Cannabis Deficiency Diagnosis
Yellow leaves? Brown spots? Curling tips? Find the cause and the fix — with look-alikes to avoid misdiagnosis.
Nitrogen Deficiency
- •Yellowing starts on bottom/oldest leaves first
- •Progresses upward over days
- •Leaves turn fully yellow then drop
- •Plant looks washed out overall
Insufficient nitrogen, pH lockout (most common), overwatering preventing uptake
Check pH first (6.0–7.0 soil). If correct, add nitrogen-rich fertilizer: fish meal, blood meal, or N-heavy base nutrient.
Phosphorus Deficiency
- •Dark green leaves developing purple/red underneath
- •Purple or red stems and petioles
- •Bronze or brown spots on older leaves
- •Delayed flowering and poor bud development
pH too low (below 6.0 locks out P), cold temperatures below 15°C, overwatering
Raise pH to 6.2–7.0 in soil. Warm root zone. Add P supplement: bat guano, bone meal, or P-heavy bloom nutrient.
Potassium Deficiency
- •Brown burnt-looking leaf tips and edges
- •Yellowing between veins on older leaves
- •Leaves curl upward at edges
- •Weakened stems and branches
pH imbalance, competing nutrients blocking K uptake, low-quality soil
Correct pH first. Add K-rich supplement: kelp meal, greensand, or K-heavy base nutrient.
Calcium Deficiency
- •Brown spots with yellow halos on newer leaves
- •Spots appear random, not along veins
- •New growth curls and distorts
- •Bud sites show brown dead areas
pH too low, using RO/distilled water without cal-mag, coco coir without calcium supplement
Add cal-mag product. Raise pH slightly. Coco growers: cal-mag is non-optional every watering.
Magnesium Deficiency
- •Yellow areas between green leaf veins on older leaves
- •Interveinal chlorosis — veins stay green, leaf turns yellow
- •Progresses to complete yellowing then death
Low pH, calcium competing with Mg uptake, flushing without replacing cal-mag, RO water
Foliar spray Epsom salt (1 tsp per litre) for fast correction. Add cal-mag to feed. Raise pH to 6.0–6.5.
Overwatering
- •Leaves droop downward (not just tips)
- •Leaves feel heavy and swollen
- •Soil stays wet for extended periods
- •Yellowing similar to nitrogen deficiency
Watering too frequently, pots without drainage, oversized pots relative to plant size
Let medium dry out significantly between waterings — lift pots to judge weight. Only water when pot feels light.
Light Burn
- •Yellowing or bleaching only on leaves closest to light
- •Bleached white buds at very top of canopy
- •Lower leaves look completely healthy
Light too close to canopy. LED lights especially can burn without much heat.
Increase distance between light and canopy. LED: minimum 30–45cm for most quantum boards.
Nutrient Burn
- •Brown crispy tips across the whole plant
- •Tips look scorched
- •Dark green leaf color before tips burn
- •Affects many leaves throughout canopy
Overfeeding — too many nutrients. More is not better. Cannabis has a saturation point.
Flush with 3x pot volume in plain pH-adjusted water. Reduce nutrient concentration by 25–50%.
Powdery Mildew
- •White powdery coating on leaves and stems
- •Starts as small circular white spots
- •Spreads rapidly if unchecked
- •Musty smell in grow room
Humidity above 60% in flower, poor airflow, plants touching each other
Isolate affected plants. Spray diluted hydrogen peroxide (3%) or potassium bicarbonate. Reduce humidity below 50%.