Nitrogen-fixing plants are those whose roots are colonized by certain bacteria that extract nitrogen from the air and convert or “fix” it into a form required for their growth. When the bacteria are done with this nitrogen, it becomes available to the plants, themselves. It is an example of a symbiotic relationship (between plant and bacteria), and the name for the process is "nitrogen fixation."
The Importance of Nitrogen
Landscapers, gardeners, and farmers value nitrogen-fixing plants for their ability to contribute an essential plant nutrient (namely, nitrogen) to the soil. Nitrogen is one of "the big three," being the "N" in NPK, the three letters that form a virtual stamp of approval for a complete fertilizer. Rodale's Organic Life, in discussing the importance of this element to plant growth, observes that "when leaves contain sufficient nitrogen, photosynthesis occurs at high rates. That’s why one of the important warning signs of nitrogen deficiency is yellowing, pale green leaves...."
By exploiting the process of nitrogen fixation, you can obtain this plant nutrient for your soil without resorting to chemical fertilizers. For best results, inoculate your legume seeds with Rhizobium bacteria. Your local county extension office may be able to tell you where, in your area, you can purchase the type of Rhizobium bacteria suited to the legume you are growing.
What Do "Cover Crops" Have to Do With Nitrogen-Fixation?
The nitrogen-fixing plants that people speak of most often are cover crops of the pea, or "legume" family, because these plants are easy to work with (they can simply be rototilled under for you to release the valuable nitrogen). Certain trees and shrubs can also fix nitrogen, but they are seldom mentioned in this connection because they are not as easy to work with (you cannot turn them under with a rototiller).
When you till a cover crop under, whether by hand or with a garden tiller, the nitrogen becomes available to your lawn or garden or landscape plants. Clover is perhaps the most common example of a nitrogen-fixing plant. Explained in a separate article is why leaving some clover in the lawn is not a bad idea. Nitrogen-fixing plants that are well-known thanks to their use in agriculture include:
- Vetch (Vicia spp.)
- Alfalfa (Medicago sativa)
- Peas (Pisum sativum)
- Beans (Phaseolus spp.)
Other Examples of Nitrogen-Fixing Plants
Below are listed several examples of plants that qualify that may not come to mind immediately when speaking of nitrogen-fixing plants. Take note that the criterion for making the list is based on science, not desirability. That is, not every plant listed is a desirable plant to grow, even though it meets the definition, scientifically, of a nitrogen-fixing plant. Some nitrogen fixers are listed as invasive plants in North America; such cases are indicated in brackets:
- Lupine flowers (Lupinus spp.)
- Bayberry shrub (Myrica pensylvanica)
- Golden chain tree (Laburnum spp.)
- Sweetfern (Comptonia peregrina)
- Bird's-foot trefoil (Lotus corniculatus) [invasive]
- False indigo (Baptisia spp.)
- Scotch broom (Cytisus scoparius) [invasive]
- Autumn olive (Elaeagnus umbellata) [invasive]
- Silk tree (Albizia julibrissi) [invasive]
- Wisteria vines [alien species are invasive, so grow the native species]
- Sweetpea vines (Lathyrus spp.)