Short answer. Manganese runs the water-splitting step of photosynthesis and activates many enzymes [6]. Deficiency 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 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 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 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 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
- pH. High pH → deficiency; low pH → toxicity. Measure runoff or reservoir.
- Iron dose. Very high Fe can suppress Mn.
- Media. Some peat and soil mixes release Mn when they go acidic or waterlogged (reduced, low-oxygen soils make Mn²⁺ more soluble).
- 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 with a balanced solution at normal EC, not plain water (see the flush debate).
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].
