# Molybdenum (Mo) in cannabis: the trace element nitrate depends on

> Molybdenum is a cofactor for nitrate reductase, so a nitrate-fed plant cannot use its nitrogen without it. Cannabis needs a minuscule amount (about 0.01–0.05 mg/L Mo); in a 9-week trial, cannabis grown with no Mo showed no visible symptoms at all despite 96% lower leaf Mo. Unlike the other micronutrients, Mo becomes more available as pH rises, so deficiency is a low-pH, acid-soil problem.

- Canonical: https://genetixbay.com/knowledge/nutrients/molybdenum.html
- Category: Nutrients · Level: Advanced
- 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

**Does cannabis need molybdenum?** Yes, cannabis needs molybdenum, but only in minuscule amounts, about 0.01-0.05 mg/L in solution. Molybdenum is a cofactor for [nitrate](https://genetixbay.com/knowledge/glossary/nitrate.md) reductase, the enzyme that turns nitrate into usable nitrogen. A complete [micronutrient](https://genetixbay.com/knowledge/glossary/micronutrient.md) mix supplies plenty in [soilless](https://genetixbay.com/knowledge/glossary/soilless.md) and hydro systems, and [deficiency](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) is rare, mainly a problem of acidic soils below about [pH](https://genetixbay.com/knowledge/glossary/ph.md) 5.5.

**What does molybdenum deficiency look like?** Molybdenum deficiency has not produced visible symptoms in cannabis: in one 9-week trial, plants given no molybdenum showed no symptoms or biomass loss despite 96% lower leaf molybdenum. In other crops it [looks](https://genetixbay.com/knowledge/glossary/bag-appeal.md) like nitrogen deficiency, with pale yellowing of older and middle leaves, marginal scorch and cupped or narrow leaves, called whiptail in brassicas.

**How much molybdenum in a hydroponic solution?** A cannabis hydroponic solution needs about 0.01-0.05 mg/L molybdenum; a cannabis trial used about 0.01 mg/L and greenhouse standards use about 0.05 mg/L. Sodium molybdate dihydrate is about 39.7% molybdenum, so 0.1 mg/L of product gives roughly 0.04 mg/L. Make a concentrated stock, because weighing a few milligrams for a tank is impossible on kitchen scales.

**Why would a plant fed nitrate still look nitrogen deficient?** A cannabis plant fed plenty of nitrate can still look nitrogen deficient if root-zone pH is off, roots are failing, or, rarely, molybdenum is short. Without molybdenum, nitrate reductase cannot work, so nitrate accumulates in leaves unused. Check pH and roots first; molybdenum sits near the bottom of the list and matters mainly in acidic soils.

**Is molybdenum available at low pH?** No, molybdenum becomes less available at low pH, the opposite of most micronutrients. As the molybdate anion it binds to iron and aluminium oxides in acidic soils, so shortages appear below about pH 5.5 in acid mineral soils. Liming the soil raises pH and releases molybdenum, which is the real fix in that situation.

**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](https://genetixbay.com/knowledge/glossary/phosphate.md), 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](https://genetixbay.com/knowledge/glossary/phenotype.md) 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.
- **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](https://genetixbay.com/knowledge/glossary/nutrient-toxicity.md):** 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

1. **N symptoms that don't respond to N:** consider pH, roots and only then Mo.
2. **Acidic soil:** raise pH with lime; that releases Mo [1].
3. **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](https://genetixbay.com/knowledge/glossary/ammonium.md) 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.

## Evidence

- Mo as a cofactor for nitrate reductase and other enzymes: *Established* [1][3].
- Cannabis showing no visual Mo deficiency in a 9-week trial: *Supported* (one study [2]).
- Mo targets for cannabis: *Grower practice* based on general greenhouse standards [4].

## Sources

1. Kaiser BN, Gridley KL, Brady JN, Phillips T, Tyerman SD. 2005. The role of molybdenum in agricultural plant production. Annals of Botany 96(5):745-754. https://doi.org/10.1093/aob/mci226
2. Cockson P, et al. 2019. Characterization of nutrient disorders of Cannabis sativa. Applied Sciences 9(20):4432. https://doi.org/10.3390/app9204432
3. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. https://doi.org/10.1016/C2009-0-63043-9
4. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6