Cannabis nitrogen deficiency is the most common reason a marijuana plant turns yellow, and it shows up first on the oldest, lowest leaves. Nitrogen is a mobile nutrient, so the plant moves it out of older foliage to keep new growth alive, which is why the bottom yellows while the top stays green. The fix is to check the pH of your root zone, then feed a nitrogen-rich fertilizer at a strength the plant can actually use.
What nitrogen does for a cannabis plant
Nitrogen is a building block of chlorophyll, the pigment that makes leaves green and drives photosynthesis. It is also part of amino acids and proteins, so it supports stem growth, leaf expansion, and the general vigor of a plant in the vegetative stage. When supply falls short of demand, chlorophyll production drops and leaves fade from deep green to lime yellow.
Symptoms of nitrogen deficiency in cannabis
- Lower and middle leaves turn pale green, then yellow, while upper leaves stay darker
- Yellowing starts at the leaf tip and moves inward toward the stem
- Affected leaves may curl down or feel soft and limp
- Growth slows, stems thin out, and new leaves come in smaller than normal
- In late stages, lower leaves dry, brown, and drop off the plant
The bottom-up pattern is the key clue. A nutrient that cannot move through the plant, such as iron or calcium, tends to discolor new growth at the top instead.
Common causes
- pH out of range. In soil, nitrogen is best absorbed around 6.0 to 7.0, and in coco or hydro around 5.5 to 6.5. Outside that window the nutrient can be present but unavailable.
- Underfeeding. Fast-growing plants in veg can outpace a light feeding schedule.
- Small container or depleted soil. A root-bound plant runs through available nitrogen quickly.
- Overwatering. Soggy roots take up water and nutrients poorly, which mimics a deficiency.
- Cold root zone. Chilly media slows root activity and nutrient uptake.
How to fix it
- Test the pH of your runoff or reservoir and adjust it into range before adding anything.
- Mix a vegetative-stage nutrient blend that lists nitrogen as part of its NPK ratio, and feed at half the label rate first.
- Water until you see a little runoff so salts do not build up in the pot.
- Wait three to five days. Existing yellow leaves will not turn green again, so watch the new growth for a return to color.
- Repeat a normal feeding schedule rather than a heavy dose. Excess nitrogen causes dark, clawing leaves and weak stems.
Do not confuse it with flowering fade
Some yellowing late in flower is normal. As buds ripen, the plant stops pushing nitrogen into leaves and directs energy to the flowers. If leaves yellow evenly across the canopy in the final weeks, that is senescence, not a deficiency, and adding nitrogen can stall ripening. Judge the plant by stage, pattern, and speed of change.
Prevention
Keep a written feeding log, water on a consistent schedule, and check pH every time you mix nutrients. Repot before plants become root-bound, and match container size to plant size. If you grow in living soil, top-dress with compost or a nitrogen-rich amendment between runs so the medium does not run empty mid-veg.