# 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](https://genetixbay.com/knowledge/glossary/dry-weight.md).

**Why are the edges of my lower leaves yellow and crispy?** Yellow, crispy edges on lower cannabis leaves often mean potassium deficiency, but check runoff EC 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 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 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 edges. In a DWC flowering trial it appeared in week 3 as dark brown lesions on [sugar leaves](https://genetixbay.com/knowledge/glossary/sugar-leaf.md), then browned fan-leaf tips; inflorescence [fresh weight](https://genetixbay.com/knowledge/glossary/wet-weight.md) 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, 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 [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](https://genetixbay.com/knowledge/glossary/nutrient-burn.md)). 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.** 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