Short answer. Molybdenum is a cofactor for nitrate reductase, the enzyme that converts nitrate into usable nitrogen, and for a few other enzymes including one in the abscisic-acid pathway [1]. Cannabis needs a minuscule amount: about 0.01–0.05 mg/L Mo in solution. In a 9-week trial, cannabis grown with no Mo showed no visible symptoms and no biomass loss, even though leaf Mo was 96% lower [2]. Deficiency is rare. Unlike most micronutrients, Mo gets less available as pH falls.
What's happening
Mo is taken up as the molybdate anion (MoO₄²⁻). Like phosphate, it binds to iron and aluminium oxides in acidic soils, and liming releases it [1]. In soilless and hydro, a complete micronutrient mix supplies plenty. Mo is moderately mobile in the plant [3].
When Mo is short, nitrate accumulates in leaves but cannot be reduced, so a plant fed plenty of nitrate looks N-deficient. Kaiser et al. note the phenotype is often misdiagnosed because symptoms appear as downstream effects of the enzymes it supports [1].
How to recognise it
- Cannabis: no visual symptoms after 9 weeks without Mo; leaf Mo 0.06 vs 1.46 mg/kg in controls [2]. Longer crops or seed-grown plants from Mo-poor parents could behave differently.being reviewed
- Other crops (for reference): general pale, N-deficiency-like yellowing of older and middle leaves, marginal scorch, cupped or narrow leaves ("whiptail" in brassicas) [1][3].
- Toxicity: plants tolerate high Mo; toxicity is very rare. The concern with excess Mo is for grazing animals, not cannabis.
Numbers that matter
| Parameter | Target | Conditions / notes |
|---|---|---|
| Solution Mo | 0.01–0.05 mg/L | Cannabis trial used ≈0.01 mg/L [2]; greenhouse standard ≈0.05 mg/L [4] |
| Leaf Mo, survey | 0.5–1.5 mg/kg | Survey range cited in [2]; NC State AG-904 gives no Mo range |
| Leaf Mo at 9 weeks with none supplied | ~0.06 mg/kg, no symptoms | [2] |
| Soil pH where Mo gets scarce | Below ~5.5 | Acid mineral soils [1] |
What to check first
- N symptoms that don't respond to N: consider pH, roots and only then Mo.
- Acidic soil: raise pH with lime; that releases Mo [1].
- DIY recipe: does your micronutrient mix include any Mo source?
How to fix it
- Sodium molybdate dihydrate (≈39.7% Mo): 0.1 mg/L product ≈ 0.04 mg/L Mo. Ammonium molybdate (≈54% Mo) also works.
- Make a concentrated micronutrient stock; weighing 4 mg for a 100 L tank is impossible on kitchen scales. For example, 1 g sodium molybdate in 1 L water gives a stock where 1 mL per 10 L ≈ 0.04 mg/L Mo.
- In acidic soil, liming is the real fix.
How to confirm it worked
If Mo was limiting, pale leaves green within 1–2 weeks of a feed containing Mo. If nothing changes, the problem was elsewhere.
Common mistakes
- Blaming Mo for any yellowing. It is near the bottom of the differential list.
- Overdosing because the number looks tiny.
