Whether you practice permaculture, agriculture or traditional gardening, you will come across the words nitrogen fixing more than once. The practical application of using nitrogen fixers is universal. Whether you are a small traditional gardener or a sustainable permaculture operation, the same rules apply.
A healthy garden or food forest is all about the levels, and planting so the individual levels (layers) support each other.
What Is a Nitrogen Fixer Exactly?
Nitrogen fixing plants can be trees, shrubs and edible plants. These plants take the nitrogen from the air through their leaves and send it to the roots where it turns into ammonia.
Now, these plants don’t just pull the nitrogen from the air on their own, they need a friendly bacteria called rhizobium and brady rhizobium to assist.
The rhizobium infects nitrogen fixing plants such as pod trees like Black Alder or Honey Locust, shrubs like Russian Olive, or Caragana, and legumes such as peas and beans, and uses the plant to help it draw nitrogen from the air. The rhizobium bacteria then converts this nitrogen gas, and stores it in the roots of the plant. This then permeates the soil providing nitrogen to all the other plants.
Why Is Nitrogen Fixing Important?
Nitrogen fixing plants has very distinct roles in soil fertility and nutrient recycling. Nitrogen is what plants need to grow, and is the key to a strong vegetative diversity. In addition to creating soil fertility, the process of nitrogen fixing also aids in pest control and water treatment by helping filtration. This make this process one of the most important things on earth.
In a proper functioning food forest or ecosystem all this nitrogen fixing takes place naturally leaving a healthy environment that will continue to sustain itself over a very long period of time.
Unfortunately, most of our food is grown through cropping systems, which require industrial produced nitrogen fertilizers to provide nutrients to the plants. This has left us with some pretty huge ecological problems.
The reason for this is that, through the practice of monoculture, the soil becomes depleted of nutrients ,and requires artificially produced fertilizer to feed the crops.
This fertilizer is created by combining nitrates and phosphates which then enter our water systems, contaminating our fish and aquatic resources, and also enters the air, creating cardio vascular problems in us and animals on a huge scale.
Having read and understood the difference between natural and industrial nitrogen fixing, the only way forward is to let nature balance our food resources using of nitrogen fixing trees, shrubs, and plants.
By using these correctly we can bring fertility to almost every soil no matter its current condition. Let’s have a look at what plants are considered the best nitrogen fixers and how we can utilize them, whether you are growing a food forest or just an urban kitchen garden.
We should start the process by creating levels (i.e. trees), then shrubs, then ground plants. You might not have the room for trees but can fit in a shrub or maybe you only have room for ground plants. The principles remain the same. The first level, of course, would be trees, the next level is shrubs followed by your ground covering plants.
I have tried to include as many native species as I could. If you are located in the North American climate zones, most of your high nitrogen fixing trees are nonnative, but flourish. Almost all of the shrubs and ground cover plants are native.
For the purpose of keeping this article short and an easy read, I will list the 10 of each nitrogen fixing trees, shrubs and cover plants. These are in my opinion the best for the North American and European climates.
These are only a few, once you decide to learn even more, the internet or your local nursery is a vast source of information on nitrogen fixing. Once we know what we should plant we will discuss how to do it. The trees, shrubs and plants I list are suitable for rural and urban planting.
- Black Alder (Alnus Glutinosa). A great starter or nurse trees to help your soil quickly. Alder is easily grown, like damp soil and has many other uses other than nitrogen fixing. For instance, it is a great wood to use for smoking meats.
- Black Locust (Robinia Psuedoacacia). This tree is one that is native to North America. The Black locust grows best in lime rich soils it is a tall growing tree and has very hard wood. So it also has many other great uses. This is perfect for fence posts and is a honey producer for example.
- Honey Locust or Thorny locust (Gleditsia Trycanthos). This is also a native to North America and prefers moist soil but is very adaptable, a fast grower often considered invasive. The wood is beautiful for crafts as it polishes up beautifully, and also a good forage for animals.
- Foxglove or Empress tree (Paulownia tomentosa). This tree is native to China but has since been introduced to North America and Europe. This is the fastest growing nitrogen fixer and also can survive in cold climates. Its other uses are fodder for animals like cows, goats and sheep as it is high in protein, and also it is used to make musical instruments.
- Mimosa (Albizia julibrissin). This is a cold tolerant tree that can get along in almost any soil. It is a great shade provider and has a beautiful aroma.
- Common Laburnum (Laburnum anagyroides). A beautiful tree and top nitrogen fixer. The only thing about this tree is that it is very poisonous.
- Redbud (Cercis Canadensis). This is a beautiful ornamental tree but does need more water than other nitrogen fixers, so you want to plant this where there is good precipitation.
- Velvet Mesquite (Prosopis Velutina). This is an important tree for humans and animals in very dry climates. Not only does this mesquite provide life giving shade but is also a valuable food source.
- Kentucky Coffee (Gymnocladus dioicus). A beautiful shade tree. You can roast the mature pods and it’s a great alternative to coffee beans.
- False Acacia (Robinia Psuedoacacia). As seen in the pictures below the nitrogen rich pods are very visible. This variety of Acacia is found in warm temperate climates.
All the trees above will grow quite tall, and are great shade providers as well as nitrogen fixers. You will want to plant these at the edges of your food forest or gardens.
This way you can utilize the shade for your next layer of growth. If in an urban setting there is no problem to aggressively trim these trees to keep them within the confines of your space.
- Autumn olive (Elaeagnus Umbellate). This shrub is originally from Asia, but has now found its way all over North America and Europe. This is a very hardy shrub, good in any soil. It grows quickly, and in some instances is considered invasive in North America. This is a bushy shrub and produces berries that can be eaten when ripe.
