What is nitrogen fixation (simple terms)?

Nitrogen fixation is a really fascinating process that is often misunderstood. We often say, for simplicity’s sake, that certain plants fix nitrogen. This understandably leads people to the conclusion that the plants absorb the nitrogen from the air on their own.

We need to look below the surface - literally, below the surface of the soil - to see what’s truly happening.

In simple terms, nitrogen fixation actually occurs because of a mutual relationship between the plants and soil microbes. It’s the soil fungi and bacteria that are able to transform the nitrogen from the atmosphere and offer it to plants.

You may have seen the nitrogen in the form of tiny white-pink nodules on the roots of bean plants or even clover.

This connection with the root system is essential to understand when if you aim to work with nitrogen fixing plants (or, more accurately, nitrogen fixing soil bacteria).

If you pull up the whole plant, the nitrogen will not be made available in the soil because you’re actually removing it.

This is one of the many reasons why regenerative no-till gardening principles, such as leaving roots in the ground after harvest, support the vitality of our soils and gardens.

Let’s dive deeper into the fascinating soil ecosystem service of nitrogen fixation, and learn how regenerative no-till farming enhances this cycle in our land.

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Why is nitrogen fixation important? What’s the benefit of nitrogen for plants?

Nitrogen is an essential nutrient for plant growth - in fact, many farmers purchase nitrogen fertilizer to add to their crops in order for their plants to grow lush and green.

You may have seen NPK ratings on fertilizer. The N stands for nitrogen, so the first number tells you the proportion of nitrogen that the fertilizer has.

Plants might not be getting enough nitrogen if they start to have yellow leaves or stop growing. This in turn can cause the plants to be vulnerable to pest damage, disease, and reduced harvests.

Learn more about the importance of nutrients for your crops and why you should only use organic fertilizer.

The Importance of Nitrogen fixation bacteria & fungi in the soil

Nitrogen fixing bacteria and fungi live in the soil and form relationships with the root systems of certain plants.

According to Jeff Lowenfels in his amazing book series, Teaming with Microbes, perennial plants are often found in soil that has a high fungal population, and annuals are often in soil with a majority of microbes are bacteria.

The fungi convert the atmospheric nitrogen into ammonium, whereas the bacteria form nitrates.

How does bacteria transform atmospheric nitrogen?

According to Dr. Christopher S. Baird, the nitrogen found in the atmosphere is in the form N2. This form is two nitrogen atoms that are tightly bonded together into a molecule.

Dr. Baird goes on to explain that plants on their own are not able to break this strong bond, but at the same time, they cannot use the N2 form of the nitrogen. Plants instead need the singular nitrogen atom.

However, bacteria and fungi are able to break the N2 bond and “fix” the nitrogen from the air into NH3 or ammonia, where the nitrogen atom bonds to three hydrogen atoms a usable form for the plants.

Then, bacteria goes a step further and transforms ammonia into nitrates, which is the form that annual plants prefer (Teaming with Microbes).

Some bacteria live on the roots of plants. For example, legumes will often have nodules of nitrogen on their roots, formed by the bacteria.

According to Nature, other bacteria in the soil fix nitrogen that don’t attach themselves to the roots of plants, and thus are called “free living.”

How does fungi transform atmospheric nitrogen?

Jeff Lowenfels in Teaming with Microbes points out that while most plants benefit from relationships with fungi, trees and other perennials benefit from living in fugally dominated soil, as they prefer their nitrogen in the form of ammonium.

Since fungi turn atmospheric nitrogen into ammonium, we often find perennials growing in soils that are robust with mycorrhizae, such as the forest.

What’s so fascinating about fungi is that they not only fix atmospheric nitrogen, but help mine nitrogen from dead plants and animals as well. According to Utah State University, the fungi act as decomposers and bring that nitrogen back into a bioavailable form for plants in the soil.

One kind of fungi that help to deliver nitrogen to plants in a usable form are the powerhouses known as arbuscular mycorrhizal fungi, which also happen to be a type of fungi that captures carbon in the soil.

