How Nitrogen-Fixing Plants Enrich the Soil

Biological nitrogen fixation is the process of converting atmospheric nitrogen into a soluble form usable by plants as fertilizer; bacteria living on the roots of leguminous plants perform this essential ecological function.

When plants don't receive sufficient nitrogen, they're unable to produce enough amino acids to make the proteins that plant cells need to grow. Nitrogen fixation is thus an incredibly important part of maintaining the environment and providing humans with food crops. Farmers and gardeners can use chemical nitrogen fertilizer to keep crops thriving, but synthetic fertilizer can pollute drinking water and poses a threat to fish and other wildlife. Biological nitrogen fixation helps crops flourish without introducing pollutants to the environment.

Nitrogen-fixing plants form a mutually beneficial symbiotic relationship with soil bacteria. These microorganisms serve as a microbial inoculant, infecting the host plant’s root system and causing it to form nodules where the bacteria can thrive. Inside these root nodules, the bacteria draw nitrogen gas from the air, turning it into fixed nitrogen that is able to be absorbed and used by the plant host.

Once the host plant dies, the bacteria are released back into the soil where they either stay put or infect another legume. The decomposed legume also becomes a form of green manure for farms and vegetable gardens, releasing its stored nitrogen into the soil along with other nutrients and organic matter. This green manure then serves as a nitrogen source for the next crop rotation.

Legumes (members of the plant species Fabaceae) are common nitrogen-fixing plants. Legume plants form a symbiotic relationship with a type of nitrogen-fixing bacteria called Rhizobium. Actinorhizal plants are certain species of non-legume trees and shrubs that have a symbiotic association with a nitrogen-fixing bacteria called Frankia. Popular types of nitrogen-fixers for home gardens include:

1. 1. Ground cover plants: Vetch, cowpea, lupine flower, soybean, clover, peanut, alfalfa, and Austrian winter pea
2. 2. Short trees and shrubs: Russian olive, autumn olive, seaberry, acacia, and Siberian pea shrub
3. 3. Tall trees: Black locust, black alder, and empress tree

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