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Nutrient Lockout
Definition: Nutrient lockout is a condition where cannabis roots cannot absorb nutrients that are physically present in the growing medium, usually because pH has drifted out of range or salts have built up. The plant shows deficiency symptoms even though it is being fed, and adding more fertilizer usually makes the problem worse.
How Nutrient Lockout works
Lockout is frustrating because it looks like hunger. Leaves yellow, spot, or curl as if the plant is starving, so the natural reaction is to feed more. But the food is already there; the plant just can’t use it.
Common causes
- pH out of range: the most frequent culprit. See pH for target ranges by medium.
- Salt buildup: repeated feeding without enough runoff leaves mineral salts concentrated around the roots.
- Nutrient antagonism: an excess of one element blocks another. Too much potassium can limit calcium and magnesium uptake; heavy phosphorus can interfere with zinc and iron.
- Cold or waterlogged roots: roots without warmth or oxygen absorb poorly no matter what’s in the medium.
Diagnosing it
Suspect lockout when several deficiencies appear at once, growth slows despite regular feeding, and symptoms get worse after you increase nutrients. Measure input and runoff pH and EC. Runoff that’s far from your input pH, or runoff EC well above what you’re feeding, confirms the problem.
Resetting the root zone
- Water with pH-adjusted plain water, around 6.5 for soil or 6.0 for coco, until runoff EC drops close to your input.
- In hydro, dump and refill the reservoir with a fresh, correctly balanced solution.
- Let soil dry slightly, then resume at half-strength nutrients and watch new growth.
Older leaves damaged by lockout won’t recover, but the plant usually bounces back within a week once roots can feed again. If the leaf tips were also burning, the root cause may be overlapping nutrient burn.