Nitrogen Fixing Plants and Trees

Nitrogen is one of the key elements in all life forms on Earth; it is part of protein and many other biological molecules. Nitrogen is the limiting factor in many ecosystems, which is why agriculture and gardening often requires some use of nitrogen-rich fertilizers.

Some of you may know that nitrogen is the most abundant gas in the Earth's atmosphere. If it is so abundant, why is it in short supply? The answer lies in the fact that it is hard to break apart nitrogen gas molecules to get the nitrogen into a biologically useful form.

Nitrogen fixers, which include bacteria, and certain plants that contain these bacteria in their roots, are the only life forms able to extract nitrogen from the air and convert it to a form useful for life. Nitrogen fixing plants working with these bacteria are an example of symbiosis, two different species that work together to benefit both of them.

You don't need to do much to benefit from having nitrogen fixers in your yard and garden. Nitrogen-fixing plants tend to bleed nitrogen through their root systems, so they will often benefit plants that grow near them.

However, there are a few additional things you can do to benefit from nitrogen fixers. Because they have essentially an unlimited supply of nitrogen, all parts of these plants tend to be very nitrogen rich. Nitrogen rich plant matter tends to decompose quickly, so this brings us to a key point:

Whenever you have nitrogen-fixing plants, collect and leave the plant material on your garden, flower-beds, or add it to your compost.

This material includes any part of the plant, and especially the leaves, stems, and twigs, as these make up the bulk of most plants. Nitrogen-fixing trees like locusts will drop a great deal of leaf litter in the fall--make sure to leave these leaves on your garden or flower beds. They will break down much faster than larger, tougher leaves, and they will enrich the soil for future growing seasons.

Some people already have nitrogen-rich soils in their yard, such as people who live in areas with extremely fertile soil, or people who practice backyard composting.  These gardeners often have little need to for nitrogen fixing plants, as sunlight or water are often more of limiting inputs into plant productivity.

Many people treat their lawns with chemicals to remove plants they consider "weeds". One of these common "weeds" is clover, along with a variety of other similar plants. I think this is a shame, because clover is a legume and a nitrogen fixer. You also might increase your luck if you find a four-leaf clover!

Clover in lawns exists in a natural balance. It tends to thrive in areas that are lower in nitrogen. If the nitrogen level increases beyond a certain point, the clover will be out-competed by grass and other plants that have higher nitrogen needs.

To get the most benefit from clover in your lawn:

• Do not apply herbicides - Most selective herbicides used on lawns will quickly kill clover and other legumes.
• Leave grass clippings on the lawn - Clover leaves and all plant parts are very nitrogen rich; if you collect them, you are taking nitrogen out of the system. Instead, leave the clippings on the lawn, and the nutrients will go back into the soil, ensuring your lawn will stay lush and green.
• Be cautious about applying nitrogen-rich fertilizer - Clover thrives in relatively low nitrogen levels; if you add too much nitrogen, your grass may look green temporarily, but you will kill off the clover, and you may be contributing to nutrient pollution when the nitrogen fertilizer runs off into streams and the water table.  Once established, a healthy lawn does not ever require fertilization.

Most nitrogen fixers are legumes, a family of plants that includes most plants we call beans and peas, peanuts, clover, as well as a number of trees, such as locust, acacia, and many others.

The legume family also includes numerous flowers and ornamental plants. Legume flowers tend to be quite showy, coming in a wide range of colors, including yellow, white, pink, purple, and blue.

Whether you are looking to grow food plants, ornamental flowers, or a large tree, there are numerous nitrogen-fixing legumes to choose from. You can plant these plants together with other plants with high nutrient needs, just like how farmers rotate crops of legumes like soybean or alfalfa, with corn.

Although most of the nitrogen fixing plants are legumes, there are a few other examples. Pictured above on the left is Sweetfern, a plant native to eastern North America. The closely related Bayberry can also fix nitrogen. These plants, native to sandy areas and beaches on the east coast of the U.S., have the added benefit of attracting Yellow-rumped warblers in winter.

Alders, which include both trees and shrubs, also are able to fix nitrogen.  Alders are often found at northern latitudes and higher elevations.  Top right is a grey alder, a species distributed widely across the northern hemisphere in Europe and North America.

The remaining non-leguminous nitrogen fixers are esoteric, scattered through a handful of other plant families, and include a few types of Coriaria, Chamaebatia foliolosa, and a few others.

These plants all have a similar symbotic relationship with nitrogen-fixing bacteria in their roots, just like legumes, although these plants are associated with different bacteria species from the ones symbiotic with legumes.

Eastern redbud (above, left), a small tree native to Eastern North America, is a legume, with characteristic pea-like pods, but is not a nitrogen fixer. Another example is the Kentucky coffeetree (above, right), a large tree whose seeds were historically roasted to produce a (caffeine-free) coffee substitute.

These plants naturally grow in nutrient-rich environments, so they have no need to fix nitrogen. Although they are descended from a common lineage with other legumes, somewhere along the line, they lost this ability! Keep this in mind if you plant them--both are beautiful landscape plants in their native range, and grow well under the right conditions, but both require relatively rich soils and will not do much for adding nitrogen to the soil.