# Potassium (K) in cannabis: role, deficiency, excess and K-Ca-Mg balance

> Potassium runs water relations, stomata and sugar transport. Cannabis grows well on roughly 175–240 mg/L K in veg, and a flowering DWC trial found no yield response anywhere between 60 and 340 mg/L K, so more is not better. Deficiency scorches the margins and serrations of older leaves; excess K mainly does harm by suppressing calcium and magnesium uptake.

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

**How much potassium does cannabis need in flower?** Cannabis in flower does well on about 150-300 mg/L potassium in the solution. A flowering [DWC](https://genetixbay.com/knowledge/glossary/dwc.md) trial found no yield response anywhere from 60 to 340 mg/L, so adequate matters but extra does not; the upper part of the range is grower practice. Healthy recent mature leaves test about 1.8-2.7% potassium by dry weight.

**Why are the edges of my lower leaves yellow and crispy?** Yellow, crispy edges on lower cannabis leaves often mean [potassium deficiency](https://genetixbay.com/knowledge/glossary/potassium.md), but check runoff [EC](https://genetixbay.com/knowledge/glossary/ec.md) first, because [salt build-up](https://genetixbay.com/knowledge/glossary/salt-build-up.md), drought, heat, and manganese or boron toxicity cause similar marginal scorch. Then check [pH](https://genetixbay.com/knowledge/glossary/ph.md) and roots. If solution potassium is below about 100 mg/L in flower, raise it in steps of 20-40 mg/L with [potassium sulfate](https://genetixbay.com/knowledge/glossary/potassium-sulfate.md) or [potassium nitrate](https://genetixbay.com/knowledge/glossary/potassium-nitrate.md).

**Can too much potassium cause magnesium or calcium deficiency?** Yes, too much potassium can cause magnesium and calcium deficiency in cannabis, because these cations compete for root uptake; one cannabis study found calcium and magnesium uptake fell as potassium supply rose. Watch for [interveinal yellowing](https://genetixbay.com/knowledge/glossary/interveinal-chlorosis.md) on old leaves and distorted new growth. Grower practice keeps potassium to calcium near 1:1 to 1.5:1 by mass.

**Do PK boosters help cannabis?** PK boosters generally do not help cannabis on a full bloom feed. Yield showed no response to potassium from 60 to 340 mg/L in a flowering trial, and adding a booster on top of a complete base can raise potassium to 400+ mg/L, inducing [magnesium deficiency](https://genetixbay.com/knowledge/glossary/magnesium.md). Crowding out calcium and magnesium is the main practical risk of these products.

**What does potassium deficiency look like?** Potassium deficiency in cannabis starts as yellowing along the margins and serrated tips of lower, older leaflets, moving inward towards the midrib and turning into tan, [necrotic](https://genetixbay.com/knowledge/glossary/necrosis.md) edges. In a DWC flowering trial it appeared in week 3 as dark brown lesions on sugar leaves, then browned fan-leaf tips; inflorescence fresh weight fell about 52%.

**Short answer.** Potassium controls water movement, stomatal opening and sugar transport into flowers. In veg, cannabis does well at about 175–240 mg/L K [1][2]; in a flowering DWC trial, yield did not respond to K anywhere from 60 to 340 mg/L [3]. So adequate matters; extra does not. [Deficiency](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) scorches the margins and serrated tips of older leaves. Excess K mostly hurts by crowding out calcium and magnesium [1].

## What's happening

K⁺ is the main cation in plant cells. It is not built into structures; it works as an osmotic and charge regulator. It drives turgor and cell expansion, opens and closes [stomata](https://genetixbay.com/knowledge/glossary/stomata.md), activates dozens of enzymes, and moves sugars through the phloem into sinks such as flowers [7].

K is highly **mobile**, so older leaves show shortage first. At the root, K⁺, Ca²⁺, Mg²⁺ (and NH₄⁺, Na⁺) compete for uptake. In cannabis, Saloner et al. reported competition between K and both Ca and Mg uptake as K supply increased [1]. That is the main practical risk of PK boosters.

## How to recognise it

**Deficiency.** Yellowing of the leaf margin, especially the saw-teeth of leaflets, on lower and older foliage; it moves inward toward the midrib and turns into tan necrosis along the margin [4]. In DWC flower, K deficiency showed in week 3 as dark brown lesions on sugar leaves, then fan-leaf tip and margin browning; most non-flower leaves were brown by week 7 and inflorescence fresh weight was about 52% lower [5]. In veg, 15 mg/L K was clearly deficient with visible symptoms in two [genotypes](https://genetixbay.com/knowledge/glossary/genotype.md) [1].

