The 'N' in NPK

Part of our series on NPK Plant Nutrients

Read part 2 here

Read part 3 here

NPK stands for nitrogen, phosphorus, and potassium respectively. These are plant macronutrients - the three nutrients needed in the largest amounts by plants. The NPK ratio found on fertilizers, such as 15:15:15 or 5:20:20, indicates the percentage of these nutrients inside the fertilizer. A 15:2:2 fertilizer would be made up of 15% nitrogen.

After carbon, hydrogen, and oxygen, nitrogen is the element that is most abundant in plants. Concentrations of nitrogen in the plants fluctuate depending on the season. During the the early growth season, nitrogen concentrations will peak.

Over the last 50 years, there has been a massive increase in food production globally. Much of this is thanks to the increased use of synthetic nitrogen fertilizer which is necessary to replace the large amounts that crops remove  from the soil. 

There are many chemical forms of nitrogen but plants can take up only some of them. The common ones used by plants are ammonium (NH⁴+), nitrate (NO^3-), and urea, CO(NH₂)₂. Mostly, higher plants will take up nitrate (NO^3-) from the soil. Once NO3 enters a root cell it can diffuse from cell to cell. For the nitrate to become usable for the plant, the plant must reduce it to ammonium. This happens in the chloroplasts using energy from photosynthesis. The ammonium can be metabolized into amino acids where they are stored or transported to other parts of the plants through xylem vessels.

Plants can store nitrogen so that it can be distributed to the plant organs when needed. When the plant loses leaves or other parts, re-growth draws on nitrogen reserves in the parts of the plant that remain and from the root system. Once the plant part re-grows and becomes capable of photosynthesis, it will accumulate its reserves of nitrogen.

What does nitrogen do for plants?

Nitrogen is a major component of chlorophyll, a molecule essential for photosynthesis. Because chlorophyll is found in leaves, between 25-75% of the plant’s total nitrogen is stored here.

Chlorophyll gives plants their green color so nitrogen deficiency turns leaves pale. Chlorophyll sits within structures called chloroplasts, a component of plant cells. Usually, chloroplasts are elliptical but nitrogen deficiency can make them circular and swollen.

Nitrogen is essential for healthy green foliage Erol Ahmed https://unsplash.com/photos/wKTF65TcReY

Nitrogen also makes up the amino acids that enable protein synthesis. Around 85% of the total nitrogen in plants will be found in proteins. 5 % will be found in nucleic acids (DNA and RNA). Seeds contain reserves of polysaccharides, proteins, and lipids. Synthesizing these requires both carbon and nitrogen.

Nitrogen deficiency restricts plant organ growth - roots, stems, leaves, flowers, fruits, and seeds will be affected. It also causes chloroplast disintegration and, eventually, death. If the plant is deprived of nitrogen as it grows, lower leaves on the stem will become pale. This is because nitrogen reserves from the base of the plant will be transported to younger leaves at the very top if their nitrogen levels are low. If the plant has been deprived of nitrogen since it was a seedling, the plant will look pale, weak, and spindly overall. 

The pale, yellow-ish leaves on this plant indicate a nitrogen deficiency Rasbak https://commons.wikimedia.org/wiki/File:Phaseolus_vulgaris_nitrogen_deficiency.jpg

However, be wary of over-fertilizing your houseplants with nitrogen. Nitrogen can boost plant growth but growth that is too rapid can be weak and more susceptible to disease. Excess nitrogen can accumulate in soils and can burn the edges of leaves and kill the plant. Always err on the side of caution with fertilizers. Make sure to follow manufacturer's instructions and if your plant is small, dilute the fertilizer more than is recommended on the packet.

This drawing shows signs of nutritional deficiency in apple leaves. Figure 6 (bottom left) shows a nitrogen deficient leaf. Figure 7 (bottom center) shows a leaf burnt by excess nitrogen. Ottawa : Dept. of Agriculture, https://commons.wikimedia.org/wiki/File:Apple_nutrition_(1941)_(19124665773).jpg

Where to get Nitrogen Fertilizers 

Here are some of our favorite Nitrogen heavy fertilizer products. (as an Amazon associate we earn from qualifying purchases)

Make Your Own Nitrogen Fertilizer

To make your own organic nitrogen fertilizer at home with natural ingredients, see our recipe. 

Learn More

• What happens when a plant gets too much Nitrogen?
• 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