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 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 (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.being reviewed |
What to check first
- 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.
- pH and roots. Wet, cold or rotted roots absorb poorly.
- Coco. Unbuffered coco releases K, so K deficiency in coco is rare; Ca/Mg shortage is common (see buffering coco).
- 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 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].