**Excess.** Rarely toxic on its own. Watch for induced Mg deficiency (interveinal yellowing of old leaves) and Ca problems (distorted new growth, spotting) when K is pushed high, and for [salt stress](https://genetixbay.com/knowledge/glossary/sodium.md) (high EC, burnt tips). The response is genotype-dependent: 240 mg/L K harmed one cultivar and stimulated another [1].

## Numbers that matter

| Parameter | Target | Conditions / notes |
|---|---|---|
| Solution K, seedlings/clones | 60–120 mg/L | *Grower practice* |
| Solution K, veg | 175–240 mg/L | Pot trials, two genotypes [1][2] |
| Solution K, flower | 150–300 mg/L | No yield response 60–340 mg/L in DWC [3]; upper part is *grower practice* |
| Leaf K, healthy (most recent mature leaf) | 1.8–2.7% dry weight | NC State floral hemp [6] |
| Leaf K at first deficiency symptoms | ~0.41% | Cannabis, sand culture [4] |
| K:Ca ratio in solution (mass) | ~1:1 to 1.5:1 | *Grower practice*; above ~2:1 watch for Ca/Mg problems. |

## What to check first

1. **Is it really K?** Marginal scorch also comes from high EC/salt build-up, drought, Mn or B toxicity (both gave marginal yellowing of lower leaves in trials [4]), and heat. Check runoff EC first.
2. **pH and roots.** Wet, cold or rotted roots absorb poorly.
3. **[Coco](https://genetixbay.com/knowledge/glossary/coco-coir.md).** Unbuffered coco releases K, so K deficiency in coco is rare; Ca/Mg shortage is common (see [buffering coco](https://genetixbay.com/knowledge/ph-ec/buffering-coco.md)).
4. **Feed.** Is your K below ~100 mg/L in flower?

## How to fix it

- **Mineral:** raise K with potassium sulfate (K₂SO₄, ~41–45% K, also supplies S) or potassium nitrate (KNO₃, ~38% K, 13% N); MKP adds K with P. Raise in steps of about 20–40 mg/L.
- **Soil/organic:** sulfate of potash, kelp meal, wood ash (very alkaline; small amounts and only in acidic soil).
- **Excess:** reduce K and check Ca and Mg are at 150–200 and 40–70 mg/L respectively.

## How to confirm it worked

No new marginal scorch on leaves above the damage within 7–10 days; damaged edges stay brown. Tissue K lands at 1.8–2.7% [6].

## Common mistakes

- Adding a [PK booster](https://genetixbay.com/knowledge/glossary/bloom-booster.md) on top of a full bloom feed, raising K to 400+ mg/L and inducing Mg deficiency.
- Blaming K for tip burn that is really overfeeding.
- Using wood ash in alkaline soil.

## Evidence

- K deficiency symptoms on older leaf margins: *Established* [4][5].
- Veg optimum ~175–240 mg/L K: *Supported* (two genotypes, one lab [1][2]).
- No flower yield response across 60–340 mg/L K in DWC: *Supported* (one trial, one cultivar [3]).
- K–Ca–Mg uptake competition: *Established* in plant nutrition [7], observed in cannabis [1].

## Sources

1. Saloner A, Sacks MM, Bernstein N. 2019. Response of medical cannabis (Cannabis sativa L.) genotypes to K supply under long photoperiod. Frontiers in Plant Science 10:1369. https://doi.org/10.3389/fpls.2019.01369
2. Saloner A, Bernstein N. 2022. Effect of potassium (K) supply on cannabinoids, terpenoids and plant function in medical cannabis. Agronomy 12(5):1242. https://doi.org/10.3390/agronomy12051242
3. Bevan L, Jones M, Zheng Y. 2021. Frontiers in Plant Science 12:764103. https://doi.org/10.3389/fpls.2021.764103
4. Cockson P, et al. 2019. Applied Sciences 9(20):4432. https://doi.org/10.3390/app9204432
5. Llewellyn D, et al. 2023. Plants 12(3):422. https://doi.org/10.3390/plants12030422
6. Suchoff D, et al. 2026. NC State Extension AG-904. https://content.ces.ncsu.edu/hemp-leaf-tissue-nutrient-ranges
7. Marschner P (ed.). 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press. https://doi.org/10.1016/C2009-0-63043-9