Nitrogen Nodules And Nitrogen Fixing Plants

Nitrogen Nodules And Nitrogen Fixing Plants

By: Heather Rhoades

Nitrogen for plants is vital to the success of a garden. Without sufficient nitrogen, plants will fail and will be unable to grow. Nitrogen is abundant in the world, but most of the nitrogen in the world is a gas and most plants cannot use nitrogen as a gas. Most plants must rely on the addition of nitrogen to the soil in order to be able to use it. There are a few plants that love nitrogen gas, though; they are able to draw the nitrogen gas from the air and store it in their roots. These are called nitrogen fixing plants.

How Do Plants Fix Nitrogen?

Nitrogen fixing plants don’t pull nitrogen from the air on their own. They actually need help from a common bacteria called Rhizobium. The bacteria infects legume plants such as peas and beans and uses the plant to help it draw nitrogen from the air. The bacteria converts this nitrogen gas and then stores it in the roots of the plant.

When the plant stores the nitrogen in the roots, it produces a lump on the root called a nitrogen nodule. This is harmless to the plant but very beneficial to your garden.

How Nitrogen Nodules Raise Nitrogen in Soil

When legumes and other nitrogen fixing plants and the bacteria work together to store the nitrogen, they are creating a green warehouse in your garden. While they are growing, they release very little nitrogen into the soil, but when they are done growing and they die, their decomposition releases the stored nitrogen and increases the total nitrogen in soil. Their death makes nitrogen available for plants later on.

How to Use Nitrogen Fixing Plants in Your Garden

Nitrogen for plants is essential to your garden but can be difficult to add without chemical assistance, which is not desirable for some gardeners. This is when nitrogen fixing plants are useful. Try planting a winter cover crop of legumes, such as clover or winter peas. In the spring, you can simply till under the plants into your garden beds.

As these plants decompose, they will raise the total nitrogen in the soil and will make nitrogen available for plants that are unable to get nitrogen from the air.

Your garden will grow greener and more lush thanks to plants that fix nitrogen and their beneficial symbiotic relationship with bacteria.

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Read more about Soil, Fixes & Fertilizers

Nitrogen Fixing Bacteria with Peas, Beans and Family

Nitrogen fixing bacteria are nature's main method of changing nitrogen to plant available forms. It occurs underground in a very friendly symbiotic relationship of legume plant with Rhizobium types of bacteria. So Nature's nitrogen factory could look something like this lupine field in Glacier National Park.

It's only been in the last century that man was even able to make synthetic nitrogen fertilizer using a process developed by Fritz Haber. To manage that we need to create super high temperatures and pressures in large fertilizer plants such as the one at Severnside in the UK shown here.

Recent research at the Center for Ecology and Evolutionary Biology at the University of Oregon by Dr. Jennifer Fox, found that agrichemicals such as synthetic nitrogen fertilzers and pesticides, interfere with the communication between plants and nitrogen fixing bacteria. The result is less nitrogen is fixed by the bacteria and of course more synthetic nitrogen is needed to maintain yields. YIKES! another vicious cycle.

Farmers and Gardeners Harness Nature's Nitrogen Production

Ancient farmers wouldn't have known about nitrogen fixing bacteria, but they did know that growing legumes yielded good food and helped other crops grow. In fact, of the eight Neolithic founder crops - the first plants domesticated by man - four were legumes and included lentils, peas, chickpeas, and bitter vetch.

Like our ancient ancestors, modern farmers plant legumes to in effect grow a type of nitrogen rich fertilizer for their other crops. It isn't the peas and beans that are making the fertilizer though, it's the nitrogen fixing bacteria, various Rhizobium, that actually do the nitrogen fixing.

These bacteria colonize the roots of their preferred plant partner. From their homes in the plant roots they take nitrogen from the air and convert it into ammonia, a form of nitrogen that plants can use. For this valuable service the legumes feed the bacteria a steady diet of plant sugars.

Your soil may already have several varieties of rhizobia present that live on from year to year. Pull up a bean plant and check it for white or pinkish white nodules on the roots.

I just did this myself in the community garden plot where I grow vegetables. The whole garden here is regularly rototilled at both the beginning and end of the season. Unfortunately, while this looks neat, tidy and ready to plant, it really does a number on the soil life. The bean plant roots didn't have a single nodule.

While you will still get a crop from the seed you sow both soil and crop are improved if the right nitrogen fixing bacteria are present in the soil.