Unlike the nitrogen fixing bacteria, which only has relationships with the roots of certain plants, arbuscular mycorrhizal fungi can build relationships with most plants (source).

As gardeners, homesteaders, and farmers, it’s important to note that brassicas and the beet family do not form relationships with mycorrhizae, so if you were going to innoculate your crops, it’s best to skip these plant families. I first learned this from Jesse Frost in his book The Living Soil Handbook.

A study done in 2019 estimated that mycorrhizal fungus contributes an excess of 70 Tg of assimilated nitrogen by plants each year!

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Why do fungi and bacteria offer this nitrogen-fixing ecosystem service?

Fungi and bacteria offer fixed nitrogen to plant in exchange for carbon. When plants go through the process of photosynthesis, they make sugars in the form of carbon. They then offer a portion of this carbon to bacteria and fungi in exchange for other nutrients.

The carbon is food for the bacteria and fungi, and is how soil is such a big storage center for atmospheric carbon dioxide.

In other words, in order to sequester more carbon in our gardens and on farms, we need to maximize photosynthesis and soil health. This can be done with organic regenerative no-till practices.

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How does organic no-till regenerative agriculture + gardening practices enhance nitrogen fixation?

If you’re unfamiliar with regenerative agriculture, learn the full picture in our regenerative agriculture knowledge base here.

Organic gardening increases soil microbes, some of which in turn fix nitrogen

Regenerative agriculture is an organic method of farming.

As mentioned in Teaming with Microbes, synthetic (non-organic) fertilizers deliver the nutrients directly to the plants.

While on the surface this sounds great, it actually skips the beneficial relationship between the plants and microbes.

When the plants no longer need the microbes to offer them fixed nitrogen, then they no longer offer carbon as food from photosynthesis.

So since the microbes aren’t in an optimal situation, their numbers decrease.

Similarly, synthetic pesticides and herbicides can decimate the soil microbiome.

On the other hand, organic fertilizers work with soil microbes, and let them be the ones to deliver the nutrients to the plants. This keeps the relationship active and helps soil health to thrive.

One thing to note is that even though copper fungicide is utilized in organic agriculture systems, since it is a fungicide it reduces the amount of arbuscular mycorrhizae, as evidenced in this 2022 study published in Frontiers in Agronomy.

No-till gardening maintains the soil microbiome instead of damaging it

Regenerative agriculture upholds a core principle of not tilling the soil.

When you think of what tilling does to the soil, it’s no wonder that tilling damages the soil microbiome, breaks fungal networks and damages soil bacteria.

This study out of South Dakota State University highlights how both long-term and medium-term no-till farms had far better soil health with stronger microbial populations.

Kristine Nichols, who was a chief scientist at the Rodale Institute, was interviewed for an article published by Civil Eats. In this article, she shares that “Tilling can start to erode the diversity of the fungi in the soil over time. So you’re going to start getting the loss of certain keystone organisms for providing amino acids and antioxidants that can be very important for human health.”

She collaborated with Penn State University and found that the amount of antioxidant and amino acid that is made by the fungi in the soil had a direct correlation with tillage, where it was more available in no-till soil.

Keeping roots in the ground makes the nitrogen nodules available for the next crop and offers bacteria and fungi food as the root material breaks down

Regenerative agriculture encourages keeping plant roots in the ground whenever possible. This means that after harvest, plants are not pulled up fully but are cut at the plant base.

This process supports nitrogen fixation in the soil. Nitrogen fixing plants need to be cut back so that the roots stay in the ground in order for nitrogen to be available for the next crop.

The nitrogen is stored in the root nodules, so if you pull up the plant by the root, you are essentially removing the nitrogen.

Cover crops help the soil microbiome to thrive

An essential component of regenerative agriculture is cover cropping, which is the process of planting beneficial crops into the soil. This usually happens in the off-season, when the fields would otherwise be unplanted.

Cover crops offer the soil microbiome a supply of food throughout the off-season. When cover crops are growing in the soil, they are engaging in photosynthesis and making carbon to feed the soil microbiome.

This article from the USDA highlights how the residue from the cover crops also gets taken down into the soil by earthworms, which the soil microbes feed on.

