# Manganese (Mn) in cannabis: netted chlorosis, low-pH toxicity and the iron balance

> Manganese is needed to split water in photosynthesis. Deficiency gives a bright yellow, netted interveinal chlorosis on upper and middle leaves with small tan spots; toxicity, usually from low root-zone pH, gives yellowing and necrosis of older leaf margins. Supply about 0.3–0.8 mg/L Mn and keep pH in range, because Mn availability swings strongly with pH.

- Canonical: https://genetixbay.com/knowledge/nutrients/manganese.html
- Category: Nutrients · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review
- 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 [manganese deficiency](https://genetixbay.com/knowledge/glossary/manganese.md) look like in cannabis?** Manganese deficiency in cannabis shows as bright yellow, netted [interveinal chlorosis](https://genetixbay.com/knowledge/glossary/interveinal-chlorosis.md) on upper and central leaves, starting at the leaflet midrib and spreading outward while the veins stay green. Advanced leaves develop small tan [necrotic spots](https://genetixbay.com/knowledge/glossary/necrosis.md) between the veins. Onset took about 6 weeks in a sand-culture study, with leaf manganese near 7.6 mg/kg at first symptoms.

**Can low [pH](https://genetixbay.com/knowledge/glossary/ph.md) cause manganese toxicity?** Yes, low root-zone pH is the usual cause of manganese toxicity in cannabis, because manganese solubility rises steeply as pH falls, so a feed that is fine at pH 6 can become toxic near pH 5. [Toxicity](https://genetixbay.com/knowledge/glossary/nutrient-toxicity.md) shows as yellowing and dying margins on lower leaves, then leaf drop. Keep [soilless](https://genetixbay.com/knowledge/glossary/soilless.md) pH around 5.5-6.3; risk rises below about 5.3.

**How much manganese in a hydroponic solution?** A cannabis hydroponic solution should carry about 0.3-0.8 mg/L manganese, a grower practice and greenhouse standard range; cannabis trials used about 1.0 mg/L, and toxicity appeared near 10 mg/L. About 1.5 mg/L of [manganese sulfate](https://genetixbay.com/knowledge/glossary/manganese-sulfate.md) monohydrate supplies roughly 0.5 mg/L manganese. Healthy recent mature leaves test about 40-158 mg/kg manganese.

**Iron or manganese deficiency, how do I tell?** Iron and manganese deficiency in cannabis differ in pattern and position. Iron [chlorosis](https://genetixbay.com/knowledge/glossary/chlorosis.md) is smoother and paler and concentrates on the very newest leaves. Manganese chlorosis is more netted, sits a little lower on upper-middle leaves, and develops small tan spots. Correct pH first, and never treat manganese deficiency with extra iron, because the two compete.

**Why do I have yellow netting between veins on top leaves?** Yellow netting between green veins on upper and middle cannabis leaves is the classic sign of manganese deficiency, often triggered by high root-zone pH, which cuts manganese availability. Very high iron doses can also suppress manganese uptake. Measure runoff or reservoir pH, bring soilless media back to about 5.5-6.3, then add manganese if the feed lacks it.

**Short answer.** Manganese runs the water-splitting step of [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) and activates many enzymes [6]. [Deficiency](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) shows as bright yellow, *netted* interveinal chlorosis on upper and central leaves, later with small tan spots [1]. Toxicity usually comes from a [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md) that has gone too acidic and shows as yellowing and dying margins on lower leaves [1][4]. Supply about 0.3–0.8 mg/L Mn and hold pH in range.

## What's happening

Mn²⁺ is the plant-available form. Its solubility rises steeply as pH falls, so the same feed can be deficient at pH 7 and toxic at pH 5 [4][5]. Mn and Fe compete: high Mn can induce [Fe deficiency](https://genetixbay.com/knowledge/glossary/iron.md) and the reverse [6]. Mn is **poorly mobile**, so deficiency appears on newer and middle leaves.