It's worth using a legume inoculant in the following cases:

  • Where the topsoil has been removed. The rhizobium bacteria are certain to be completely nonexistent or very reduced in this case.
  • Where the soil has not had any legume ground cover for a long time. Rhizobium can lie dormant for a few years, however, they will eventually die off unless they are united with their appropriate legume host.
  • In soils where synthetic fertilizers and pesticides have been heavily used. These agrichemicals are toxic or caustic to rhizobium bacteria.
  • When planting a variety of legume that hasn't been grown in your garden before.
  • As insurance. Inoculants aren't expensive and it never hurts to supply your seed with a fresh batch of the right bacteria. Share the cost with your neighbor if cash is really tight.

How to Use Legume Inoculants

The Right Stuff

- Make sure you have the right type of nitrogen fixing bacteria inoculant for the crop you are growing. Many seeds used for cover crops are "rhizocoated" meaning it is already coated with the right nitrogen fixing bacteria. If the seed is raw - meaning not coated - buy the appropriate inoculant when you buy your seeds.

Often garden stores stock an inexpensive inoculant containing several types of nitrogen fixing bacteria. This mix most often contains the bacterium strains for peas and beans and will work well for them. However, if you are planting peanuts for example it won't work for them.

It's Alive - But Not Forever

- The powder containing the nitrogen fixing bacteria is a live product. It has an expiry date and needs to be stored properly to ensure it is viable. If in doubt get a fresh batch.

Moisten Seed to Apply

- Moisten the seed with either a little water or milk. Add the powder to the seed and mix thoroughly so that the seed is completely covered.

Use Lots

- You can't use too much of the rhizobacteria, but you can use too little, so pile it on.

Maybe You've Got 'em, Maybe Not

- Once you have a specific nitrogen fixing bacteria in the soil it may be present for future crops. But you must plant its host regularly so the bacterium have a place to live. Otherwise they will eventually die out. It's probably worth inoculating every year to be sure.

List of Common Legumes and Their Rhizobium Bacteria

  • Peanut - Bradyrhizobium sp.
  • Milkvetch - Rhizobium sp. (Astragalus)
  • Chickpeas - Mesorhizobium sp.
  • Lassen crownvetch - Rhizobium sp.
  • Soybeans - Bradyrhizobium japonicum
  • Chickling vetch, grass pea - R. leguminosarum bv. viciae
  • Lentils - R. leguminosarum bv. viciae
  • Bird's foot trefoil - Mesorhizobium loti
  • Lupines - Bradyrhizobium sp. (Lupinus)
  • Alfalfa - Sinorhizobium meliloti
  • Sweet Clover - S. meliloti
  • Common Beans - R. leguminosarum bv. phaseoli
  • Field or Garden Peas - R. leguminosarum bv. viciae
  • Crimson, Red, White and Alsike Clover - R. leguminosarum bv. trifolii
  • Hairy Vetch, Fava Beans, Broad Beans - R. leguminosarum bv. viciae

Won't My Compost Supply the Nitrogen Fixing Bacteria I Need?

In a word No. This is one of the rare situations where compost isn't going to help.

Your compost pile is a home for decomposer bacteria and fungi. The decomposers can circulate and help maintain plant available nitrogen in your garden. But they can not take nitrogen from the air and turn it into fertilizer.

Rhizobium, the nitrogen fixing bacteria equipped to do that, need live hosts. If the legume along with its bacteria partner have not grown in the soil before it doesn't matter how much compost you add it's not going to put that microbe in your garden.

How nitrogen fixing works

Before planting a cover crop, Gaskin said it helps to understand how legumes fix nitrogen into the soil. The bacteria with which legumes have a symbiotic — mutually beneficial — relationship are Rhizobia bacteria, little bacteria that infect legume roots and exist naturally in soil, said Gaskin. “The bacteria are able to do this miraculous conversion by taking nitrogen gas and converting it into a chemical form, ammonium that plants can use. In return, the plant supplies the bacteria with carbohydrates, which gives them energy to function.”

One of the key goals with cover crops, she said, is to keep a living root in the soil at all times. “It’s how we keep that whole ecosystem down there in the soil growing. Roots exude carbohydrates and other things, and they keep those little micro-organisms down there alive and healthy.”

