# Magnesium (Mg) in cannabis: interveinal yellowing, targets and the K-Ca competition

> Magnesium sits at the centre of chlorophyll. Cannabis trials put the optimum at about 35–70 mg/L Mg in veg, and Mg supply raised cannabinoid and terpene production in flower. Deficiency shows as interveinal yellowing on older leaves with green veins; it is often induced by low pH or by too much potassium or calcium rather than an actual lack of Mg.

- Canonical: https://genetixbay.com/knowledge/nutrients/magnesium.html
- Category: Nutrients · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Veg, Flower · Systems: Any system
- 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

**Why are my lower leaves [yellow between the veins](https://genetixbay.com/knowledge/glossary/interveinal-chlorosis.md)?** Lower cannabis leaves that yellow between green veins usually show [magnesium deficiency](https://genetixbay.com/knowledge/glossary/magnesium.md), because magnesium is mobile and the plant moves it from old leaves to new growth. The pattern starts faint and becomes a stark green-vein, yellow-blade contrast with necrotic patches. Iron and manganese cause similar chlorosis but on new growth, and potassium deficiency starts at leaf margins instead.

**How much magnesium does cannabis need?** Cannabis needs about 35-70 mg/L magnesium in veg, the optimum found in a controlled dose-response trial, and about 40-70 mg/L in flower. Grower practice for seedlings and clones is 25-40 mg/L, with calcium to magnesium kept near 3:1 to 4:1 by mass. Healthy recent mature leaves test about 0.30-0.65% magnesium by dry weight.

**Can I use [Epsom salt](https://genetixbay.com/knowledge/glossary/magnesium-sulfate.md) on cannabis? How much?** Yes, Epsom salt (magnesium sulfate heptahydrate, about 9.9% magnesium) is a standard cannabis magnesium source. In the root feed, 0.5 g/L supplies about 50 mg/L magnesium and 65 mg/L sulfur. As a veg foliar spray, grower practice uses about 2-5 g/L with a wetting agent at lights-off or low light. Never spray flowers, and test before repeating doses in soil.

**Is it magnesium deficiency or too much potassium?** Magnesium deficiency in cannabis is often induced by too much potassium, because magnesium uptake is weak compared with potassium and calcium; in cannabis, rising potassium supply lowered calcium and magnesium uptake. Check whether a [PK booster](https://genetixbay.com/knowledge/glossary/bloom-booster.md) or extra calcium was added recently. If solution potassium is above about 300 mg/L, bring it down rather than adding more magnesium.

**Does magnesium increase THC or [terpenes](https://genetixbay.com/knowledge/glossary/terpene.md)?** Research suggests magnesium supply can increase [cannabinoid](https://genetixbay.com/knowledge/glossary/cannabinoid.md) and terpene production in flowering cannabis: one controlled study spanning 2-140 mg/L magnesium found higher production as supply rose from [deficiency](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md). This is supported by a single study, and the exact flower optimum still needs confirming. Practical flower targets are about 40-70 mg/L magnesium in the solution.

**Do [LED](https://genetixbay.com/knowledge/glossary/led.md) lights need more magnesium?** LED lights do not make cannabis need more magnesium; the claim that LEDs create a [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md) need is a myth and the GenetixBay house position. Magnesium demand tracks the cultivar, the water source, [coco](https://genetixbay.com/knowledge/glossary/coco-coir.md)'s [cation exchange](https://genetixbay.com/knowledge/glossary/cation-exchange-capacity.md) and root-zone [pH](https://genetixbay.com/knowledge/glossary/ph.md). Genotypes on the same feed take up and move magnesium differently, which explains most plants that seem magnesium-hungry.

**Short answer.** Magnesium is the atom at the centre of every [chlorophyll](https://genetixbay.com/knowledge/glossary/chlorophyll.md) molecule. A cannabis dose-response trial found about 35–70 mg/L Mg optimal in veg [1], and a follow-up found Mg supply increased cannabinoid and terpene production in flower [2]. Deficiency shows as yellowing *between* the veins of older leaves while the veins stay green [3]. It is often induced by low pH or by too much potassium, calcium or ammonium, so check those before adding more Mg.

## What's happening

Mg²⁺ is part of chlorophyll, activates enzymes including Rubisco and ATPases, and is needed to load sugars into the phloem [7]. It is **mobile**, so the plant pulls it from old leaves to supply new ones; symptoms begin low on the plant.

Mg uptake is weak compared with K⁺ and Ca²⁺. When K or Ca is high, or NH₄⁺ is a large share of N, Mg uptake drops. In cannabis, rising K supply was linked to lower Ca and Mg uptake [6]. That is why a PK booster or heavy Cal(cium)-only supplement can produce a Mg deficiency.

