Short answer. Zinc activates many enzymes and is needed to make auxin, the hormone that drives stem and leaf expansion [3]. In cannabis, deficiency 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 cropsbeing reviewed
- 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 (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) |
What to check first
- pH of the root zone (soil above ~7.0, soilless above ~6.5).
- P level in the feed: is a bloom booster pushing P above 100 mg/L?
- Water and plumbing: galvanised pipe (excess) or RO with a micronutrient-free feed (shortage).
- 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 products.
- Using galvanised steel trays or pipe in recirculating hydro.
