Nitrogen Fixers for the Organic Garden

A plant’s roots draw the nutrients from the soil and the plant uses these nutrients to grow and produce fruit and seeds. When we pick vegetables from our garden, the nutrients are removed and the soil becomes depleted with each harvest. Our goal as good gardeners and land stewards is to return nutrients to the soil in natural and environmentally friendly ways.

To help us, many plants will actually put nitrogen, an essential nutrient for healthy plant growth, back into the soil. These plants are referred to as nitrogen fixers. They take nitrogen from the atmosphere and put it into the soil through a process called nitrogen fixation where it can be used by the plants.

The most commonly grown plants that fix nitrogen are legumes, such as peas and beans, but there are many other nitrogen fixers including trees, shrubs, and flowers. Nitrogen fixers can be grown for edible crops, tilled under as green manures, or included as part of the perennial landscape to naturally improve land health and soil fertility. Nitrogen fixation is often used in agriculture but can also be integrated into a small garden with great results.

Our goal as good gardeners and land stewards is to return nutrients to the soil in natural and environmentally friendly ways.

Let’s look at how nitrogen fixation occurs, and how you can incorporate these plants into your garden plan.

How Plants Fix Nitrogen (for Non-Science Majors)

Nitrogen is essential for good plant growth and our gardens can easily become nitrogen deficient even though about 78% of the atmosphere is composed of nitrogen. Atmospheric nitrogen (N2) is a very stable compound and cannot be used by living organisms. Atmospheric nitrogen has to be brought into the soil and converted into a form that plants can use. This is what nitrogen fixers do.

It All Starts with a Bacteria Infection

Nitrogen fixation is a symbiotic relationship between some plants and certain soil-dwelling bacteria. A few weeks after a seed is planted, these bacteria infect the roots and create small growths called nodules where they dwell and multiply. The plant expends quite a bit of energy to feed the bacteria. In exchange, these bacteria convert atmospheric nitrogen into ammonia (NH3) and ammonium (NH4) which feed the plant. These compounds are further converted into nitrates (NO2 and NO3) by nitifying bacteria.

The nodules on the roots are generally fairly small, about 50mm (1/4 inch) in diameter, but some can grow to the size of your fist. Some are round, while others are elongated, or even look like a hand. While they are young, nodules are white or gray inside and do not fix nitrogen until they mature. Mature nodules will have reddish-pink insides indicating that nitrogen fixation is underway.

There are two broad classifications of nitrogen-fixing plants based on the type of bacteria that grows in the nodules:

Legumes. Probably the most common type of nitrogen fixers, legumes interact with the bacteria rhizobia. Legumes include peas, beans, vetch, clover, alfalfa, lentils, and peanuts. Each plant interacts with a different species of rhizobia and healthy soil will be alive with millions of bacteria.

Actinorhizal Plants: This group of plants work together with the bacteria Frankia to fix nitrogen into the soil. Actinorhizal plants include alder trees, sea buckthorn, Russian Olive, and buffaloberries, and many others.

How Much Nitrogen Do Plants Fix?

Legumes and actinorhizal plants grow wild in many undisturbed places around the globe and can contribute 11kg to 34kg (25–75 lb) of nitrogen per acre in a natural ecosystem every year. In artificial environments, such as farm fields or gardens, green manuring legumes can incorporate nearly a hundred kilograms (several hundred pounds) of nitrogen each acre.

Of course, growing a few bean plants won’t fix this much nitrogen, but even a few nitrogen fixers can benefit your soil. Here are some ideas on how to make the most of nitrogen fixers in your garden:

Use Legumes as Companion Plants: Companion planting is the practice of growing two or more crops side by side to form a mutually beneficial environment, and legumes and actinorhizal plants make great companion plants.

Most of the nitrogen produced in the nodules is consumed by the plant but any extra nitrogen seeps into the surrounding soil. While the amount of excess nitrogen seeped out might be small, it will be easily absorbed by neighbouring plants. In the garden, legumes can be planted alongside most other vegetables with good results but it is best to avoid planting them near alliums such as onions and garlic.

Here is a fairly comprehensive list detailing which companion plants benefit each other.

