# 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?** Lower cannabis leaves that yellow between green veins usually show magnesium deficiency, 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](https://genetixbay.com/knowledge/glossary/potassium.md) 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](https://genetixbay.com/knowledge/glossary/dry-weight.md).

**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?** Research suggests magnesium supply can increase cannabinoid and terpene production in flowering cannabis: one controlled study spanning 2-140 mg/L magnesium found higher production as supply rose from deficiency. 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 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's [cation exchange](https://genetixbay.com/knowledge/glossary/cation-exchange-capacity.md) and root-zone pH. 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](https://genetixbay.com/knowledge/glossary/fan-leaf.md) and later spread to younger leaves by weeks 5–6, with midribs and margins staying green; inflorescence [fresh weight](https://genetixbay.com/knowledge/glossary/wet-weight.md) 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](https://genetixbay.com/knowledge/glossary/general-hardness.md) 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 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