# Dark olive or brown sunken spots on older leaves: likely causes and fixes

> Dark olive or brown sunken spots on older leaves on cannabis is most often Phosphorus deficiency or High pH reducing phosphorus availability. How to tell them apart, the fix for each and what recovery looks like.

Also called: dark spots on bottom leaves, bronze blotches lower leaves, phosphorus spots, olive green blotches.

Run the checker with this sign: https://genetixbay.com/knowledge/diagnose/?at=old&s=olive-spots-old

## Likely causes, most likely first

### Phosphorus deficiency (Possible)

- Kind: Nutrient; urgency: Deal with it this week
- Fix: Warm the root zone to 18-22 °C (64-72 °F) and nights above ~15 °C (59 °F) first; cold blocks P uptake. Correct pH (soilless 5.5-6.5, soil 6.0-7.0). If genuinely short, supply about 30-60 mg/L P; controlled cannabis trials found no yield or cannabinoid gain above roughly 25-60 mg/L, so skip 'bloom boosters'. Soil: a balanced liquid feed; living soil: a slow P amendment acts in weeks.
- Recovery: No new olive spots on leaves that open after the fix, within 7-10 days; growth rate picks up. Spotted leaves stay damaged.
- 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: Measure runoff EC against input EC: Water to 10-20% runoff, collect the runoff in a clean cup and measure EC of both with a calibrated meter (1.413 mS/cm standard). Runoff more than ~0.5 mS/cm above input means salt build-up; well below input means the plant is hungry or you are over-leaching. In hydro, compare reservoir EC with the EC you mixed.
- 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: 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: [Phosphorus (P) in cannabis: what it does, deficiency, excess and how much is enough](https://genetixbay.com/knowledge/nutrients/phosphorus.md)

### High pH reducing phosphorus availability (Possible)

- Kind: pH; urgency: Deal with it this week
- Fix: Bring soil pH to 6.2-6.8 (soilless 5.8-6.2) and neutralise alkaline water; above about pH 7, phosphate precipitates with calcium. Do not add more P to compensate.
- Recovery: No new olive spots on leaves that open after pH correction.
- 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: Slurry (1:2) test of the medium: Take a sample from the root zone (not the top 2 cm), mix 1 part medium with 2 parts distilled water by volume, stir, wait 30 minutes and measure pH and EC of the liquid. Use it for soil and living soil where runoff is unreliable. Slurry EC reads lower than pour-through EC; compare like with like.
- Confirm: Get a water report (or test your water): Ask your water utility for the annual report or test well water: alkalinity (mg/L CaCO3), pH, EC, Ca, Mg, Na, Cl, B, and whether it uses chlorine or chloramine. Alkalinity above ~100-150 mg/L as CaCO3 pushes root-zone pH up; sodium above ~50 mg/L and chloride above ~70 mg/L are a problem; softened water is high in sodium.
- Article: [pH and nutrient availability: what pH really controls, by medium](https://genetixbay.com/knowledge/ph-ec/ph-nutrient-availability.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.