Nitrogen Deficiency

Nitrogen deficiency is the plant running out of its main building block and pulling what is left out of its oldest leaves to keep the new growth alive. It is the most common deficiency in cannabis and also the most commonly misdiagnosed.

What you are seeing#

It starts at the bottom. The lowest, oldest fan leaves go from green to a flat, uniform pale green, then to yellow. The whole leaf yellows evenly, veins included, rather than yellowing between the veins. Those leaves then go limp, turn papery and drop off with a light tug.

The yellowing marches upward over days. New growth at the top stays green at first, which is the tell: nitrogen is mobile, so the plant strips its old leaves to feed the new ones.

Later you get slow growth, smaller new leaves, and sometimes red or purple coloring in the stems and petioles.

Why it happens#

Nitrogen is the element the plant uses to build chlorophyll and protein. When there is not enough in the root zone, the plant breaks down the chlorophyll in old leaves and moves the nitrogen to the growing tips. The old leaves go yellow because the green is literally being removed.

Common beginner causes, in order of how often they actually happen:

• Pre-amended soil ran out. Bagged soil carries a plant three to five weeks. Week five is when people panic.
• Feeding too light, or feeding plain water for weeks because the plant looked fine.
• pH out of range, so the nitrogen is in the pot but the roots cannot take it up.
• Root damage from overwatering, which stops uptake whatever is in the soil.

One honest note: some lower leaf yellowing in the last two to three weeks of flowering is normal. The plant is supposed to cannibalize itself at the end.

How to fix it#

1. Test runoff pH today. Soil wants 6.0 to 7.0, coco 5.5 to 6.5, hydro 5.5 to 6.2. If you are outside that band, this is pH lockout, not a deficiency, and adding fertilizer will make it worse.
2. If pH is fine, feed a nitrogen-forward grow nutrient at half the bottle's recommended strength. For most beginner lines that is roughly 300 to 500 ppm on the 500 scale, or EC 0.6 to 1.0.
3. Feed every watering in coco and hydro. In soil, feed every second or third watering.
4. Over the next week, watch the top three nodes. If the new growth comes in a deeper green, hold at that strength. If the yellow keeps climbing, step up to three quarter strength.
5. Do not push nitrogen after week three of flowering. Late nitrogen makes leafy, grassy smelling buds.

How to tell it apart#

Versus pH lockout. Identical looking in soil. The discriminator is your meter, not your eyes. Collect the water that runs out the bottom of the pot and read its pH. In range means real deficiency. Out of range means lockout, and the fix is flushing and correcting pH, not feeding.

Versus magnesium deficiency. Both hit old lower leaves. Nitrogen yellows the whole leaf uniformly, veins and all. Magnesium yellows the tissue between the veins and leaves the veins themselves green, so the leaf looks like a green skeleton on a yellow background. Magnesium also throws rusty brown spots. Nitrogen does not.

Versus sulfur deficiency. Sulfur is immobile, so it yellows the new top growth first while the bottom stays green. That is the exact opposite pattern. If the top of your plant is the yellow part, stop reading this page.

How long until it looks better#

The already-yellow leaves will not turn green again. That tissue is spent and the plant has taken what it needed from it. Leave them attached unless they are fully dead.

What you are watching is new growth. Within three to five days of a corrected feed, the newest leaves at the top should come in noticeably darker. Within seven to ten days the yellowing should stop advancing up the plant. If it is still climbing after a week of proper feeding at correct pH, your problem is in the root zone, not the bottle.