Nitrogen (N) in Cannabis – Deficiency and Requirements

Nitrogen deficiency begins with the lowest leaves turning light green and then yellow, and the yellow climbs upwards one tier at a time. In most trials, nitrogen above about 160 mg/L brought no extra yield. The figure means milligrams per litre of feed solution, the finished mix in the watering can. What nitrogen does and how much of it sits in the leaf is explained in the article on nutrient functions.

How do I recognise nitrogen deficiency?

The plant withdraws nitrogen from the old leaves and sends it to the new growth, so the lower leaves go hungry first and turn light green, later yellow. How far the yellow reaches shows the severity. At 80 mg/L in vegetative growth it stayed on the lower leaves, but at 30 mg/L it took over the whole plant. Sulphur deficiency looks different, because it starts in the middle of the plant and leaves the lower leaves green. In vegetative growth, magnesium deficiency keeps the veins green, whereas nitrogen deficiency lightens the whole leaf. Towards the end of flowering, the oldest fan leaves, meaning the large fingered leaves on the stem and branches, yellow even when the plant is fully fed.

Stage	Appearance
First sign	The plant grows more slowly than its neighbours, and the lowest leaves turn evenly light green, often first at the tips of the leaflets.
Advanced	The yellow grows strong and climbs into the middle of the plant. Within the leaf it runs from the base of the leaflets to the tip.
Advanced	The leaf stalks of the worst cases turn reddish brown, and veins and stalks may take on a red or purple tint.
Late	The lowest leaves turn brown, die and drop, and the plant stays clearly smaller.
In flowering	In week three the tips of the lower fan leaves yellow, and a week later the fan leaves of the lower two thirds are yellow.
In flowering	Main stem and lower branches turn yellow and finally brown. Last of all the sugar leaves, the small leaves inside the buds, yellow until only the youngest stay green.

Read the diagnostic path with root colour and non-nutrient look-alikes in the article on identifying nutrient deficiency.
Look at the bottom of the plant to see whether the yellow starts on the oldest leaves.
If the lower leaves stay green and the middle yellows, check for sulphur.
If the veins stay green in vegetative growth, check for magnesium.
Towards the end of flowering, separate ageing from deficiency as described in the article on leaf senescence.

What do I do about nitrogen deficiency?

The remedy is to feed heavily once or twice and then raise the standing value. The NC State group, the plant nutrition specialists at North Carolina State University, waters for this with 300 to 400 mg/L nitrogen. The nitrogen comes from a complete fertiliser that supplies all macronutrients. This corrective dose comes from advisory practice and has not been tested on cannabis as a standing value. It stops the progression, but strongly yellowed leaves do not recover.

The cause can be too little nitrogen in the fertiliser. In soil, an unsuitable pH can also make nitrogen scarce, the pH being the measure of how acidic a solution is. How this happens is explained in the article on nutrient lockout. In the runoff, the water draining out of the bottom of the pot, you can also measure the EC. This electrical conductivity captures all dissolved salts together and is given in mS/cm.

Check where the yellow sits and which phase the plant is in.
Measure the pH in the root zone. Look up the target range per medium and the runoff EC in the article on nutrient diagnosis by growing medium.
Water once or twice with 300 to 400 mg/L nitrogen, letting about 10 % run off.
Then give about 160 mg/L as a standing value.
Watch the new growth. If it stays green, give no further shock doses.

How much nitrogen does cannabis need?

Cannabis needs about 160 mg/L in the feed solution, in vegetative growth and in flowering alike. In vegetative growth, plants at 30 mg/L lacked three quarters of their mass, and at 240 mg/L the roots already grew worse. The trial tested no level between 160 and 240 mg/L.

In flowering, yield rose up to about 160 mg/L in three trials in substrate, meaning the material in the pot. Above that, bud mass kept rising in only one of the three trials. A THC-rich cultivar in perlite, a heat-expanded volcanic rock that binds hardly any nutrients, was not harmed by even 320 mg/L in flowering. In hydroponics, where the roots hang in the nutrient solution, the calculated optimum for flowering was 194 mg/L with weekly changes of the solution. As standing values from practice, the NC State group gives 150 to 200 mg/L in vegetative growth and 200 to 225 mg/L before flowering. For peak flowering it gives 100 to 150 mg/L, which contradicts the trial value of 160 mg/L, and the conflict is unresolved. Nitrate, one of the two nitrogen forms in fertiliser, also occurs in tap water.

In vegetative growth give 160 mg/L, not more.
In flowering stay at about 160 mg/L.
Count the nitrate from the tap water and follow the calculation in the article on calculating a nutrient solution.
For the question of flowering onset, read the article on feeding, root zone and flowering onset.

Does a lot of nitrogen in flowering cost potency?

In most flowering trials it does, but in one hydroponic trial it did not. Potency here means the cannabinoids such as THC and CBD. One test used a cultivar with about 7 % each of THC and CBD. Between 30 and 320 mg/L, its THCA, the acid form of THC in the plant, fell by 69 % and its CBDA by 63 %. In aeroponics, where freely hanging roots are sprayed with nutrient solution, 250 instead of 150 mg/L lowered the THC content of two cultivars by 9.5 %. Only one of the two cultivars gained 29 % more yield from it.

In a CBD cultivar in pots, bud mass and content moved in opposite directions, as the table shows. There 160 mg/L gave 95 % of the highest CBD yield, with a third less fertiliser than 240 mg/L. At 80 mg/L the cannabinoid yield fell clearly, because bud mass dropped more than content rose.

