Cannabis Yellow Leaves and Deficiency: The Short Answer

Yellow leaves on cannabis point to a nutrient deficiency, but the deficiency is often a symptom rather than the root cause. Mobile nutrients such as nitrogen, magnesium, potassium, and phosphorus move from old tissue to new, so shortages show on lower leaves first. Immobile nutrients such as iron, calcium, and sulfur cannot move, so shortages appear on new growth at the top. Before you add anything to the reservoir or the soil, test pH. Cannabis in soil takes up most nutrients between pH 6.0 and 7.0, and in coco or hydro between 5.5 and 6.5. When pH drifts outside that window, nutrients stay in the medium and the plant starves even though you fed it. Correct pH first, then treat the nutrient.

Cannabis Yellow Leaves Diagnosis: Pictures and Guide

Prerequisites

  • A calibrated pH meter or pH test drops
  • An EC or PPM meter for coco and hydro
  • Your current nutrient schedule and feed strength
  • A magnifier or phone camera for close leaf inspection

Step-by-Step: Diagnose and Correct Yellow Leaves

  1. Note where the yellowing starts. Lower leaves point to nitrogen, magnesium, potassium, or phosphorus. Upper leaves and new tips point to iron, calcium, sulfur, or zinc.
  2. Check the pH of the feed water and of the runoff or root zone. Record both numbers.
  3. Measure EC or PPM in the feed and in the runoff. Runoff higher than feed means salt buildup. Runoff lower means the plant is eating more than you supply.
  4. Compare the leaf pattern to the deficiency list below.
  5. Correct pH into range before you add nutrients. Water with plain pH-adjusted water if salt has built up in the medium.
  6. Apply the single nutrient that matches the pattern at half strength. Wait 3 to 5 days.
  7. Watch new growth, not old leaves. Damaged leaves will not turn green again. Recovery shows as green new leaves.
  8. Log pH, EC, feed strength, and the date. Compare the next feeding against it.

Deficiency Cheat Sheet

Nitrogen

Uniform pale yellow from the bottom up, older leaves first, slow growth. Common in late vegetative plants that outgrew their feed.

cannabis yellow leaves curling

Magnesium

Yellow between the veins while the veins stay green, starting on middle and lower leaves. Common under LED lights and in coco.

cannabis yellow leaves nutrient lockout

Potassium

Yellow or brown leaf edges and tips on older leaves, weak stems. Often paired with high EC in late flower.

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Iron

Bright yellow new growth with green veins at the top of the plant. Almost always a pH problem, not a shortage in the medium.

Calcium

Yellow or brown spots on new leaves, curled tips, stunted new growth. Common in coco and in reverse-osmosis water with no mineral content.

Yellow Leaves That Are Not a Deficiency

  • Overwatering: droopy leaves, yellowing across the whole plant, heavy pots.
  • Light burn or heat stress: yellowing and crisp tops closest to the lamp.
  • Root problems: yellowing with brown, slimy roots and a sour smell.
  • Natural senescence: late flower fade that moves from the bottom up while buds swell.

Prevention

Keep a written feed log, calibrate your pH meter every two weeks, water to runoff in soil and coco, and change reservoir water on a set schedule. Consistency prevents most yellow-leaf problems before they start.