# Zinc (Zn) in cannabis: small leaves, margin burn and the phosphorus link

> Zinc is needed for many enzymes and for making the growth hormone auxin. Deficiency in cannabis shows late, as yellowing and then tan necrosis along the toothed margins of new leaves, often with small leaves and short internodes elsewhere in the literature. Supply about 0.1–0.5 mg/L Zn; high pH and very high phosphorus are the usual reasons a fed plant runs short.

- Canonical: https://genetixbay.com/knowledge/nutrients/zinc.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

**What does zinc deficiency look like in cannabis?** Zinc deficiency in cannabis shows late, after about 9 weeks without zinc in one controlled study, as yellowing along the [toothed margins](https://genetixbay.com/knowledge/glossary/serration.md) of newer and expanding leaves that develops into tan, irregular necrotic patches along the edge. Leaf zinc at first symptoms was about 10.7 mg/kg. Small leaves and [short internodes](https://genetixbay.com/knowledge/glossary/node-stacking.md) are reported in other crops but not yet confirmed in cannabis.

**Why are my new leaves small with burnt edges?** Small new cannabis leaves with burnt, yellow-to-tan edges along the leaf teeth can point to zinc deficiency, usually caused by high root-zone pH or very high phosphorus rather than a feed lacking zinc. Check pH, and whether a [bloom booster](https://genetixbay.com/knowledge/glossary/bloom-booster.md) pushes phosphorus above 100 mg/L. [Potassium deficiency](https://genetixbay.com/knowledge/glossary/potassium.md) also burns margins, but on old leaves; [broad mites](https://genetixbay.com/knowledge/glossary/broad-mite.md) distort new growth.

**How much zinc in a hydro feed?** A cannabis hydro feed needs about 0.1-0.5 mg/L zinc, a grower practice range; cannabis trials used about 0.2 mg/L. Zinc sulfate heptahydrate is about 22.7% zinc, so 1.3 mg/L of product supplies roughly 0.3 mg/L. Healthy recent mature leaves test about 33-60 mg/kg zinc. Keep phosphorus at or below about 60 mg/L to protect zinc uptake.

**Can too much phosphorus cause zinc deficiency?** Heavy phosphorus is one of the better-documented [antagonisms](https://genetixbay.com/knowledge/glossary/nutrient-antagonism.md) in crops, depressing zinc uptake, although it has not been tested in cannabis. Since phosphorus above about 60 mg/L adds no yield, running high phosphorus 'for buds' only risks inducing zinc and iron shortages. Trim phosphorus back to 30-60 mg/L and correct pH when new-leaf margins burn.

**Is zinc toxic to cannabis?** Excess zinc can harm cannabis: in hydroponic hemp, about 6.5 mg/L zinc for one week was used as a stress treatment and raised stress markers, and in many crops excess zinc induces [iron deficiency](https://genetixbay.com/knowledge/glossary/iron.md) with interveinal chlorosis on new leaves. Galvanised pipe, trays or fittings in [recirculating systems](https://genetixbay.com/knowledge/glossary/recirculating-system.md) can leach zinc, so remove them and replace the solution.

**Short answer.** Zinc activates many enzymes and is needed to make auxin, the hormone that drives stem and leaf expansion [3]. In cannabis, [deficiency](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) showed late (after about 9 weeks without Zn) as yellowing and then tan necrosis along the toothed margins of new leaves [1]. Supply about 0.1–0.5 mg/L Zn. When a fed plant runs short, high pH or very high phosphorus is usually to blame.

## What's happening

Zn²⁺ is taken up by roots and moves poorly once in a leaf, so new growth shows shortage first [3]. Classic Zn deficiency in other crops causes "little leaf" and "rosetting": small, narrow leaves on short internodes, because auxin production falls [3]. In cannabis the documented sign was marginal, along the leaf teeth [1]. Zn availability drops as pH rises, and heavy P fertilisation is one of the better-documented antagonisms in crops (P–Zn) [3][4].

