Understanding Nitrogen Toxicity and How to Prevent It

Author: Kevin Espiritu, Epic Gardening

Hydroponic gardening is wonderful for so many reasons, but we growers still run into problems from time to time. In fact, as a beginner there can be more problems than gardening in soil!

Nitrogen toxicity is one of these annoying problems. Nitrogen is one of the most important nutrients for plants, but too much can cause serious harm to your garden. If left unchecked, nitrogen toxicity can completely kill your plants.

What is Nitrogen Toxicity?

Plants need a lot of nitrogen, especially in their vegetative stage of growth. Most of the time it’s hard to give them too much, but every now and then you can get too excited and flood your nutrient reservoir with excess nitrogen.

When that happens, your plants cannot handle the surplus and begin to struggle. This happens most often in the flowering stage of growth, when plants need less nitrogen than they did in earlier stages. If you keep nitrogen levels stable and don’t properly adjust your nutrient mixture, you could be setting your garden up for nitrogen toxicity.

How Much Nitrogen Does Your Plant Need?

Note: These are general rules of thumb, not specific prescriptions for particular types of plants. Every type of plant is different and requires a different mixture of nutrients for optimal growth. However, there are consistent patterns between all plants that you should know.

Vegetative Stage

During this stage, your plant will feast on nitrogen as it develops the stems, roots, and foliage to support energy production during flowering phase. Almost any type of hydroponic nutrient will do well here.

Flowering / Blooming Stage

During this stage, your plants are transitioning from developing a lot of leaves and stems and focusing their energy on buds, flowers, and fruit. If you give your plants too much nitrogen during this phase you can seriously impact its development.

Signs of Nitrogen Toxicity

An example of the opposite problem, nitrogen deficiency. If you take the “normal” appearance and darken it, you have a classic symptom of nitrogen toxicity.

While the signs of nitrogen toxicity vary slightly based on the plant that you’re growing, there are some universal signals that point to nitrogen toxicity as the problem you’re facing:

  1. Extremely dark green leaves
  2. “Burning” of leaf tips, causing them to turn brown
  3. Some leaves turning yellow, due to abundance of nitrogen but lack of other nutrients

These symptoms are exacerbated if the pH of your solution is off or if you are over or under-watering your plant. All of these cause your plant to stress easier, making the effects of too much nitrogen more pronounced.

Ruling Out Other Potential Plant Problems

Before you jump to treating your plant for nitrogen toxicity, it’s a good idea to rule out any other potential causes.

First, make sure you aren’t over or under-watering your plant. If you’re growing hydroponically, you shouldn’t have to worry about this because the roots are likely sitting in a nutrient solution of some kind.

Make sure your nutrient solution is being properly oxygenated with air stones, and that the pH and temperature of your nutrient solution are within normal ranges.

Finally, check the air temperature of your grow tent or grow room and make sure it’s not too hot or cold.

If your garden passes all of these checks, it’s very likely you have a nitrogen toxicity problem and you can move on to treatment and prevention options.

Treating Nitrogen Toxicity

Nutrient burn in a tomato plant.

If you guessed that the solution to nitrogen toxicity is “stop giving your plants so much nitrogen,” you’d be right! But treating an existing garden suffering from nitrogen toxicity isn’t quite that simple, though it’s still pretty easy to fix.

Check the PPM / EC of your solution and ensure that is within normal ranges for your plant’s stage of growth. If it isn’t, you can:

  • Dilute the solution with fresh water, being sure to calibrate your pH afterwards
  • Change out your nutrient reservoir completely with fresh water and nutrients

If your plant is in flowering phase, nutrient levels look good, and you’re still suffering from symptoms of nitrogen toxicity, you should probably do a complete reservoir change and add fresh nutrients. It might be the case that you have too much nitrogen as a percentage of total nutrients in the solution, causing poor results during flowering phase.

If you’re growing in soil but don’t feel that you’re over-feeding your plants, you may just have soil that has a higher concentration of nitrogen than normal. To fix this, filter and pH some water and pour it in the soil that houses the affected plants. This will flush out some of the nutrient contents of your soil and hopefully solve the problem.