# Cal-Mag

> Cal-Mag is a supplement supplying calcium and magnesium, usually as nitrates; need depends on cultivar, water source (RO vs tap) and medium (coco), not on LED lighting.

- Canonical: https://genetixbay.com/knowledge/glossary/cal-mag.html
- Also written: CalMag, Cal Mag, calcium magnesium supplement, Ca-Mg supplement
- Glossary category: Nutrients · Evidence: Myth
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Cal-Mag is a class of supplement providing calcium and magnesium, typically [calcium nitrate](https://genetixbay.com/knowledge/glossary/calcium-nitrate.md) and [magnesium nitrate](https://genetixbay.com/knowledge/glossary/magnesium-nitrate.md) in liquid form, sometimes with iron. It also adds nitrogen, which counts towards the feed total.

## In practice

Growers use it to make up for calcium and magnesium missing from [RO](https://genetixbay.com/knowledge/glossary/ro-water.md) or very soft water, to buffer [coco](https://genetixbay.com/knowledge/glossary/coco-coir.md) before planting, and with cultivars that genuinely demand more Ca and Mg. With hard tap water, a complete base often already supplies enough. Too much raises [EC](https://genetixbay.com/knowledge/glossary/ec.md), pushes nitrogen up and can suppress potassium. Dose to a target in mg/L (ppm) of Ca and Mg, not by habit.

## Don't confuse with

- **Epsom salt**: magnesium and sulfur only, no calcium.
- **Calcium nitrate**: calcium only.
- **Coco buffer**: a Cal-Mag use, not the product itself.

## Myth check

"[LEDs](https://genetixbay.com/knowledge/glossary/led.md) need more Cal-Mag" is a Myth (GenetixBay house position). LEDs do not create a Ca or Mg requirement. Demand is driven by cultivar, water source and coco [cation exchange](https://genetixbay.com/knowledge/glossary/cation-exchange-capacity.md); [leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md) differences under LED (Nelson and Bugbee) affect transpiration and environment settings, not nutrient need in themselves. See [Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md) and [Mythbusters](https://genetixbay.com/knowledge/mythbusters.md).

## Related terms

[Calcium](https://genetixbay.com/knowledge/glossary/calcium.md), [Magnesium](https://genetixbay.com/knowledge/glossary/magnesium.md), [Magnesium nitrate](https://genetixbay.com/knowledge/glossary/magnesium-nitrate.md), [Calcium nitrate](https://genetixbay.com/knowledge/glossary/calcium-nitrate.md), [Secondary nutrient](https://genetixbay.com/knowledge/glossary/secondary-nutrient.md)

## Read more

- [Cal-Mag: what it is, when you need it and when you don't](https://genetixbay.com/knowledge/nutrients/cal-mag.md)
- [Buffering coco coir: why it matters and how to do it](https://genetixbay.com/knowledge/ph-ec/buffering-coco.md)

## Sources

1. 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
2. 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
3. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6