The more food available, the more robust the soil ecosystem is, which in turn captures more carbon and benefits the next crop.

This was proven by a study in 2019, where cover cropping increased the amount of bacteria and fungi in the soil, and had more diverse populations within the microbiome. These effects were enhanced even more when combined with no-till farming.

Polyculture/Interplanting offers an opportunity to plant crops with nitrogen fixing capabilities

Similar to how permaculture encourages plant guilds, which are communities of plants that benefit one another, interplanting is a process that is implemented by regenerative farmers.

In this way, plants that have relationships with nitrogen-fixing bacteria can be planted near crops that have higher nitrogen requirements. As the plants with the nitrogen root nodules are cut back, the nitrogen becomes available to the nearby crops.

Related: Learn how biodiversity in the garden fights climate change

Keep the soil covered by mulching to protect the microbiome

Regenerative farmers often will add mulch of some kind to their soil once their seeded crops have started to grow or upon planting seedlings.

The mulch helps the soil to retain water, reduces erosion, adds organic matter, and offers food to the soil microbes.

The fact that fungi work in relationship with perennials is great knowledge to know and understand. If you think about a forest ecosystem, you can see the fungal root network (aka the mycorrhizae) as white strands if you turn over some of the forest soil. The forest hosts a wide variety of perennials, and the trees are interconnected through this mycorrhizal system.

This is why we often mulch with wood chips around perennials and trees - the wood chips encourage fungi to form to break down the wood, and in turn the fungi enrich the soil around the plant.

Likewise, it’s important to understand that soil bacteria prefer non-woody material, such as grass clippings, spent hay and straw. For annual plants, this type of mulch is preferred.

Related: A full guide to organic mulch

What plants are nitrogen fixing?

Cover crops for nitrogen fixation

If you practice regenerative gardening, take note of these cover crops to get planted in the fall or early spring.

According to the University of Georgia, crops such as:

• alfalfa
• berseem clover
• perennial peanut
• red clover
• birdsfoot trefoil
• white clover

all have the potential to work with the soil microbiome to fix 200+ pounds of nitrogen per acre planted.

Alfalfa has the most potential, with the possibility of an acre of alfalfa fixing up to 300 pounds of nitrogen.

Which cover crop should you sow in your garden? Learn how to choose the best cover crop for your situation.

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Trees, Bushes, and Shrubs for nitrogen fixation

If you work with permaculture guilds, agroforestry, or just want to integrate more perennials into your garden, here are some of the plants to note for nitrogen fixation.

According to Plants for a Future,

• Sea Buckthorn
• Autumn/Russian Olive (invasive in some areas, check before planting)
• Alder
• Silverberry
• Rose Acacia
• Black Locust
• Buffaloberry
• Pink Fuzzy Bean
• Perennial Sweet Pea
• Butterfly Pea
• Sweet Fern
• Wisteria

all support the soil microbiome to make nitrogen available.

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Annual Plants for nitrogen fixation

**Annual plants that fix nitrogen due to their relationship with the soil microbiome are:**

• beans, including fava, bolita and soy
• peas, including snap, snow, and sweet
• peanut
• fenugreek
• alfalfa

Regenerative agriculture, climate change + the nitrogen cycle go hand in hand

It’s important to note that synthetic nitrogen fertilizer is applied with such vigor in conventional gardening that it’s a major source of greenhouse gases. According to the Soil Association, the fertilizer is applied in excess, which causes it to go back into the atmosphere as nitrous oxide. Nitrous oxide is just under 300 times more potent then carbon dioxide as a greenhouse gas.

To put the scope of this into perspective, the UN environmental program reports that half of nitrogen fertilizer applied in Scotland is lost.

If instead we can implement regenerative practices, which would forbid the application of synthetic nitrogen fertilizer and instead work with organic applications when necessary, we could create a shift towards a more sustainable environment.

Before you go, dive deeper into some related articles:

• Regenerative agriculture: a nature-based solution to climate change
• Grow healthier plants with organic fertilizers
• Start a no-till garden
• Learn the basics of building soil health

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