# 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 look like in cannabis?** Manganese deficiency in cannabis shows as bright yellow, netted interveinal chlorosis 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 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 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 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 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](https://genetixbay.com/knowledge/glossary/cannabinoid.md) 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](https://genetixbay.com/knowledge/glossary/overwatering.md) (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, 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](https://genetixbay.com/knowledge/glossary/nutrient-antagonism.md): *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