and magnesium nitrate, sometimes with iron. You need it when your water and base nutrient together supply too little Ca and Mg: RO or very soft water, new or unbuffered coco, or a base nutrient designed for hard water. How much a plant needs is driven mainly by its cultivar, the water source, the medium (coco's cation exchange), root-zone pH and transpiration. The claim that LED lights create a Cal-Mag need is a myth. You do not need it on hard tap water with a complete base feed, and overusing it adds nitrogen and pushes potassium and magnesium out of balance.

- Canonical: https://genetixbay.com/knowledge/nutrients/cal-mag.html
- Category: Nutrients · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Veg, Flower · Systems: Coco, DWC, RDWC, NFT, Drip, Rockwool, Soil
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**Do I need Cal-Mag with RO water?** With RO water, cannabis usually needs Cal-Mag or another calcium and magnesium source, because RO supplies essentially none and many base nutrients assume some comes from the tap. Add only the gap between your base and the targets of about 150-200 mg/L total calcium and 40-70 mg/L magnesium. Grower practice is roughly 40-60 mg/L Ca and 15-25 mg/L Mg from the supplement.

**How much Cal-Mag should I use in coco?** Cannabis in coco should get enough Cal-Mag to bring total solution calcium to about 150-200 mg/L and magnesium to about 40-70 mg/L, at a calcium to magnesium mass ratio near 3:1 to 4:1. Buffer new coco first, because its exchange sites hold potassium and sodium and soak up calcium and magnesium until saturated. After buffering, a smaller ongoing top-up is often enough.

**Do [LED](https://genetixbay.com/knowledge/glossary/led.md) lights need more Cal-Mag?** LED lights do not make cannabis need more Cal-Mag; that claim is a myth and contradicts the GenetixBay house position. A photon does not consume calcium. LEDs emit less radiant heat, so leaves can run cooler, [transpire](https://genetixbay.com/knowledge/glossary/transpiration.md) less and deliver less calcium to new growth. That is a VPD setting, so raise air temperature a degree or two or lower humidity before adding a supplement.

**Why does one strain need Cal-Mag and another doesn't in the same room?** One cannabis cultivar can need Cal-Mag while another does not in the same room because calcium and magnesium demand is largely genetic. In thirteen [hemp cultivars](https://genetixbay.com/knowledge/glossary/hemp.md) grown side by side, leaf calcium varied more than any other nutrient, and two medical [genotypes](https://genetixbay.com/knowledge/glossary/genotype.md) moved nutrients differently on the same potassium feed. When only one plant shows symptoms, suspect the genetics before the bottle.

**I switched from HPS to LED and got rust spots: is it Cal-Mag?** Rust spots after switching a cannabis room from HPS to LED are not caused by the LED itself, so do not assume a Cal-Mag shortage. Cooler leaves under LED lower leaf VPD and slow calcium delivery to new growth, so adjust temperature or humidity first. Many growers changed to coco and RO water at the same time, so check water, medium, pH and potassium too.

**Can I use too much Cal-Mag?** Yes, too much Cal-Mag harms a cannabis feed. Most products are nitrate-based, adding roughly 1 mg/L nitrogen for every 1.2 mg/L calcium, and overuse pushes potassium and magnesium out of balance; the extra nitrogen is unwanted late in flower. Avoid full-rate doses on [hard tap water](https://genetixbay.com/knowledge/glossary/tap-water.md) 'just in case', and never use Cal-Mag to treat broad-mite damage or pH problems.

**Should I add Cal-Mag if I have hard tap water?** On hard tap water with a complete base feed, cannabis growers usually should not add Cal-Mag. Water above about 60 mg/L calcium or 25 mg/L magnesium is classed as hard and can supply much of what the plant needs. Read the water report and the base nutrient label, then add only the gap to reach about 150-200 mg/L calcium and 40-70 mg/L magnesium.

**Is Cal-Mag the same as [Epsom salt](https://genetixbay.com/knowledge/glossary/magnesium-sulfate.md)?** Cal-Mag is not the same as Epsom salt. Epsom salt is magnesium sulfate only and contains no calcium. Cal-Mag supplements are usually [calcium nitrate](https://genetixbay.com/knowledge/glossary/calcium-nitrate.md) plus [magnesium nitrate](https://genetixbay.com/knowledge/glossary/magnesium-nitrate.md), sometimes with a little iron, so they also add nitrogen to the feed. Check the label of any Cal-Mag product to see how much nitrogen it contributes before using it late in flower.

**Short answer.** Cal-Mag is a calcium-plus-magnesium supplement, usually calcium nitrate and magnesium nitrate, sometimes with a little iron. You need it when your water plus base nutrient supply too little Ca and Mg: **RO or very soft water, new or unbuffered coco, or a base formulated for [hard water](https://genetixbay.com/knowledge/glossary/general-hardness.md).** You do not need it on hard tap water with a complete base feed. Overuse adds nitrogen and pushes K and Mg out of balance. **"LEDs need more Cal-Mag" is a myth.** Demand is set mainly by the **cultivar**, then by your **water**, your **medium**, **root-zone pH** and **transpiration**, not by the light source.

## What's happening

Many base nutrients are designed assuming some Ca and Mg come from the tap. Hard water commonly supplies tens of mg/L of each; Purdue classes water above about 60 mg/L Ca or 25 mg/L Mg as hard [3]. RO water supplies essentially none. A feed that is balanced on tap water can therefore be Ca- and Mg-short on RO.

Coco makes it worse when new: its exchange sites hold K and Na and soak up Ca and Mg from the feed until they are saturated [4] (see [buffering coco](https://genetixbay.com/knowledge/ph-ec/buffering-coco.md)).