Before planting cover crops, Gaskin urges home gardeners to take an extra step she likens to an insurance policy to ensure cover crops fulfill their nitrogen-fixing role. “We recommend that you inoculate your legume seeds with these Rhizobia bacteria. Then you know [the bacteria is] right there when the seeds germinate and it’s ready to infect the root.” The inoculant is often available where cover crop seeds are sold. But, Gaskin added, it’s important to remember when you buy the inoculant that it is a living bacteria. “Don’t go buy a bag of inoculant and throw it on the dashboard of your car and go run a bunch of errands,” she advised. “High heat will kill the bacteria.” It should be stored in a cool place such as a refrigerator until you are ready to use it. While it may seem unnatural to store bacteria with food, it is not going to “escape” and cause harm.

Growing cover crops also requires gardeners to do something else that is unnatural: kill the plants before they set seed. Legumes need nitrogen they have obtained from the Rhizobia to produce seeds. The nitrogen that has been “fixed” from the air into the ground is used to make proteins in the seed. To get the most nitrogen form a cover crop, it needs to be killed before it sets seed. That’s why legume food crops don’t supply much nitrogen for subsequent crops.

Create Homemade Fertilizer

Antranias / Pixabay

In contrast, composting involves creating fertilizer at home so you can add vital nutrients to the soil each year. However, fertilizer will only feed your actively growing plants, while compost adds nutrients directly to the soil. The latter is the best way to grow the healthiest plants over long periods of time.

Instead of buying a large bag of potting mix from your local garden center, add your own compost to the soil. This will promote plant growth, as well as allowing you to recycle biodegradable waste such as leaves or vegetable scraps.

A nitrogen-rich compost as bacteria and other symbiotic life forms that allow veggies to feed themselves, fight off diseases, and boost flavor. In addition, compost allows soil to retain moisture levels. You can purchase bagged compost, but a homemade version is the best source of nitrogen.

If you’ve never made your own compost before, you can learn how to make a pile at home. Alternatively, you can purchase it from a garden supply store. Many gardeners choose to make their own for small projects, and buy commercial compost as needed for large-scale grows.

Multi-use Nitrogen Fixers For Your Food Forest

With all these benefits, you need to plant these highly useful plants in your own yard or forest garden! Here are some great choices organized by other benefits. And the more uses, the more you can do in smaller spaces!

Edible Nitrogen Fixers

All of these plants can be used both to build your soil, but also as a food source.

  • Ahipa
  • Alfalfa
  • American Licorice
  • Beans
  • Black-eyed Pea
  • Breadroot
  • Carob
  • Chickpea
  • Fava Beans
  • Goumi
  • Ice Cream Bean Tree
  • Jicama
  • Lentils
  • Peanut
  • Peas
  • Pigeon Pea
  • Prairie Acacia
  • Sea Buckthorn
  • Siberian Pea Shrub
  • Silver Birch
  • Tamarind

Insectiary Nitrogen Fixers

These plants are are host plants or food sources for beneficial insects. And of course they may be attractive to humans as well!

  • Bird’s-foot Trefoil
  • Black Locust
  • Clover
  • Comfrey
  • False Indigo
  • Goumi
  • Honey Locust
  • Linden Tree
  • Lupine
  • Silk Tree
  • Silver Birch
  • Sweet Fern

Ornamental Nitrogen Fixers

Surrounding ourselves with beauty is important too. It’s okay for some of our plants to be merely ornamental, and these are still good for the soil.

  • Bladder Senna
  • Carob
  • California Lilac
  • Golden Chain Tree
  • Honey Locust
  • Linden Tree
  • Sidebeak Pencil Flower
  • Silk Tree
  • Silver Birch
  • Silver Wattle
  • Sweet Pea
  • Wisteria
  • Yellow Wood Tree

Livestock Feed and Pasture

If you have enough space for pastured livestock, here are some nutritional nitrogen fixers to add into your pasture and browse.

  • Alfalfa
  • Bird’s-foot Trefoil
  • Black Locust
  • Clover
  • Kudzu
  • Pigeon Pea
  • Siberian Pea Shrub
  • Tagasaste
  • Vetch


If you are into herbal medicine, here are some plants that may be good to have on hand in addition to their nitrogen fixation abilities.

  • Alder
  • Alfalfa
  • Birch
  • Black Locust
  • Clover
  • Honeybush
  • Mountain Mahogany
  • Prairie Acacia
  • Redbush
  • Scotch Broom
  • Sea Buckthorn
  • Sweetgale
  • Tamarind

Black alder is a good wood crop and has medicinal benefits.

