Most 'deficiencies' are actually pH lockout. Check and correct pH first before adding anything. Mobile nutrients (N, P, K, Mg) show on old/lower leaves; immobile ones (Ca, Fe, S, Zn, B) show on new/top growth.
1Nitrogen (N) — lower leaves yellow
Uniform yellowing starting at the bottom, moving up. Leaves drop.
Fix: increase grow nutrient / N. Normal in late flower.
Too much N: very dark green, shiny leaves with tips curling down ('the claw').
2Phosphorus (P) — dark, purple, bronze patches
Dark blotches, purple stems (not genetics), slow growth, usually lower leaves.
Often caused by cold roots (<15°C) or pH above 7.
Fix: warm the root zone, correct pH, bloom nutrient.
3Potassium (K) — burnt, crispy edges
Leaf tips and edges brown and crispy, yellowing between, mid/lower leaves.
Fix: check pH, add bloom/PK booster carefully.
4Calcium (Ca) — rusty spots on new growth
Brown irregular spots, twisted or stunted new leaves.
Common with coco and RO water. Fix: Cal-Mag 1–2 ml/L, pH correctly.
5Magnesium (Mg) — interveinal yellowing
Yellow between the veins while veins stay green, mid to lower leaves.
Fix: Cal-Mag or Epsom salt (1 tsp/gal as a feed or ¼ tsp/L foliar in veg).
6Iron (Fe) — yellow new growth with green veins
Top leaves turn pale yellow/white, veins green.
Almost always pH too high. Correct pH; chelated iron if needed.
7Nutrient burn
Brown, crispy tips that slowly creep in. Starts small.
Fix: drop feed strength 25%; flush with pH'd water if severe.
8Light burn / heat stress
Top leaves bleach yellow/white or 'taco' upward while lower leaves look fine.
Fix: raise the light or dim it; keep canopy under 28°C.
9Overwatering vs underwatering
Overwatered: droopy, firm, fat leaves; pot heavy. Let it dry out more.
Underwatered: limp, papery, thin leaves; pot light. Water thoroughly and slowly.
10Pests
Spider mites: tiny dots and webbing under leaves → neem (veg only), predatory mites.
Fungus gnats: flies around soil → let top dry, sticky traps, BTi (Mosquito Bits).
Powdery mildew: white flour patches → lower humidity, more airflow, potassium bicarbonate in veg.