## How to recognise it

- **Deficiency:** bright yellow netted interveinal chlorosis on upper and central foliage, starting at the leaflet midrib and spreading outward; veins stay green; advanced leaves develop small tan necrotic spots between veins [1]. Onset took about 6 weeks in sand culture [1]. In a [DWC](https://genetixbay.com/knowledge/glossary/dwc.md) flowering trial, withholding Mn gave no visible symptoms and no yield or cannabinoid loss even though upper-canopy Mn was very low [2]; reserves from veg may carry plants through a short flower cycle.
- **Toxicity:** in a 10× Mn treatment (about 10 mg/L Mn), marginal yellowing of lower leaves moved inward toward the midrib, followed by marginal necrosis and leaf drop [1]. Leaf Mn in those plants was only 48 mg/kg, within normal survey ranges, so [tissue tests](https://genetixbay.com/knowledge/glossary/tissue-test.md) may not flag it; the authors called for higher-dose work [1]. In other crops, Mn toxicity also shows as brown speckles on older leaves and induced Fe deficiency on new ones [6].
- **Differential vs iron:** Fe chlorosis is smoother and paler, concentrated on the very newest leaves; Mn chlorosis is more netted, sits a little lower (upper-middle), and develops tan spots.

## Numbers that matter

| Parameter | Target | Conditions / notes |
|---|---|---|
| Solution Mn | 0.3–0.8 mg/L | *Grower practice*/greenhouse standards; cannabis trials used ≈1.0 mg/L [1] |
| Toxicity seen at | ~10 mg/L in solution | Cannabis, sand culture, 7 weeks [1] |
| Root-zone pH, soilless | 5.5–6.3 (accepted range 5.5–6.5) | Below ~5.3 Mn and Fe toxicity risk rises [4][5] |
| Leaf Mn, healthy (most recent mature leaf) | 40–158 mg/kg | NC State floral hemp [3] |
| Leaf Mn at first deficiency symptoms | ~7.6 mg/kg | [1] |

## What to check first

1. **pH.** High pH → deficiency; low pH → toxicity. Measure runoff or reservoir.
2. **Iron dose.** Very high Fe can suppress Mn.
3. **Media.** Some peat and soil mixes release Mn when they go acidic or waterlogged (reduced, low-oxygen soils make Mn²⁺ more soluble).
4. **Fungicides.** Mn-containing fungicides (mancozeb-type) add Mn; never use on cannabis flowers.

## How to fix it

- **Deficiency:** correct pH first. Then add manganese sulfate monohydrate (≈32% Mn; 1.5 mg/L product ≈ 0.5 mg/L Mn) or Mn-EDTA (≈13% Mn).
- **Toxicity:** raise root-zone pH into range (in soil, lime; in soilless, raise feed pH and stop acidic inputs). [Root-zone flush](https://genetixbay.com/knowledge/glossary/root-zone-flush.md) with a balanced solution at normal [EC](https://genetixbay.com/knowledge/glossary/ec.md), not plain water (see [the flush debate](https://genetixbay.com/knowledge/harvest-cure/flush-debate.md)).

## How to confirm it worked

New growth emerges green without netting within 7–10 days. For toxicity, no new marginal necrosis on lower leaves after pH correction.

## Common mistakes

- Treating Mn deficiency with iron, which can make it worse.
- Letting a peat-based mix drift acidic and then adding more acid.
- Relying on tissue tests alone to rule out Mn toxicity [1].

## Evidence

- Mn deficiency symptoms in cannabis: *Established* (one sand-culture study [1]); not reproduced in a short DWC flower trial [2].
- Mn toxicity symptoms at ~10 mg/L: *Supported* (one study; threshold not refined [1]).
- pH control of Mn availability and Fe–Mn antagonism: *Established* in plant nutrition [4][5][6].

## 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. Llewellyn D, et al. 2023. Plants 12(3):422. https://doi.org/10.3390/plants12030422
3. Suchoff D, et al. 2026. NC State Extension AG-904. https://content.ces.ncsu.edu/hemp-leaf-tissue-nutrient-ranges
4. Michigan State University Extension. pHix it! Getting substrate pH just right. https://www.canr.msu.edu/news/phix-it-getting-substrate-ph-just-right
5. Lopez RG, Mickelbart MV, Pasian C. Purdue Extension HO-242-W. https://www.extension.purdue.edu/extmedia/HO/HO-242-W.pdf
6. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. https://doi.org/10.1016/C2009-0-63043-9