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Nitrogen Deficiency
Definition: Nitrogen deficiency is a shortage of the nutrient cannabis uses most heavily to build leaves, stems, and chlorophyll. Because nitrogen is mobile, the plant pulls it from older growth to feed new shoots, so the first sign is uniform yellowing of the lowest fan leaves that gradually moves up the plant.
How Nitrogen Deficiency works
Nitrogen is the first number in NPK for a reason: fast-growing cannabis in veg burns through it quickly. When supply falls short, the plant sacrifices old leaves to protect new ones.
What it looks like
- Lower fan leaves turn pale green, then evenly yellow across the whole leaf, veins included.
- Yellowed leaves eventually drop off.
- Yellowing climbs the plant if left uncorrected, and overall growth slows.
Telling it apart
Magnesium shortage yellows the tissue between green veins. Light burn starts at the top of the plant. Nitrogen deficiency starts at the bottom and colors the whole leaf.
When yellowing is normal
In the last 2 to 3 weeks of flower, cannabis naturally draws down stored nitrogen and lower leaves fade to yellow. That’s expected and often welcome. Yellowing in veg or early flower is the problem.
Common causes
- Underfeeding during veg, or soil nutrients used up after 3 to 4 weeks.
- A plant root-bound in a pot that’s too small.
- pH outside the right range, which can lock nitrogen out even when it’s present.
- Cold, wet roots that absorb poorly.
Fixing it
Check pH first. If it’s fine, feed with a nitrogen-forward veg nutrient at a moderate dose. Organic growers can use fish emulsion for quicker results or blood meal and other slow-release amendments for the longer term. Yellow leaves won’t turn green again, but the spread should stop within 3 to 5 days.