Nitrogen might be the most important nutrient for a developing plant. However, overzealous growers may occasionally find that they’ve gone a bit overboard in their feeding, resulting in nitrogen toxicity. Since this can be fatal if left untreated, knowing how to spot the signs early can save your plant before any damage is done.
Nitrogen toxicity symptoms
The most common symptom of nitrogen toxicity in weed plants is dark green and shiny leaves. Since nitrogen is one of the key components of chlorophyll, too much nitrogen can cause a plant to develop too much chlorophyll, leading to even further problems. While chlorophyll is required for photosynthesis, there are limits to how much a plant actually needs.
Too much chlorophyll can, counterintuitively, make cannabis plants less capable of absorbing sunlight as it blocks out other compounds that are also required for a healthy plant. This can make the plant more susceptible to things like light burn or any other stressor.
Besides turning dark green, the leaves of a weed plant suffering from nitrogen toxicity will likely begin to curl downwards at the tips. This “clawing” is perhaps the most notable sign of nitrogen toxicity, though it can also be caused by overwatering (more on that below). Both of these symptoms are more likely to start at the bottom of the plant before working their way up. If left unchecked, these leaves will slowly turn brown and burn away rendering them completely useless.
During the flowering phase of weed plants, nitrogen toxicity will cause them to develop small, lightweight, and airy buds that lack any real density. It’s worth noting that leaves during flowering (Particularly in the later stages) will naturally begin to turn brown and die off, especially in the lower sections of the plant. While this can look like a nitrogen problem (nitrogen deficiency and toxicity can both cause this issue), it’s a natural part of plant development. Don’t adjust your feeding just because of this.
Don’t confuse cannabis nitrogen toxicity with other issues
1. Nitrogen toxicity vs overwatering
As briefly mentioned above, the symptoms of nitrogen toxicity (dark green leaves that claw downwards) can also be caused by overwatering. Knowing which problem you’re treating is the most important thing at this point so being able to tell the two apart just might save your weed plant.
Nitrogen toxicity will lead to very dark shiny leaves that curl downward at the tips. On top of this, you’ll likely notice severely stunted growth and—in extreme cases—leaf burn. On the other hand, overwatering will lead to leaves that are only slightly darker (if at all), less pronounced curling, and severe wilting and yellowing of the leaves.
How can you tell the difference between nitrogen toxicity and overwatering?
The most obvious way is to simply check the soil; is it waterlogged or muddy? Well, there’s your problem. Check out our complete guide on overwatering to learn how to prevent and treat this potentially catastrophic issue.
2. Nitrogen toxicity vs light burn
Overwatering isn’t the only issue that can be confused with nitrogen toxicity, though. As I stated above, excess nitrogen can cause a weed plant to be less able to absorb light and this can lead to light burn. If you’re seeing brown crispy leaves or bleached buds at the top of your cannabis plant, you might be dealing with light burn.
While your first instinct might be to raise the light (this would help), if you’re not treating the root cause, you’re unlikely to see much change. If light burn is the only problem you’re dealing with, the lower parts of your plant will be unaffected. You won’t see any clawing and the lower leaves will remain a vibrant green.
3. Nitrogen toxicity vs other nutrient imbalances
Lastly, nitrogen toxicity can be confused with a handful of other nutrient imbalances. Iron, sulphur, and nitrogen deficiencies can all cause yellowing leaves, though in slightly different ways:
- Nitrogen toxicity typically involves yellowing of the entire leaf starting at the bottom of the weed plant and climbing upwards slowly.
- Iron deficiency commonly presents itself as yellowing in the areas between the veins on a leaf while the veins stay green. This is known as interveinal chlorosis and is a symptom of a range of micronutrient deficiencies.
- Sulphur deficiency causes yellowing among lower leaves.
- Potassium deficiency commonly causes yellowing along the outer areas of leaves at the bottom of the plant.
