Short answer. Iron is needed to make chlorophyll and to run photosynthetic and respiratory electron transport [7]. Deficiency shows as yellowing between the veins of the newest leaves [1]. It is almost always caused by high root-zone pH or alkaline water, not by a feed with no iron [4]. Supply about 1.5–3 mg/L Fe as a chelate that stays stable at your pH: Fe-EDTA up to about pH 6.5, Fe-DTPA to about 7.5, Fe-EDDHA to about 9 [5].
What's happening
Free Fe³⁺ is nearly insoluble at neutral and alkaline pH; it precipitates as hydroxides and with phosphate. Chelates (EDTA, DTPA, EDDHA) are organic cages that keep iron in solution until a root takes it. Each cage has a pH ceiling above which iron is displaced [5].
Fe is immobile in the phloem: once laid down in an old leaf it stays there, so new growth shows the shortage first [4][7]. High bicarbonate (alkaline water) is the classic trigger, because it raises root-zone pH and directly impairs iron uptake [4]. High P, Mn, Zn, Cu and Ca can also compete with or precipitate iron (see nutrient antagonism).
How to recognise it
- Deficiency: begins as slight marginal yellowing at the base of new leaflets, then interveinal chlorosis on new and expanding leaves while lower and middle foliage stays dark green [1]. Severe cases: almost white new leaves [4]. In a sand-culture cannabis trial, symptoms took about 9 weeks to appear [1]; in a DWC flowering trial, withholding Fe produced no striking symptoms and no significant yield loss, probably because the plants had iron reserves and some contamination [2].
- Toxicity: rare in soilless at normal pH. At low pH (below ~5.3) Fe and Mn become very soluble; in iron-efficient crops this causes bronzing, dark speckling and necrotic spots [4]. Cannabis Fe toxicity has not been characterised.being reviewed
- Differential: Mn deficiency (similar but more netted, spots, upper and central leaves), Zn (narrow, small leaves, margins), S (whole leaf pale, not interveinal), light bleaching (top only, near lamp).
Numbers that matter
| Parameter | Target | Conditions / notes |
|---|---|---|
| Solution Fe | 1.5–3 mg/L | Grower practice and greenhouse standards; modified Hoagland used in cannabis trials ≈4 mg/L [1] |
| Root-zone pH, soilless | 5.5–6.3 | Above ~6.5, choose DTPA or EDDHA |
| Fe-EDTA stable to | ~pH 6.5 | [5] |
| Fe-DTPA stable to | ~pH 7.5 | [5]; common hydro choice |
| Fe-EDDHA stable to | ~pH 9 | [5]; soil and alkaline water; stains red |
| Leaf Fe, healthy (most recent mature leaf) | 70–150 mg/kg | NC State floral hemp [3] |
| Leaf Fe at first deficiency symptoms | ~60 mg/kg | Cannabis, sand culture [1] |
| Irrigation water alkalinity flag | >100–150 mg/L CaCO₃ | Acidify (see water quality) |
What to check first
- pH of the root zone, not just the input. Runoff or reservoir pH above ~6.5 explains most cases.
- Water alkalinity. Get a water report; bicarbonate is the usual culprit [4].
- Root health and temperature. Cold, waterlogged or rotting roots fail to take up iron.
- Chelate type. EDTA in a reservoir drifting to 7 is not doing its job.
- Light. Very high-intensity lamps photodegrade Fe-EDTA and Fe-DTPA in clear reservoirs and lines; keep stock and reservoirs dark.being reviewed
How to fix it
- Bring pH back to range (5.5–6.2 hydro, 5.8–6.3 coco). This alone often fixes it in 5–7 days.
- Acidify alkaline water before mixing nutrients.
- Switch chelate to DTPA (hydro, coco) or EDDHA (soil, high-pH water). Typical products: Fe-DTPA ≈ 11% Fe, Fe-EDDHA ≈ 6% Fe; 2 mg/L Fe from an 11% DTPA product ≈ 18 mg/L of product.
- Foliar iron (Fe-DTPA or Fe-EDTA, about 50–100 mg/L Fe) greens existing new leaves quickly in veg; grower practice. Not on flowers.
How to confirm it worked
New leaves emerge green within 5–10 days; already-yellow leaves may partly re-green over 1–2 weeks. Tissue Fe 70–150 mg/kg [3].
Common mistakes
- Adding more iron while pH sits at 7.
- Using Fe-EDTA in soil or hard-water systems.
- Leaving nutrient stock in clear bottles in the sun.
- Confusing Fe (new growth) with Mg (old growth).
Evidence
- New-leaf interveinal chlorosis as the Fe deficiency signature in cannabis: Established [1].
- High pH/alkalinity as the usual cause: Established in container horticulture [4]; Supported for cannabis.
- Chelate pH limits: Established [5].
- Extra Fe above standard levels improving cannabis yield: Not supported (augmented Fe/P/K regime did not substantially increase biomass [6]).
