Leaves are the plant's dashboard. The key diagnostic question is always where did it start — old growth at the bottom, or new growth at the top? Mobile nutrients (N, P, K, Mg) get pulled from old leaves first; immobile ones (Ca, Fe, and most micros) show up in new growth.
Before you diagnose: check pH first
Most "deficiencies" are actually pH lockout. Nutrients can be present in the medium but chemically unavailable outside the absorption window:
- Soil: pH 6.0–6.8
- Coco / hydro: pH 5.5–6.1
If runoff pH is out of range, fix that and wait three days before treating any specific deficiency. Adding more nutrients to a locked-out plant makes it worse.
The chart
| Symptom | Starts | Likely cause | Fix |
|---|---|---|---|
| Even yellowing, whole leaves | Bottom, moves up | Nitrogen (N) | Up veg feed; normal in late flower — don't treat |
| Purple stems, dark dull leaves, red-bronze patches | Bottom | Phosphorus (P) | Bloom feed; check pH, cold roots make it worse |
| Yellow-brown scorched leaf edges | Bottom-mid | Potassium (K) | Bloom feed; flush first if salt buildup suspected |
| Yellow between veins, veins stay green | Bottom-mid | Magnesium (Mg) | Cal-Mag or Epsom salt (1 tsp/gal) |
| Rusty brown spots, new growth twisted | Upper/new | Calcium (Ca) | Cal-Mag; very common under LED in coco |
| New leaves yellow-white, veins green | Top/new | Iron (Fe) | Almost always pH — fix pH, rarely feed iron |
| Burnt brown leaf tips curling back | Everywhere | Overfeeding, not deficiency | Feed at 50 %, plain water next round |
The three most common cases
1. Yellowing bottom leaves in late flower. Not a problem. The fade is the plant spending its stored nitrogen exactly as designed. Enjoy it.
2. Rusty spots under LED. Modern LEDs run cooler canopies and plants transpire less, moving less calcium. If you grow in coco under LED, run Cal-Mag as a default, not a treatment.
3. "I fed more and it got worse." Burnt tips plus dark green means overfed. The fix is subtraction: flush with plain pH'd water, resume at half strength.
Deficiency vs. everything else
- Spots that wipe off = pests or mildew, not nutrition (see the pest guide)
- Symmetrical patterns across the whole plant = nutrition or pH
- One branch only = physical damage or localized root issue
- Overnight droop with no color change = watering or root zone, not feed
Fix one variable at a time and give it 3–4 days. Leaves that are already damaged don't heal — judge recovery by the new growth.