## How to recognise it

- **Deficiency:** marginal yellowing on newer and expanding leaves, concentrated along the toothed margins; it develops into tan, irregular necrotic patches along the edge [1]. Biomass was not significantly reduced at symptom onset [1]. Small, crinkled or narrow new leaves and shortened internodes are reported in other crops
- **Toxicity:** in hydroponic hemp, 100 µM Zn (≈6.5 mg/L) for a week was used as a stress treatment, raising stress markers [5]. Excess Zn induces Fe deficiency (interveinal chlorosis on new leaves) in many crops [3]. Galvanised fittings in recirculating systems can leach Zn.
- **Differential:** K deficiency (margins too, but on *old* leaves), Ca and B (distorted new growth), heat or [light stress](https://genetixbay.com/knowledge/glossary/light-stress.md) (tip curl at canopy top), and broad mites.

## Numbers that matter

| Parameter | Target | Conditions / notes |
|---|---|---|
| Solution Zn | 0.1–0.5 mg/L | *Grower practice*; cannabis trials used ≈0.2 mg/L [1] |
| Stress-level Zn | ~6.5 mg/L | Hemp, hydroponic, 1 week [5] |
| Leaf Zn, healthy (most recent mature leaf) | 33–60 mg/kg | NC State floral hemp [2] |
| Leaf Zn at first deficiency symptoms | ~10.7 mg/kg | [1] |
| Solution P | ≤60 mg/L | Higher P adds no yield and may depress Zn uptake (see [phosphorus](https://genetixbay.com/knowledge/nutrients/phosphorus.md)) |

## What to check first

1. **pH** of the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md) (soil above ~7.0, soilless above ~6.5).
2. **P level** in the feed: is a bloom booster pushing P above 100 mg/L?
3. **Water and plumbing:** galvanised pipe (excess) or RO with a micronutrient-free feed (shortage).
4. **Pests:** broad mites mimic new-growth distortion.

## How to fix it

- Correct pH and trim P back to 30–60 mg/L.
- Add zinc sulfate heptahydrate (≈22.7% Zn; 1.3 mg/L of product ≈ 0.3 mg/L Zn) or Zn-EDTA (≈14% Zn).
- **Foliar (veg only):** zinc sulfate at about 0.1–0.5 g/L of product; *grower practice*. Test on one leaf first.
- **Toxicity:** remove the Zn source (galvanised parts), replace the solution, keep pH in range.

## How to confirm it worked

New leaves emerge full-size with clean margins within 7–14 days. Tissue Zn 33–60 mg/kg [2].

## Common mistakes

- Running very high P "for buds" and inducing Zn and Fe shortages.
- Over-correcting with multiple [micronutrient](https://genetixbay.com/knowledge/glossary/micronutrient.md) products.
- Using galvanised steel trays or pipe in recirculating hydro.

## Evidence

- Zn deficiency symptoms in cannabis (new-leaf margins): *Supported* (one controlled study [1]).
- P–Zn antagonism: *Established* in crops [3][4]; *untested* in cannabis.
- Zn targets: *Supported* (standard solutions; no cannabis Zn rate study found).

## Sources

1. Cockson P, et al. 2019. Characterization of nutrient disorders of Cannabis sativa. Applied Sciences 9(20):4432. https://doi.org/10.3390/app9204432
2. Suchoff D, et al. 2026. NC State Extension AG-904. https://content.ces.ncsu.edu/hemp-leaf-tissue-nutrient-ranges
3. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. https://doi.org/10.1016/C2009-0-63043-9
4. Rietra RPJJ, Heinen M, Dimkpa CO, Bindraban PS. 2017. Effects of nutrient antagonism and synergism on yield and fertilizer use efficiency. Communications in Soil Science and Plant Analysis 48(16):1895-1920. https://hub.ifdc.org/handle/20.500.14297/2410
5. Luyckx M, Hausman JF, Isenborghs A, Guerriero G, Lutts S. 2022. Silicon and zinc stress in industrial hemp (hydroponic study). UCLouvain repository. https://research.dial.uclouvain.be/handle/2078.5/105352