What does a potassium deficiency look like?
Potassium is a mobile nutrient inside the plant: when it's in short supply, the plant redistributes it from older, mature leaves to active growth points. That's why symptoms show up first on lower- or mid-canopy leaves — never on new growth.
The classic pattern is necrosis (dead tissue, brown or tan) on the edges and tips of leaves, slowly working inward toward the midrib. Before the necrosis sets in, you'll often see a yellowish chlorosis along the margins, and sometimes irregular spotting or mottling. The center of the leaf and the veins stay green the longest.
- Necrosis on the edges and tips of older leaves (lower or mid canopy).
- Necrosis spreading inward over time.
- Yellowing chlorosis along the margins before necrosis appears.
- Leaf margins curling upward on affected leaves.
- New growth looks normal in a genuine K deficiency.
Why is it almost always induced deficiency rather than a true K shortage?
The most common cause of potassium symptoms isn't that your feed is low on K — it's that K is in the solution but unavailable to the plant. Two mechanisms explain this:
First, pH. Potassium has maximum availability between pH 5.5 and 6.5 in hydro. Above that range, K⁺ ions start to precipitate or compete with other ions in solution, and the plant can't take them up even though K is in the reservoir.
Second, cation antagonism. Potassium, calcium, and magnesium compete for the same uptake sites at the root. When Ca or Mg are relatively excessive, they displace K and trigger an induced deficiency even though K levels in solution are fine. Bevan et al. (2021) documented this cation antagonism in cannabis: high K doses (> 175 ppm) suppressed Ca and Mg uptake — and the mechanism works both ways.
How do you tell K deficiency apart from nutrient burn?
Burnt leaf edges also appear when your solution EC is too high (nutrient burn / salt burn). The difference is in the distribution and context:
- K deficiency: brown tips and edges on OLD leaves only, normal or low EC, possibly high pH.
- Nutrient burn / salt burn: brown tips and edges on leaves at ALL canopy levels including new growth, high EC (> 2.0–2.2 mS/cm in veg, > 2.4–2.8 in bloom depending on stage).
- General lockout: if EC is fine but pH is severely out of range, you may see symptoms on multiple nutrients at the same time.
Always measure EC and pH before diagnosing. Runoff with pH > 7.0 or < 5.0 rules out almost everything else — the issue is lockout, not your formula.
What are the correct potassium ranges for cannabis?
K in cannabis nutrient solution typically runs around 150–250 ppm depending on stage and cultivar. Cockson et al. (2019) tested multiple K levels in cannabis and found the plant tolerates a wide range, but that the K:Ca:Mg ratio matters as much as the absolute concentration of any one of them.
Saloner and Bernstein (2023) confirmed that cannabis adjusts macronutrient uptake based on the relative supply of each cation in solution: when K rises beyond what's needed, Ca and Mg drop; when Ca or Mg are in excess, K drops. That's why symptom-based diagnosis always has to include a full ratio review — not just a look at whichever nutrient seems deficient.
How do you fix a potassium deficiency?
The protocol is: fix the environment first, then decide whether you actually need to add K.
- Check solution and runoff pH. If it's outside 5.8–6.2 in hydro (or 6.0–6.5 in coco/soil), correct it before touching your formula.
- Check EC. If it's above the target for your stage, dilute and run a gentle flush.
- Check the K:Ca:Mg ratio. If Ca or Mg are overdosed relative to K, the problem is balance — not K concentration.
- Only if pH and EC are dialed in and the ratio shows no cation excess should you consider raising K within the 150–250 ppm range.
- Never raise K and Ca or Mg at the same time: cation antagonism will just shift the problem from one nutrient to another.
