Cannabis Nitrogen Deficiency and Yellow Leaves
Nitrogen deficiency turns the oldest cannabis leaves yellow first. Color starts at the leaf tip and the margins, then moves along the midrib toward the stem. Leaves fade from green to pale green to yellow. In later stages they dry, curl, and drop. New growth at the top of the plant holds its green at the start. That pattern is the main clue. It separates nitrogen from iron and sulfur, which discolor new leaves before old ones.
Cannabis Yellow Leaves During Flowering: Causes and Fixes
Why Lower Leaves Go Yellow
Nitrogen is a mobile nutrient. When supply runs short, the plant pulls nitrogen out of old tissue and moves it to new growth. Nitrogen also forms part of chlorophyll, the pigment that makes leaves green. A leaf that gives up its nitrogen breaks down chlorophyll and loses color. Magnesium and potassium move the same way, but each one leaves a different mark on the leaf.
Causes
- Feed strength too low. Vegetative plants in coco or hydro often need 100 to 200 ppm nitrogen.
- pH out of range. In soil, nitrogen uptake works at pH 6.0 to 7.0. In coco and hydro, pH 5.8 to 6.2. A reservoir at pH 5.0 blocks uptake even when nitrogen sits in the water.
- Overwatering. Soggy media holds no oxygen at the roots, and uptake drops.
- Cold root zone. Roots at 60 F or below take up less nitrogen.
- Root damage from fungus gnat larvae, root rot, or a transplant that tore the root ball.
- Leaching. Heavy watering in a small pot pushes nitrate below the root zone.
How to Confirm It
Measure the pH and the EC of the feed, then measure the runoff. Runoff EC below feed EC means the plant eats more than you supply. Runoff EC above feed EC means salts build up in the media. A yellow lower leaf plus high runoff EC points to lockout, not to a weak mix. Pull the pot and look at the roots. White and firm means the roots work. Brown and soft means rot.
Fix
- Set pH first. Soil at 6.0 to 7.0. Coco and hydro at 5.8 to 6.2.
- Raise the nitrogen rate by 10 to 20 percent. Use a nitrate source for a faster response.
- Flush if runoff EC runs high. Push pH-adjusted water through the pot until runoff EC sits near feed EC, then feed again.
- Warm the root zone to 68 to 75 F. Let the top inch of media dry between waterings.
- Leave the damaged leaves on the plant for now. A yellow leaf will not turn green again. New growth shows whether the fix worked.
Look-Alikes
- Magnesium: yellow between the veins on middle leaves, veins stay green.
- Potassium: brown tips and brown margins on older leaves.
- Iron: yellow new leaves with green veins.
- Sulfur: even yellow across new leaves, top of the plant.
Recovery Timeline
New growth shows color 5 to 10 days after the correction. Full green returns in 2 to 3 weeks. If new leaves stay pale after 10 days, recheck pH and runoff EC. Two problems often run together. Low pH causes nitrogen lockout and magnesium lockout at the same time, and one fix handles both.