- Russian olive or Silver Berry (Elaeagnus Angustifolia). This shrub grows in any soil, originating in central Asia, and it is now widely grown throughout North America and Europe. It’s drought tolerant, and can survive cold temperatures. The Russian olive is a great wind break, and also used as a herbal remedy.
- Sea Berry (Hippophae Rhamnoides). This shrub prefers coastal climates, but has been grown successfully in all other regions of North America and Europe. It thrives in all types of soil, and can handle cold and drought. This shrub is a non-leguminous nitrogen producer. It yields delicious edible berries, and also has herbal properties.
- Siberian Pea (Caragana Aborescens). This shrub is extremely hardy and a top nitrogen fixer as it produces tons of small seed pods. This is a very fast growing shrub suitable to almost any soil other than very moist soil. These make fantastic wind blocks and hedges. They are great for pollinators, the bark is used for paper and the seeds for making dye.
- Spanish Broom (Spartium Junceum). As seen in the picture below is a great nitrogen fixer. This is primarily found in warm temperate climates. Notice what the fixers have in common? Pods.
- Buffalo Berry (shepherdia argentea). This is a member of the Russian olive family. This was a staple for North American indigenous people. Used as medicine, dye, and to flavor meat.
- Mountain Mahogany (Cercocarpus). This shrub grows at altitude, and prefers a hot dry climate. It is a top nitrogen fixer from the rose family, so it produces gorgeous flowers and fruit.
- Mountain Misery (Chamaebatia).This is an aromatic evergreen nitrogen fixer. Its resin is highly flammable and the leaves make a pleasant tea for health.
- Cliff Rose (Purshia). Another evergreen from the rose family. This shrub was traditionally used for making arrows and dyes.
- Honey Mesquite (Prosopis Glandulosa). This mesquite is a shrub, and a super-plant for pollinators. Like other mesquite, it is great for fire wood, and for smoking meats.
Nitrogen-Fixing Ground Cover Plants
- Peas (Pisum Sativum). Garden, field and pigeon are just a few varieties of these delicious nitrogen fixers. The varieties are numerous and their nutritional value well documented.
- Wood vetch (Vicia Slyvatica). This is only one of the vetch family, but is a hardy plant which unlike the beans or peas is a perennial, meaning you do not have to plant it every year. Wood vetch is a valuable herbal remedy for muscle pain and cramping.
- Clovers (Fabaceae).These beneficial plants are of the bean family and offer amazing ground cover. They are also a perennial plant which is just convenient. Clovers, whether white or red, offer terrific herbal remedies for blood purification respiratory ailments, and are rich in minerals. This great little nitrogen fixer is a much better alternative to grass lawns.
- Everlasting Sweet Pea (Lathyrus Latifolius). This is the perennial version of the sweet pea family. This is a sturdy perennial which can stand the cold, and is a good climber.
- Peanuts/ground nuts (Arachis Hypogaea). This is a legume so great nitrogen fixer and a delicious food source used all over the world.
- Licorice (Glycyrrhiza Glabra). This is a flowering perennial with many uses. This aromatic plant is used for flavoring candy, tobacco, also in medicines.
- Alfalfa (Medicago Sativa). This is a flowering perennial and one of the best foraging crops for herbivores or livestock.
- Dryas Drummondii (Dryas Drummondii). This is a herbaceous perennial which can grow almost anywhere and is most commonly used as an ornamental plant.
- Lupins (Lupinus). Another herbaceous perennial which is both a source of food and an ornamental.
- Beans (Fabaceae). All bean varieties are great nitrogen fixers, whether Fava, bush bean, lima bean or chickpea, but there are so many more to list. Any of these not only contribute an important part to our diets but are terrific nitrogen fixers when their season is over.
For the purpose of this article I have listed only a few of each variety of nitrogen fixers. The ones listed are some of the easiest to grow in a variety of climates. By using these varieties you will increase your chance of success in a shorter period of time.
Planting should be done in succession, starting of course with your trees. Allowing the trees to establish will provide a better soil base and shade for your shrubs, these in turn will do the same for your ground cover plants.
It is of course always easier to plant perennials but this limits what we have for our food. The main things you want to consider is how each level of these plants supports the other. The shade creates a moister environment for the smaller plants and requires much less water than a traditional exposed garden.
Are Beans and Peas Good Nitrogen Fixers?
Something that should be mentioned is, there seems to be a difference of opinion between permaculturists and gardeners as to the viability of beans and or peas as nitrogen fixers. This is easily explained.
The reason for the confusion is that legumes produce nitrogen with their roots, then that nitrogen goes into producing the fruit, beans or peas. We then harvest the fruit and the nitrogen does not return to the soil.
The solution to this, is not to completely harvest all your beans or peas and let some of them return to the soil. This is not a problem if you have a larger area, however if you are in a smaller urban setting just stack your plants. These plants don’t mind a little crowding, and many of them are creepers and will grow vertically.
The most important thing is, that we as people interested in growing food and sustainability improve the soil wherever we go, our future and our children’s future depends on it.
Dirk is Canadian permaculture designer with a certificate from Tagari farms, studying under the tutelage of the legends and founders of permaculture Bill Mollison and Geoff Lawton. Dirk has been using permaculture techniques for more than 20 years, initiating and managing projects in, Cambodia, Madagascar Montenegro and Vietnam He has been helping people use permaculture techniques in growing their own food, and in leading more sustainable lives across the world.