Potassium Deficiency in Hydroponics — Symptoms & Fix
Marginal leaf scorch, weak stems, and poor fruit set signal potassium deficiency. Diagnose Ca/Mg antagonism and fix EC in 48 hours.
BY ROOTLESS FARM
Quick answer
Brown scorched margins on older leaves, weak floppy stems, and low fruit set = potassium deficiency. The cause is usually Ca or Mg antagonism rather than missing K — when both run high, K uptake collapses. Hold K at 200–300 ppm, Ca below 200 ppm, Mg below 50 ppm, and pH 5.8–6.2. Stems firm up within a week.
Symptoms
- Marginal yellowing then brown scorched necrosis on older leaves
- Leaves curl downward at the edges
- Weak, floppy stems that bend under fruit load
- Reduced flower count and small fruit
- Slow ripening, uneven color in tomato
- Increased susceptibility to fungal disease [OSU-NUT-01]
Cause
Potassium is phloem-mobile, so the plant pulls K from the oldest leaves first and the deficiency shows there. K is the largest cation by demand in fruiting crops — tomato and pepper draw K at 1.5–2× the rate of nitrogen during fruit fill. The dominant failure mode is cation antagonism: Ca²⁺ and Mg²⁺ compete with K⁺ at the same root transport channels. A reservoir with Ca at 250 ppm and Mg at 80 ppm will starve a plant of K even when solution K reads 250 ppm [OSU-NUT-01]. The second cause is a fruiting-stage K under-spec — vegetative mixes carry 150 ppm K, which is too low once fruit set begins.
Diagnose
| Check | Target | Deficiency signal |
|---|---|---|
| Solution K | 200–300 ppm | < 150 ppm |
| Solution Ca | 150–200 ppm | > 220 ppm (antagonism) |
| Solution Mg | 40–50 ppm | > 70 ppm (antagonism) |
| pH | 5.8–6.2 | rarely the cause |
| Stem strength | rigid | floppy under fruit |
A tissue test confirms: leaf K below 2.0% dry weight in tomato or below 4.0% in lettuce is deficient. Differentiate from Mg deficiency (interveinal chlorosis, no scorch) and from salt burn (tip burn, not margin scorch).
Fix
- Mix fresh solution with K at 250 ppm using potassium nitrate (13-0-46) or potassium sulfate. KNO₃ at 0.55 g/L delivers 250 ppm K.
- Drop Ca to 180 ppm and Mg to 45 ppm in the new mix to relieve antagonism.
- Hold pH at 5.8–6.2 — K availability does not require pH adjustment, but holding the band stabilizes the whole solution.
- Replace 100% of reservoir if Ca:K ratio exceeded 1.2:1 — diluting will not rebalance fast enough [OSU-NUT-01].
- Switch to a fruiting-stage mix once flowering begins; vegetative formulas under-deliver K by 50–80 ppm.
Prevention
Run a fruiting-specific formula from the first flower truss onward. Test reservoir K weekly if you grow tomato, pepper, or strawberry — these crops can deplete K in a 40 L reservoir in five days [GROWER-LOGS]. Keep a notebook of Ca:Mg:K ratios with every mix; the target band is roughly 4:1:5 (Ca:Mg:K). Calibrate the EC meter monthly so drift does not mask K depletion.
FAQ
4 entries- Q01Why are my tomato leaves browning at the edges?
- Marginal necrosis on older leaves is classic potassium deficiency. Confirm K at 200–300 ppm and check that Ca and Mg are not crowding K out of root uptake.
- Q02How does calcium block potassium uptake?
- Ca²⁺, Mg²⁺, and K⁺ share root cation channels. When solution Ca exceeds 220 ppm or Mg exceeds 70 ppm, K uptake drops sharply.
- Q03How fast does potassium deficiency reverse?
- New growth and stem strength recover in 7–10 days once K is restored and antagonism is relieved.
- Q04What pH is best for potassium?
- K is available across pH 5.5–7.5, so pH is rarely the cause. Look at solution K and competing cations first.