Other Useful Nitrogen Fixers

  • Alder (woodworking)
  • American licorice (erosion control)
  • Bayberry (good for hedges and birds)
  • Birch (woodworking, nurse tree for seedlings, sap)
  • Black Locust (woodworking, weed control, erosion control)
  • False Indigo (dye)
  • Kudzu (prevents erosion, good for basket making)
  • Mesquite (woodworking and edible pods)
  • Siberian Pea Shrub (good for hedges)
  • Tagasaste (good for hedges)
  • Mountain Mahogany (Woodworking, mediinal)

Can Be Invasive

These plants grow eagerly and can overtake more delicate native ones, so if you plant them, feel free to prune with gusto. While many people demonize invasive plants, remember that they are often invasive because we have already disturbed and damaged the native ecosystem. These types of plants thrive in and improve poor soils and can be controlled with active management.

  • Autumn Olive
  • Bird’s-foot Trefoil
  • Black Locust
  • Cape Broom
  • Honey Locust
  • Kudzu
  • Mesquite
  • Scotch Broom
  • Silk Tree
  • Wisteria (non-native types)

There are so many different nitrogen fixers that you should be able to find the perfect ones for your individual needs. Adding in nitrogen fixers is a great way to increase the overall health of your garden, food forest, or yard.

Want more information on starting a fruit tree guild? Check out these posts:

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Nitrogen-Fixing Plants

If you’ve read my posts from this spring, you’ll have endured me going on and on about peas and beans and how they fix nitrogen in the soil. For those who nodded off during those episodes or who have just tuned in, I’ll go over it briefly.

Some plants have the ability to fix nitrogen in the soil. Actually, a type of bacteria called a rhizobia invades the roots of plants in the Fabacea family and a few others, and fixes atmospheric nitrogen in nodules on the plant’s roots. This is beneficial to both the plant and the bacteria, a process called mutulism. It also benefits whatever grows around the plants because, when the plant dies, the nodules release their nitrogen into the surrounding soil. In the case of long-lived shrubs and trees that fix nitrogen, as roots die off or are replaced, they release their nitrogen.

An edible forest garden is one where man mimicks the dynamics of an old-growth forest. Why? Because forests succeed without the aid of fertilizer, tilling, mulching, irrigation or any interference or ‘help’, as it were, from man. How does it do this? The plants that grow complement each other, providing what each other needs. These relationships are called plant guilds. You can create plant guilds, substituting plants that provide food for humans. In a guild there is a taller tree which provides shade and leaf droppings (mulch), shrubs which provide more shade, mulch and habitat for animals and insects, plants that fix nitrogen in the soil, plants that have long tap roots called ‘miner’ plants, because they take up nutrients from deep in the soil and deposit them on the soil surface when their leaves die off, plants that attract pollinators, and plants that are ground covers to regulate heat and moisture. Using permaculture practices for water harvesting and organic gardening, when the guild matures it should be almost completely self-sustaining.

Say you want to plant an apple tree. That would be your tall canopy tree for the guild, which drops leaves as mulch. Beneath it, you could plant a shrubby herb such as rosemary (another edible), daikon radishes (miners, leaving the cut leaves on the surface after harvesting the edible root), bush beans (legumes) and herbs such as dill, parsley and basil, some of which you allow to flower for pollinators. As the tree grows, the plant guild can widen and others planted.


There are many plants, trees and shrubs that fix nitrogen in the soil. All beans and peas including soybeans and fava beans do when the plants are finished cut them above the soil so the roots stay put and decay where they are to release the nitrogen. Cover crops such as clover and hairy vetch are grown and turned under to improve the nitrogen in the soil. If you are from the Southern California area, perhaps you’d be interested in knowing what native plants are nitrogen fixers.

Ceanothus (California Lilac) at Elfin Forest

The native Southern California nitrogen fixers include: ceanothus, lupine, deerweed, California peashrub (endangered) (lotus), and redbud. Non-natives that are commonly used are alders, acacias, calliandra, sweet peas, guaja, and many more, as the Fabacea family is very large. Use any of the natives in ornamental gardens and not only will you be improving the soil and the vigor of the surrounding plants, but providing much needed habitat for our native birds and insects.

Try building plant guilds it is challenging and fun. Many combinations of plants are suggested on permaculture

Watch the video: Best nitrogen fixing plants for vegetable gardens and food forests