Growing legumes with a nitrogen-hungry companion plant will not necessarily increase the overall nitrogen level in the soil, but it will keep the soil from becoming depleted of this essential element.

Till the Plants After Harvest: When a nitrogen-fixing plant flowers, almost all of the nitrogen is directed into the seeds (the nitrogen in the seeds is converted into amino acids which is why most legumes are very high in protein). Once you harvest these seeds, such as delicious fresh pea pods, there is very little nitrogen left in the rest of the plant, and nitrogen fixation stops because the plant is feeding the seed pod instead of the bacteria in the nodule.

To make the most of your nitrogen fixers, till the plant into your soil after the harvest is done. The decomposing plant will release any remaining nitrogen back into the soil. Alternatively, you can add the plant to your compost to produce nitrogen-rich humus.

Grow a Nitrogen-Fixing Green Manure: The best way to add lots of nitrogen to your soil is to grow a legume as a green manure. Green manures are plants that are grown for the sole purpose of being tilled into the soil. Green manures can either be grown on their own or undersown beneath your vegetables.

To use green manures in your garden, thickly sow a legume and let it grow until it is about to produce seeds. At this point, the plant is lush and green and nitrogen fixation in the nodules is at its peak. Then, dig in the green manure and the decomposing plant will add all the nitrogen to the soil.

Include Nitrogen Fixing Perennials: A garden should be as beautiful as it is productive, and adding trees and shrubs is a great way to improve the aesthetics of your plot. Adding nitrogen-fixing perennials creates a long-term nitrogen factory for your soil while adding colour and diversity to your garden.

Or maybe there is an unusable space in your garden. Why not try growing a nitrogen-fixing perennial such as a Sea Buckthorn that will improve the soil, provide edible berries, and feed the birds at the same time.

Nitrogen-fixing trees and bushes are also being successfully used to establish new woodlands and are an integral part of a forest garden.

How to Encourage Nitrogen Fixation

The living soil is an incredibly involved and integrated science and many factors affect how a plant fixes nitrogen, but here are a few basic ways we can help support nitrogen fixation of our plants:

Soil pH: Nitrogen fixation is most effective when the soil pH is between 6.5-6.8. Test your soil and adjust the pH as necessary.

Rhizobium Inoculant: Since nitrogen fixation is the result of bacteria, if the right bacteria isn’t there, nitrogen fixation will not occur. Many legume seeds available from seeds companies come already coated with the appropriate rhizobia, but you can also buy inoculants to add to the soil.

Trace Elements: While many gardeners test their soil regularly, many trace elements are often ignored, yet these seemingly insignificant elements can have big impacts on the garden. In particular, having the right amount of molybdenum (Mo) and cobalt (Co) has been found to encourage nitrogen fixation.

Compost: Adding compost to the garden is always a good idea. In the case of nitrogen fixation, compost and humus will improve the structure of your soil and help retain any nitrogen that might otherwise be leached or washed away.

Adding carbonaceous matter, such as digging in straw with your green manure can have a similar effect.

Avoid Stressing Your Plants: Nitrogen fixation declines if a plant becomes stressed. Common causes of stress include lack of water, excessive heat, poor nutrition, or damage to the plant or roots.

Should I Fertilize?: Many sources say that you should fertilize your nitrogen fixers with nitrogen fertilizer at planting to help the plants grow until the nodules form. Then your plants will theoretically fix more nitrogen in the long run. However, an over application of nitrogen has been found to actually reduce the amount of nitrogen fixed because it is easier for the plant to use the fertilizer rather than make its own.

In our opinion, adding artificial fertilizers is never a good thing, especially when nature has provided a perfect way to add nitrogen to the soil

30 Plants That Fix Nitrogen

There are a surprising number of plants that fix nitrogen, and here are a few to try that could make a great addition to your garden. This list is by no means comprehensive but will hopefully give you a few ideas to get started.

Nitrogen Fixing Trees

Nitrogen Fixing Bushes and Shrubs

Legumes

Nitrogen Fixing Flowers

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Bellwether Farming

Comments

Peggy Woods from Houston, Texas on September 15, 2021:

We always dig our compost into a garden at the start of each season. Reading this was very informative about certain types of plants that add nitrogen back into the soil. Thanks!