Short answer. Magnesium is the atom at the centre of every chlorophyll 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, 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.
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].being reviewed |
| 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
- 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).
- K and Ca levels. Did you add a PK booster or extra calcium recently?
- Water. RO water supplies no Mg. Hard water often supplies 10–40 mg/L.
- Coco. Unbuffered coco can bind Mg (see buffering coco).
- 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 (≈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].
