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Why Don’t Legumes Need Nitrogen Containing Fertilizers – The #1 Reason

Nitrogen is a necessary part of life. Approximately 68 percent of air is made up of nitrogen. Even water in coastal areas consists of nitrogen gas. There is no place on the face of the planet which is untouched by nitrogen.

This is why most living beings are dependent on nitrogen. Our lives are evolved with the presence of nitrogen in the environment. Humans need nitrogen for their bodies to make proteins in the muscles, skin, blood, hair, nails, and DNA. Humans get this nitrogen from protein-heavy foods.

Similarly, plants need nitrogen as well. They get the much-needed nitrogen from the soil, and when the soil is lacking in nitrogen, we feed the plants with nitrogen-containing fertilizers. But unlike many other plants, legumes don’t need nitrogen-based fertilizers.

If you’re wondering why that is, we’re here to answer the same question. Let’s find out why don’t legumes need nitrogen containing fertilizers?

Why Don't Legumes Need Nitrogen Containing Fertilizers
Legumes – via Wikimedia

What Are Legumes?

Legumes are one of the healthiest foods. They’re known for providing nourishment and being affordable and accessible. Some even call them a nutritional powerhouse. Legumes are rich in proteins, fiber, folate, iron, phosphorus, and fatty acids.

Many people use the terms legumes, beans, and pulses interchangeably but legume refers to the whole plant and pulses are the edible seeds. So a legume includes the leaves, stems, and pods, while a pulse is just the seed, like peas, beans, and lentils.

Some popular and common legumes are:

  • Red beans
  • Chickpeas
  • Black-eyed peas
  • Soy nuts
  • Fava beans
  • Anasazi beans
  • Edamame
  • Peanuts (some sources consider it a legume, while others don’t)

Legumes are a food staple in many cultures. According to studies, in Mediterranean areas, people are said to consume between 8 to 23 grams of legumes per day. And in northern Europe, people consume less than 5 grams of legumes a day.

Legumes are low in calories and high in nutrition. This is why they’re a popular choice amongst diet foods. The lowest calorie legume is Mung beans, with 12 calories per 100 grams.

All of this information about legume seeds is fascinating. We can see how legumes are different from all the other food sources, but what makes them stand out the most is that they don’t require any nitrogen-containing fertilizer!

The reason behind why don’t legumes need nitrogen containing fertilizers? is that legumes can convert nitrogen from the air into useable compounds. Fascinating, right?

Why Dont Legumes Need Nitrogen Containing Fertilizers 2
Assortment of legumes in wooden spoons on black background – via Flickr

Why Do Plants Need Nitrogen?

If a plant is healthy, its above-ground tissues will have 3 to 4 percent nitrogen. Most soil fertility management programs focus just on nitrogen in the soil.

Chlorophyll is the compound of plants that traps energy from sunlight and uses it to produce sugar from carbon dioxide and water (this process is called photosynthesis). This photosynthetic pigment is green in color and plays an important role in the growth of plants.

The major component of chlorophyll is amino acids, which are the basic building block that forms proteins. Without proteins, the plants will stop growing and eventually wither and die. Proteins are structural elements of plants and are also used in the production of enzymes.

Nitrogen is used to construct proteins. Hence, without nitrogen, the plant will die. Nitrogen is also a component of Adenosine Triphosphate (also known as ATP). ATPs capture chemical energy and use it for life processes in a living organism.

Nitrogen is also a major component of nucleic acids such as DNA. It helps the plant grow and reproduce. This is why it is surprising that legumes can grow without any supplementary nitrogen.

Nitrogen Fixation

Nitrogen fixation is a process where nitrogen gas from the air is converted into nitrogen-containing compounds such as ammonia. These compounds are useful in biochemical processes.

This process occurs in legumes when bacteria called rhizobia (present in the soil) infect the roots of the legumes (such as beans, peas, and alfalfa). After infection, the roots of the legume form a growth around the bacteria called a nodule. This process is called nodulation.

The bacteria in these nodules are responsible for converting atmospheric nitrogen into compounds such as ammonia and nitrate. The plant then flowers, and after the flowering, the nitrogen moves from the roots to the seeds.

The legumes eventually die with time, and the nitrogen decomposes into the soil. The nitrogen will release into the air, half of it within the year and the rest within the next few years. Legumes don’t need nitrogen-containing fertilizers because they make their nitrogen-based compounds using the nitrogen from the air.

The reason for legumes not needing nitrogen-based fertilizers isn’t that they don’t have any use for it. But because they’re capable of fulfilling their own nitrogen needs and they don’t depend on supplements. Legumes still need nitrogen for their growth and well-being, the same way other plants do.

Not only that, but it also fertilizes the soil with nitrogen, which can be used up by other plants later.


Many people decide to rely fully on legumes to add nitrogen to their soils. But sometimes native rhizobia bacteria found in the soil are ineffective in nitrogen fixation. That’s when inoculation comes to use.

Inoculation is a process where prepared sources of rhizobia are introduced commercially to the soil to encourage nitrogen fixation. Commercial inoculants contain rhizobia with maximum potential to fix nitrogen.

Why Don’t Legumes Need Nitrogen Containing Fertilizers?

The above-mentioned information gives us the answer. Legumes don’t need nitrogen-based fertilizers because they are capable of nitrogen fixation. Nitrogen fixation is the process that converts nitrogen from the air into usable compounds like ammonia and nitrate. These compounds help construct proteins and encourage their growth.

The roots of the legumes form nodules around the bacteria, and these chemical conversions take place in those nodules. So why would legumes need fertilizer to provide them with nitrogen-based when they can make them on their own?


All species need nitrogen. It is the building block of proteins, and without protein, any living organism would die. Legumes are the same. But unlike other living creatures, they can fix nitrogen and make their nitrogen-containing compounds, using the nitrogen in the air.

Legumes can live without nitrogen fertilizers because the nitrogen from the air is enough for their survival.

Hopefully, this answers your question. If you have any more queries, leave them in the comment section down below and we’ll get back to you!