At 80 and 160 mg/L the plant also moved nitrogen from old leaves into the buds earlier than at 240 mg/L, a shift called remobilisation. The leaf there reached its highest nitrogen content after 54 instead of 83 days, by calculation 29 days earlier. The shift showed as paler leaves and later as red and purple leaf colouring. It is still open whether a tight supply makes the plant form more cannabinoids or only spreads them over less bud mass.

Nitrogen in flowering	Bud mass per plant	CBD content	CBD yield per plant
80 mg/L	16.1 g	6.3 %	989 mg
160 mg/L	22.6 g	6.2 %	1384 mg
240 mg/L	26.3 g	5.7 %	1450 mg

Feeding for weight: stay at about 160 mg/L.
Feeding for potency: stay at the low end and do not go above 160 mg/L.
Do not drop to 80 mg/L.
Read red and purple leaf colouring in flowering together with the nitrogen rate before feeding more.

How do I recognise too much nitrogen?

Too much nitrogen shows as a dark green, smaller plant, but dark green alone proves nothing. In the trial the leaves were equally dark at 160, 240 and 320 mg/L. The roots already grew worse at 240 mg/L, while the foliage still looked normal. Above 160 mg/L the root held less phosphorus, calcium and magnesium, and leaf and stem held less manganese.

The researchers name two causes: a toxic effect of nitrate or of ammonium, the second nitrogen form in fertiliser, and a salt effect of the high concentration. Growers call leaves that curl downwards the “claw”, and in the only scientific source the total salt content rose along with the nitrogen. So it has not been shown whether nitrogen alone causes the claw. In other crops, plant nutrition specialists at Montana State University describe soft and lush new shoots, weak stems and delayed maturity. Spider mites and aphids pierce soft tissue more easily, although on peas aphids multiplied more slowly under rich feeding. Data for cannabis are missing.

Too high an EC, by contrast, cups young leaves downwards and burns the margins evenly. Phosphorus deficiency also turns leaves dark green. With a cool or wet root zone it brings purple stems, and otherwise olive-green spots on the lower leaves.

Stage	Appearance
First sign	Leaves and shoots turn darker green, and the plant puts on a lot of leaf mass.
Advanced	Plant, leaves and side shoots stay shorter. At very high rates of a complete fertiliser, glossy dark green leaves curl downwards.
Advanced	Under complete fertiliser the older lower fan leaves died from 300 mg/L upwards. The leaf tips burned slightly, and the feed solution measured 1.59 mS/cm.
Late	Under complete fertiliser at 450 and 600 mg/L, dead tissue spread through the whole plant.
In flowering	A lot of nitrogen favours leaves at the expense of buds, and given late it drives extra leaves.
In flowering	500 mg/L from the start of flowering drove leaves and thicker stems but hardly any buds, and cannabinoid contents fell clearly.

Measure EC in the runoff.
If it is high, flush with plain water.
Check cupped young leaves with evenly burned margins for salt damage.
Check purple stems or olive-green spots low on the plant as a sign of phosphorus deficiency.
Lower nitrogen to 160 mg/L.
Use the new growth to see whether the change works.

How much ammonium can cannabis tolerate?

Ammonium should make up no more than 30 % of the nitrogen, and in a perlite trial with one cultivar 0 % was best for yield. Plants take up nitrogen both as nitrate and as ammonium, and they can store nitrate in large amounts. Accumulated ammonium, by contrast, damages the cells, and that damage builds up and becomes visible late. Leaf chlorophyll fell as the ammonium share rose, so dark green, soft foliage points to too much nitrogen in total. Deformed shoot tips can also come from calcium or boron deficiency. Without any ammonium the runoff pH rose and the leaf margins developed fine dots, and 10 to 30 % ammonium prevented both.

Urea, an organic nitrogen compound, turns into ammonium in the substrate. In the researchers’ interpretation, organic liquid fertilisers carry more ammonium than mineral ones. Horticultural fact sheets give less than 40 % for ammonium and urea combined. In other crops the damage starts with lightening between the veins or at the margin, and yellow spots or lime-green foliage also occur. Later the leaf margins roll, and the root tips die off orange-brown. Potted chrysanthemums developed thick, leathery leaves with dead areas along the midrib.

On tomatoes, lesions formed on the stem, which an equally high potassium rate prevented. In ornamentals, enough potassium lowers the ammonium in the leaf, while cool, wet, poorly aerated substrate and low light favour the damage. Fact sheets name substrate temperatures below 13 to 15.5 °C or below 65 °F, by calculation 18.3 °C. In a small hydroponic system with three plants per level, the best cannabinoid values came at 40 % ammonium. That finding is methodologically weak and does not move the limit. In the perlite trial total nitrogen stayed at 200 mg/L, and only the ammonium share rose:

Ammonium share of nitrogen	Bud mass compared with 0 %	Appearance
0 %	Highest value	runoff pH rose, leaf margins with fine pale dots
10 %	35 % less	no visible damage
30 %	32 % less	no visible damage
50 %	46 % less	almost all of flowering unremarkable, then wilting, 40 % of plants dead
100 %	97 % less	curled shoot tips with brown margins after 17 days, plants died

Read nitrate nitrogen and ammonium nitrogen separately on the label.
Count urea as ammonium.
Bring ammonium to no more than 30 % of total nitrogen.
If shoot tips are deformed, check fertiliser and substrate temperature. A lot of ammonium and cold, wet substrate point to ammonium damage.
Lower the ammonium share, let the substrate dry down and keep it warmer.
Make sure there is enough potassium.
With pure nitrate, measure the pH in the runoff.