Cal-Mag is not Epsom salt. Epsom salt is magnesium sulfate only. Most Cal-Mag products are nitrate-based, so they also add roughly 1 mg/L N for every 1.2 mg/L Ca (from the calcium nitrate) plus N from magnesium nitrate. Check your label.

### What actually drives Ca and Mg demand

1. **Cultivar.** Genetic demand varies. Two cultivars in the same room, on the same feed, can show very different Ca and Mg status. In thirteen hemp cultivars grown side by side, leaf Ca had the widest spread of any nutrient [7], and two [medical cannabis](https://genetixbay.com/knowledge/glossary/medical-vs-adult-use.md) genotypes differed in how they took up and moved nutrients on the same K feed [5]. If one plant in a mixed room shows Ca/Mg signs and the rest don't, suspect the genetics before the bottle.
2. **Water source.** RO and soft water bring almost no Ca or Mg; hard tap water can bring most of what the plant needs [3].
3. **Medium.** Fresh coco's exchange sites are loaded with K and Na and pull Ca and Mg out of the feed until buffered [4].
4. **Root-zone pH.** Low pH cuts Ca and Mg availability; see [pH and nutrient availability](https://genetixbay.com/knowledge/ph-ec/ph-nutrient-availability.md).
5. **Transpiration and VPD.** Ca moves only in the transpiration stream [6]. Cool leaves in a humid room transpire little, so new growth gets little Ca whatever the feed holds.

> **Greenleaf aside: the LED myth.** "LEDs need more Cal-Mag" is a *Myth*. A photon doesn't consume calcium. The kernel of truth is transpiration: LEDs put out less radiant heat, so leaves often run cooler than under HPS at the same air temperature [8], leaf VPD drops, and Ca delivery to new growth slows [6]. That is an environment setting, not a light-source requirement. Many growers also moved to LEDs at the same time as they moved to coco and RO water, which gets the blame mixed up. If you switch to LEDs, set leaf VPD (raise air temperature a degree or two, or lower RH) and check your water and medium before you reach for a supplement.

## Numbers that matter

| Parameter | Target | Notes |
|---|---|---|
| Total Ca in solution (water + base + supplement) | 150–200 mg/L | Cannabis research solutions ran 200 mg/L [1] |
| Total Mg | 40–70 mg/L | Veg optimum 35–70 mg/L [2] |
| Ca:Mg (mass) | ~3:1 to 4:1 | *Grower practice* |
| RO base before nutrients (if your base assumes tap) | ~Ca 40–60, Mg 15–25 mg/L | *Grower practice*; roughly EC 0.3–0.4 [mS/cm](https://genetixbay.com/knowledge/glossary/ms-cm.md) from the supplement. |
| Hard-water threshold | >60 mg/L Ca or >25 mg/L Mg | [3] |

## Decision steps

1. **Read your water report** (or measure source EC: RO is usually <0.05 mS/cm, many tap waters 0.2–0.8).
2. **Read the base nutrient label**: what Ca and Mg does it supply at your dose? Many list it as % CaO and % MgO; convert (Ca = CaO × 0.715; Mg = MgO × 0.603).
3. **Add only the gap** between water + base and the targets.
4. **Coco:** buffer first; then a smaller ongoing top-up is often enough.
5. **Re-check K.** If K is very high (PK boosters), you may be fighting K [antagonism](https://genetixbay.com/knowledge/glossary/nutrient-antagonism.md) rather than a Ca/Mg shortage [5].

## Common mistakes

- Adding full-rate Cal-Mag to hard tap water "just in case".
- Using Cal-Mag to treat broad-mite damage or pH problems.
- Forgetting the extra nitrogen late in flower.
- Adding Cal-Mag after the base nutrient in the same concentrate bottle (precipitation with sulfate and phosphate). Add it to the water first, at working strength.

## Evidence

- Ca and Mg shortage on RO water and in fresh coco: *Supported* (substrate chemistry [4], water chemistry [3]; no cannabis trial specific to Cal-Mag).
- Target Ca and Mg levels: *Supported* [1][2].
- "LEDs need more Cal-Mag": *Myth* (GenetixBay house position). No study shows the light source itself raises Ca or Mg demand; the transpiration effect of cooler leaves [6][8] is a VPD setting, fixed with the environment.
- Cultivar differences in Ca and Mg uptake: *Supported* [5][7].

## Sources

1. Cockson P, et al. 2019. Characterization of nutrient disorders of Cannabis sativa. Applied Sciences 9(20):4432. https://doi.org/10.3390/app9204432
2. Morad D, Bernstein N. 2023. Response of medical cannabis to magnesium (Mg) supply at the vegetative growth phase. Plants 12(14):2676. https://doi.org/10.3390/plants12142676
3. Lopez RG, Mickelbart MV, Pasian C. Alkalinity management in soilless substrates. Purdue Extension HO-242-W. https://www.extension.purdue.edu/extmedia/HO/HO-242-W.pdf
4. Greenhouse Grower. Washed, low-EC, or buffered? How to specify coco substrate for your crop. https://www.greenhousegrower.com/?p=180860
5. Saloner A, Sacks MM, Bernstein N. 2019. Frontiers in Plant Science 10:1369. https://doi.org/10.3389/fpls.2019.01369
6. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. https://doi.org/10.1016/C2009-0-63043-9
7. Kalinowski J, et al. 2020. Augmenting nutrient acquisition ranges of greenhouse grown CBD (cannabidiol) hemp (Cannabis sativa) cultivars. Horticulturae 6(4):98. https://doi.org/10.3390/horticulturae6040098
8. Nelson JA, Bugbee B. 2015. Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS ONE 10(10):e0138930. https://doi.org/10.1371/journal.pone.0138930