# Magnesium

> Magnesium (Mg) is a mobile secondary nutrient at the centre of every chlorophyll molecule; deficiency causes interveinal yellowing on older leaves.

- Canonical: https://genetixbay.com/knowledge/glossary/magnesium.html
- Also written: Mg, Mg2+, magnesium deficiency, Mg deficiency
- Glossary category: Nutrients · Evidence: Established
- 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"

Magnesium is a secondary macronutrient absorbed as Mg2+. It sits at the centre of the chlorophyll molecule, activates the enzymes that fix carbon and handle ATP, and helps load sugars into the phloem. It is mobile, so the plant strips it from older leaves when supply runs short.

## In practice

Classic [deficiency](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) is [interveinal chlorosis](https://genetixbay.com/knowledge/glossary/interveinal-chlorosis.md) on lower and middle leaves: green veins with yellow tissue between, later turning brown. Hydro feeds usually run about 40-70 mg/L (ppm) Mg, most often from [magnesium sulfate](https://genetixbay.com/knowledge/glossary/magnesium-sulfate.md). High potassium, calcium or ammonium competes with Mg at the root, so a deficiency can appear even with Mg in the tank. Morad and Bernstein found cannabis responds to Mg supply across a range before plateauing.

## Don't confuse with

- **Iron deficiency**: also interveinal, but on new growth.
- **Potassium deficiency**: starts at leaf margins rather than between the veins.
- **Epsom salt**: a common Mg source, not a synonym for magnesium.

## Myth check

[LEDs](https://genetixbay.com/knowledge/glossary/led.md) do not create a magnesium need (Evidence: house position, Myth). Mg demand is driven by cultivar, [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md) and coco [cation exchange](https://genetixbay.com/knowledge/glossary/cation-exchange-capacity.md). See [Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md) and [Mythbusters](https://genetixbay.com/knowledge/mythbusters.md).

## Related terms

[Secondary nutrient](https://genetixbay.com/knowledge/glossary/secondary-nutrient.md), [Mobile nutrient](https://genetixbay.com/knowledge/glossary/mobile-nutrient.md), [Magnesium sulfate](https://genetixbay.com/knowledge/glossary/magnesium-sulfate.md), [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md), [Nutrient antagonism](https://genetixbay.com/knowledge/glossary/nutrient-antagonism.md)

## Read more

- [Magnesium (Mg) in cannabis: interveinal yellowing, targets and the K-Ca competition](https://genetixbay.com/knowledge/nutrients/magnesium.md)
- [Cal-Mag: what it is, when you need it and when you don't](https://genetixbay.com/knowledge/nutrients/cal-mag.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. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press. https://doi.org/10.1016/C2009-0-63043-9