# Middle leaves yellowing first: likely causes and fixes

> Middle leaves yellowing first on cannabis is most often Sulfur deficiency, Molybdenum deficiency or Very low pH limiting molybdenum. How to tell them apart, the fix for each and what recovery looks like.

Also called: middle of plant yellow, mid-canopy yellowing, yellowing in the middle leaves.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=whole&s=yellow-middle-leaves

## Likely causes, most likely first

### Sulfur deficiency (Possible)

- Kind: Nutrient; urgency: Low urgency
- Fix: Rare unless a feed is built only from nitrates and chlorides. Supply about 50-120 mg/L S as sulfate, e.g. via magnesium sulfate or potassium sulfate at normal rates.
- Recovery: New leaves emerge green within 7-10 days.
- Confirm: Leaf tissue analysis: Send 20-30 most-recently-matured leaves (no petioles) from affected plants, and separately from healthy ones, to an agricultural lab. Compare with cannabis and hemp sufficiency ranges (for example N 3.3-4.8%, Mg 0.40-0.81%, K 1.8-2.9% dry weight in recent mature leaves). Results take 1-2 weeks; use it to confirm a chronic problem, not an emergency.
- Confirm: Measure PPFD at the canopy top: Use a quantum (PAR) meter, or a calibrated phone app as a rough guide, at the top leaves in a 5-9 point grid. Compare with the stage target: seedlings 100-300, veg 300-700, flower 800-1,200 µmol/m²/s at ambient CO2. Bleaching and taco at the top above ~1,100 point to light; stretch below ~300 in veg points to too little.
- Confirm: Watch new growth for 5-10 days after one change: Make one correction at a time, mark the top node with a twist tie and photograph the same leaves every 2-3 days. Damaged leaves never recover; judge only new growth and whether the symptom stops spreading. Mobile-nutrient symptoms stop climbing within 5-7 days; immobile ones show in new leaves within 7-10 days.
- Article: [Sulfur (S) in cannabis: role, deficiency, targets and sulfate sources](https://genetixbay.com/knowledge/nutrients/sulfur.md)

### Molybdenum deficiency (Less likely)

- Kind: Nutrient; urgency: Low urgency
- Fix: Almost never the answer: cannabis grown 9 weeks without Mo showed no visible symptoms. Correct low pH (Mo availability falls as pH drops); supply about 0.01-0.05 mg/L Mo via a complete micro mix.
- Recovery: Pale middle leaves stop spreading after pH correction.
- Confirm: Leaf tissue analysis: Send 20-30 most-recently-matured leaves (no petioles) from affected plants, and separately from healthy ones, to an agricultural lab. Compare with cannabis and hemp sufficiency ranges (for example N 3.3-4.8%, Mg 0.40-0.81%, K 1.8-2.9% dry weight in recent mature leaves). Results take 1-2 weeks; use it to confirm a chronic problem, not an emergency.
- Confirm: Measure input pH and root-zone (runoff or reservoir) pH: Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
- Article: [Molybdenum (Mo) in cannabis: the trace element nitrate depends on](https://genetixbay.com/knowledge/nutrients/molybdenum.md)

### Very low pH limiting molybdenum (Less likely)

- Kind: pH; urgency: Low urgency
- Fix: Raise pH into range; Mo becomes more available as pH rises, unlike the other micros. Cannabis showed no visible Mo symptoms in a 9-week trial, so look harder for other causes.
- Recovery: Pale middle leaves stop spreading.
- Confirm: Measure input pH and root-zone (runoff or reservoir) pH: Calibrate the meter with fresh pH 7 and pH 4 buffers, then measure the mixed feed and the runoff (containers) or reservoir (hydro). Soil and living soil should sit at 6.0-7.0, coco, rockwool and peat mixes at 5.5-6.5, hydro at 5.5-6.2. Track 3+ readings; one reading is not a trend.
- Confirm: Leaf tissue analysis: Send 20-30 most-recently-matured leaves (no petioles) from affected plants, and separately from healthy ones, to an agricultural lab. Compare with cannabis and hemp sufficiency ranges (for example N 3.3-4.8%, Mg 0.40-0.81%, K 1.8-2.9% dry weight in recent mature leaves). Results take 1-2 weeks; use it to confirm a chronic problem, not an emergency.
- Article: [Molybdenum (Mo) in cannabis: the trace element nitrate depends on](https://genetixbay.com/knowledge/nutrients/molybdenum.md)

## Note

Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.