# Iron (Fe) in cannabis: yellow new growth, chelates and pH

> Iron is needed to make chlorophyll and run electron transport. Deficiency shows as interveinal yellowing of the newest leaves and is almost always caused by high root-zone pH or alkaline water, not a lack of iron in the feed. Supply about 1.5–3 mg/L Fe as a chelate matched to your pH: EDTA up to about 6.5, DTPA to about 7.5, EDDHA to about 9.

- Canonical: https://genetixbay.com/knowledge/nutrients/iron.html
- Category: Nutrients · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- 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

**Why are my new leaves yellow with green veins?** New cannabis leaves that turn yellow between green veins usually show iron deficiency, which starts as slight yellowing at the base of new [leaflets](https://genetixbay.com/knowledge/glossary/leaflet.md) while lower and middle foliage stays dark green. It is almost always caused by high root-zone [pH](https://genetixbay.com/knowledge/glossary/ph.md) or alkaline water rather than a feed lacking iron. Manganese, zinc and sulfur shortages can look similar, so check pH first.

**Which iron chelate is best for hydro: EDTA, DTPA or EDDHA?** Fe-DTPA is usually the best iron chelate for cannabis in hydro and coco, because it keeps iron available up to about pH 7.5. Fe-EDTA works only up to about pH 6.5 and fails in a reservoir drifting toward 7, while Fe-EDDHA, stable to about pH 9, suits soil and alkaline water but stains red. Keep chelate stocks and reservoirs dark.

**How many ppm iron in a nutrient solution?** A cannabis nutrient solution should carry about 1.5-3 ppm (mg/L) iron, based on grower practice and greenhouse standards; the modified Hoagland solution used in cannabis trials ran about 4 mg/L. To supply 2 mg/L iron from an 11% Fe-DTPA product, dose about 18 mg/L of product. Healthy recent mature leaves test about 70-150 mg/kg iron.

**Iron deficiency or high pH?** Iron deficiency in cannabis is usually a high-pH problem rather than a shortage of iron in the feed. Measure root-zone pH in the runoff or reservoir, not just the input; readings above about 6.5 explain most cases, and alkaline water with bicarbonate above roughly 100-150 mg/L as CaCO3 is the classic trigger. Bringing pH back to range often fixes it within 5-7 days.

**Can low pH cause iron toxicity?** Low root-zone pH, below about 5.3, makes iron and manganese very soluble, and in iron-efficient crops this causes bronzing, dark speckling and necrotic spots. Iron toxicity in cannabis has not been characterised by research, and it is rare in soilless media at normal pH. Keep soilless pH around 5.5-6.3 to avoid both iron shortage and excess.

**Short answer.** Iron is needed to make [chlorophyll](https://genetixbay.com/knowledge/glossary/chlorophyll.md) and to run [photosynthetic](https://genetixbay.com/knowledge/glossary/photosynthesis.md) and respiratory electron transport [7]. [Deficiency](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) 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](https://genetixbay.com/knowledge/glossary/phosphate.md). 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](https://genetixbay.com/knowledge/nutrients/nutrient-antagonism.md)).

## 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](https://genetixbay.com/knowledge/glossary/dwc.md) 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.
- **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](https://genetixbay.com/knowledge/glossary/light-stress.md) (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](https://genetixbay.com/knowledge/glossary/alkalinity.md) flag | >100–150 mg/L CaCO₃ | Acidify (see [water quality](https://genetixbay.com/knowledge/ph-ec/water-quality.md)) |

## What to check first

1. **pH of the [root zone](https://genetixbay.com/knowledge/glossary/root-zone.md)**, not just the input. Runoff or reservoir pH above ~6.5 explains most cases.
2. **Water alkalinity.** Get a water report; bicarbonate is the usual culprit [4].
3. **Root health and temperature.** Cold, [waterlogged](https://genetixbay.com/knowledge/glossary/overwatering.md) or [rotting roots](https://genetixbay.com/knowledge/glossary/root-rot.md) fail to take up iron.
4. **Chelate type.** EDTA in a reservoir drifting to 7 is not doing its job.
5. **Light.** Very high-intensity lamps photodegrade Fe-EDTA and Fe-DTPA in clear reservoirs and lines; keep stock and reservoirs dark.

## 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]).

## 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. Lopez RG, Mickelbart MV, Pasian C. Alkalinity management in soilless substrates. Purdue Extension HO-242-W. https://www.extension.purdue.edu/extmedia/HO/HO-242-W.pdf
5. HortScience. 2025. Iron chelates (Fe-EDTA, Fe-DTPA, Fe-EDDHA) in high-pH soilless media for calibrachoa and soybean. HortScience 60(3):404. https://journals.ashs.org/view/journals/hortsci/60/3/article-p404.xml
6. Velechovsky J, Malik M, Senkyrik JB, Tlustos P. 2024. Effect of augmented nutrient composition and fertigation system on biomass yield and cannabinoid content of medicinal cannabis. Frontiers in Plant Science 15:1322824. https://doi.org/10.3389/fpls.2024.1322824
7. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. https://doi.org/10.1016/C2009-0-63043-9