Clover is one of the few plants that can take nitrogen from the air. Most plants draw their nitrogen from the soil. Macleay Grass Man commons.wikimedia.org/wiki/File:Trifolium_repens_leaf_NC6.jpg

Is too much Nitrogen a bad thing?

Of all the plant nutrients, your plant needs nitrogen in the greatest amounts. It is required for vital life functions. These include photosynthesis, which produces energy for the plant, and protein synthesis, which builds plant tissue. Plants can absorb three different forms of nitrogen: (NH4+) or ammonium;  nitrate  (NO3-),  and urea,  ((NH3)2CO. 

As with anything, however, you can have too much of a good thing. Excess nitrogen will kill your plant. 

When the plant is taking up too much nitrogen, it accumulates in plant tissue. If the plant’s source of nitrogen is mainly NH4+ (ammonium), the chances of toxicity are increased. Too much ammonium decreases amounts of ATP which allows energy to be released from photosynthesis. 

Plants tend to be able to tolerate higher amounts of (NO3-) or nitrate than NH4+ (ammonium). However, it can still reach toxic levels. Its main effect is to cause iron deficiency in plant leaves. The leaf will turn yellow while the veins remain green. 

How much nitrogen is too much?

Different species require different levels of nitrogen for peak health. They also have varying thresholds for nitrogen toxicity. 

Nitrogen toxicity does not happen as soon as plants receive more nitrogen than it uses. When plants have just a little more nitrogen than they need, they enter what’s called the ‘luxury consumption’ state. Here, the nitrogen excess is small enough that it does not damage the plant. However, because the plant is getting more nitrogen than it needs, it won’t increase plant health or yield. The danger zone occurs when nitrogen supply increases beyond this luxury consumption phase. The plant enters a ‘critical concentration’ where nitrogen accumulates in toxic amounts. 

What are the signs of nitrogen toxicity in plants?

The main sign of nitrogen toxicity in plants will be leaf discoloration. This will start on the very outer edges of the leaf and spread inwards. The outer leaf is made of more mature tissue where nitrogen will have had more time to accumulate. After this, the veins on the leaf will start to collapse. Too much nitrogen also increases the plant’s susceptibility to bacterial and fungal diseases. 

The leaves of this blueberry bush are showing signs of nitrogen toxicity Cityside189 https://commons.wikimedia.org/wiki/File:Blueberry_nitrogen_burn.JPG

What are the causes of nitrogen toxicity in plants?

Nitrogen toxicity is common in winter months when over-zealous gardeners stick to a heavy summer fertilizing schedule even though the plants have stopped growing. You should be fertilizing much less in the winter, increasing the amount at the start of spring.

Brand new store-bought plants are also at risk of over-fertilization because the soil might already contain slow release fertilizer that will last a few weeks or months. 

How do I make sure I don’t give my plants too much nitrogen? 

Always err on the side of caution when applying fertilizers. For small plants, use fertilizers with a low NPK ratio. The NPK ratio is found on all bottles of fertilizer. It indicates the percentage of nitrogen, phosphorus, and potassium inside the fertilizer solution. A balanced 5:5:5 fertilizer that contains 5% nitrogen, 5% phosphorus, and 5% potassium is suitable for smaller plants. A fertilizer with a higher NPK ratio of 15:15:15 would be better for larger plants since it contains more of the three macronutrients. Always read the manufacturer’s instructions for how much fertilizer you should add to your plant’s water. 

For your most prized plants, you can perform a soil nutrient test. By testing how much of each nutrient is present in the soil, you can determine exactly how much fertilizer and what kind of fertilizer you should be applying. 

Here are some of our favorite balance fertilizers (as an Amazon associate we earn from qualifying purchases)

Learn More

• The N in NPK - Nitrogen: What does it do for Plants?
• The P in NPK - Phosphorus: What does it do for Plants?
• The K in NPK - Potassium: What does it do for Plants?
• Make a homemade balanced fertilizer recipe
• DIY - a Homemade potassium fertilizer recipe for flowering and fruiting plants
• How to make Organic Phosphorus fertilizer
• DIY - Homemade Nitrogen Fertilizer - Natural sources of Nitrogen for Plants