4. Nitrogen toxicity vs deficiency
If you’re worried about giving your cannabis plants too much nitrogen, know that nitrogen deficiency is also a serious concern. While the symptoms are unlikely to be confused, they’re important to keep an eye out for if you’re reducing your cannabis plant’s nitrogen intake.
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What causes nitrogen toxicity in weed plants?
The most obvious answer is simply “too much nitrogen” but, as always, there’s a little more to that. Overall, there are two major reasons why a plant might be experiencing nitrogen toxicity; overfeeding or having too much nitrogen in the soil.
Overfeeding
Overfeeding can happen when overly eager growers apply nitrogen-rich fertilisers at higher levels than necessary. Seriously, always follow the guidelines on the package of whatever nutrients you’re using. The manufacturers know what they’re doing. More food does not mean more weed.
Soil overloaded with nitrogen
On the other hand, soil can become overloaded with nitrogen when organic matter decomposes or when nitrogen-based fertilisers are overused. This can lead to an imbalance where plants can’t use the nitrogen they get causing it to build up. If you want to avoid this, you can easily make your own soil at home.
How to fix nitrogen toxicity in cannabis
Regardless of the cause, once you’ve identified that nitrogen toxicity is the problem, the solution is the same:
- Stop fertilising your weed plant
It sounds obvious, but if your plant has been getting too much nitrogen, giving it more is a bad idea. Stop feeding it immediately.
- Flush your soil
Give your plant plain, pH-balanced water until you start seeing runoff. Let the soil dry out and repeat this 2-4 times. This can take several days so don’t rush it.
- Reassess your watering practices
While flushing your cannabis plants, monitor the drainage of your pots. If they’re not draining adequately, then you’ll wind up with excess water and nutrients building up in the soil. Depending on your setup, you may need to add some more drainage holes (if you’re using a plastic pot) or simply water less frequently. It’s also worth checking your humidity, if it’s above 65% (during vegetation or above 60% during flowering) lowering it could help your drainage problems.
- Prune affected leaves
Once a weed leaf has begun yellowing from nitrogen toxicity, it’s not going to get better. Promptly remove any damaged leaves (these should be lower, older leaves anyway so it’s not a major loss).
- Adjust pH
Imbalanced pH can cause a host of nutrient deficiencies or toxicities. Without going too much into the science, cannabis plants can only absorb nutrients when the pH of the medium is in a narrow range; 6.0-7.0 for soil and 5.5-6.5 for hydro. Going above or below that is almost certain to cause nutrient issues.
- Monitor new growth
Keep an eye on what your plant does over the next few days. If everything goes well, you’ll start to notice healthy new growth and older growth should start to return to its original look within 5-7 days.
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Should I transplant a plant with nitrogen toxicity?
While it might be tempting to simply transplant to clean soil, this is not a good idea. Nitrogen toxicity is incredibly stressful for a plant; as is transplantation. Moving an already stressed cannabis plant is likely to do far more harm than good. If you absolutely must transplant your plant (e.g., it’s too large for its pot and was due for transplanting anyway), then wait at least a week after solving any nutrient issues.
Preventing nitrogen toxicity
Of course, it’s always easier to prevent a problem than fix it. Luckily, avoiding nitrogen toxicity is super simple:
- Use slow-release fertilisers
Organic fertilisers tend to offer a slower release of nutrients meaning that plants are less likely to be overwhelmed by a sudden influx.
- Reduce feeding at the end of the vegetative stage
Since cannabis plants need much less nitrogen during flowering, lowering your input in the final week or two of vegetation can help prevent issues.
- Flush your plants before flowering
Since your soil is likely full of nitrogen (it should be, at least), flushing for a day or two before switching to flowering nutrients is never a bad idea.
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Keep nitrogen toxicity at bay
The very definition of “too much of a good thing”, nitrogen toxicity is often born out of genuine over-eagerness. By taking things a little bit slower and following the guidelines on your fertiliser, this common problem is entirely preventable. Even if you do find yourself dealing with it, solutions are quick and easy.