Mg demand also varies by **cultivar**: genotypes on the same feed take up and move nutrients differently [6]. Together with [RO water](https://genetixbay.com/knowledge/glossary/ro-water.md), coco's cation exchange and root-zone pH, that explains most "Mg-hungry" plants. **"LEDs need more Cal-Mag" is a *Myth*:** the light source doesn't raise Mg demand. See [Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md).

## How to recognise it

- **Early:** faint interveinal yellowing on lower, older leaves [3].
- **Progressing:** stronger contrast; veins stay green while the tissue between them goes yellow [3].
- **Advanced:** stark green-vein, yellow-blade pattern with necrotic patches [3]. In DWC flower, symptoms began in week 4 on lower fan leaves and later spread to younger leaves by weeks 5–6, with midribs and margins staying green; inflorescence fresh weight was about 33% lower [4].
- **Differential:** iron and manganese also cause interveinal chlorosis, but on *new* growth. K deficiency starts at margins, not between veins. Late-flower senescence is general fading, not a crisp vein pattern.

## Numbers that matter

| Parameter | Target | Conditions / notes |
|---|---|---|
| Solution Mg, seedlings/clones | 25–40 mg/L | *Grower practice* |
| Solution Mg, veg | 35–70 mg/L | Optimum in a cannabis trial [1] |
| Solution Mg, flower | 40–70 mg/L | Mg raised cannabinoids and terpenes; the study spanned 2–140 mg/L [2]. |
| Ca:Mg ratio (mass) | ~3:1 to 4:1 | *Grower practice*; standard Hoagland is 200:49 ≈ 4:1 |
| Leaf Mg, healthy (most recent mature leaf) | 0.30–0.65% dry weight | NC State floral hemp [5] |
| Leaf Mg at first deficiency symptoms | ~0.12% | Cannabis, sand culture [3] |

## What to check first

1. **pH.** Soilless below about 5.5 or soil below about 6.0 reduces Mg availability. Keep soilless at 5.5–6.5 and soil at 6.0–7.0 (accepted practice; see [pH and nutrient availability](https://genetixbay.com/knowledge/ph-ec/ph-nutrient-availability.md)).
2. **K and Ca levels.** Did you add a PK booster or extra calcium recently?
3. **Water.** RO water supplies no Mg. Hard water often supplies 10–40 mg/L.
4. **Coco.** Unbuffered coco can bind Mg (see [buffering coco](https://genetixbay.com/knowledge/ph-ec/buffering-coco.md)).
5. **Roots.** Cold or waterlogged roots absorb poorly.

## How to fix it

- **Mineral:** magnesium sulfate heptahydrate (Epsom salt, ≈9.9% Mg, 13% S). 0.5 g/L supplies about 50 mg/L Mg and 65 mg/L S. [Magnesium nitrate](https://genetixbay.com/knowledge/glossary/magnesium-nitrate.md) (≈9.5% Mg, 11% N) if you also want N.
- **Foliar (veg):** Epsom salt at about 2–5 g/L with a wetting agent, at lights-off or low light; new leaves green faster. *Grower practice.* Do not spray flowers.
- **Soil:** dolomitic lime if the soil is also acidic (slow); Epsom salt dissolved in irrigation water for a quick response.
- **Rebalance K:** if K is above about 300 mg/L, bring it down instead of adding Mg.

## How to confirm it worked

Yellowing stops climbing within a week; new leaves stay fully green. Old interveinal damage stays. Tissue Mg 0.30–0.65% [5].

## Common mistakes

- Adding Epsom salt repeatedly in soil without testing; sulfate and [EC](https://genetixbay.com/knowledge/glossary/ec.md) build up.
- Missing the real cause (K excess or low pH).
- Confusing Mg (old leaves) with Fe/Mn (new leaves).

## Evidence

- Mg deficiency symptoms on older leaves: *Established* [3][4].
- Veg optimum 35–70 mg/L: *Supported* (one cultivar, one lab [1]).
- Mg increases cannabinoid/terpene production: *Supported* (one study [2]).
- Foliar Mg as a quick fix: *Grower practice*, consistent with other crops.
- "LEDs need more Cal-Mag": *Myth* (GenetixBay house position); Mg demand tracks cultivar, water, medium and pH [6].

## 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. Morad D, Bernstein N. 2025. From deficiency to toxicity: magnesium increases cannabinoid and terpene production in cannabis plants. PMC12739851. https://pmc.ncbi.nlm.nih.gov/articles/PMC12739851/
3. Cockson P, et al. 2019. Applied Sciences 9(20):4432. https://doi.org/10.3390/app9204432
4. Llewellyn D, et al. 2023. Plants 12(3):422. https://doi.org/10.3390/plants12030422
5. Suchoff D, et al. 2026. NC State Extension AG-904. https://content.ces.ncsu.edu/hemp-leaf-tissue-nutrient-ranges
6. Saloner A, Sacks MM, Bernstein N. 2019. Frontiers in Plant Science 10:1369. https://doi.org/10.3389/fpls.2019.01369
7. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. https://doi.org/10.1016/C2009-0-63043-9