Chapter 4 – Legumes and Legume Nitrogen Fixation

Nitrogen Fixation

Legumes are plants belonging to the Fabaceae plant family. They are dicots (see this resource for an overview of dicot characteristics) that have evolved a unique ability to partner with soil bacteria to convert atmospheric nitrogen gas into a plant-useable form of nitrogen. Legume species allow specific strains of bacteria to form nodules on their root hairs. Legume nitrogen fixation is a symbiotic process because both partners benefit. Plants receive access to nitrogen in a form that they need for their growth and, in exchange, the bacteria receive energy to support bacterial growth in the form of carbon from plant photosynthesis as well as physical protection within the root nodules. As a result of this symbiotic process, legumes tend to be higher in nitrogen than other plant species, which can improve soil nitrogen availability, reduce (often eliminate) the need for nitrogen fertilizers, and increase the protein content of legume seeds that can be beneficial for human health.

Examples of Legume Crops

There are a broad range of legume plants used in agriculture. Legumes that have been selected for their seed production, such as dry beans, are also known as pulses. Pulse crops are important components of human diets globally due to their high protein and fiber content. Some legumes, such as soybean, have also been selected for high oil content in their seeds. Other legumes have been selected for their vegetative growth and are fed to livestock as forages, such as the perennial legume alfalfa. Some legumes are also grown as cover crops that are not harvested to increase soil nitrogen availability for the next crop.


These videos provide an overview of the legume family, the nitrogen fixation process and an overview of different types of legume pulse and forage crops. To view, click on the video play button which will open in the full image.  Then you can click play to start it. Once you are finished, click on the “X” in the upper right corner to bring you back to the image with all video links. If you wish to expand the video to fill your entire screen, clicking “Watch On YouTube” will open a new browser window to give you that option. All videos have closed captioning. (Photo credit: Gayle Volk, USDA )


Click through these interactive questions to review the important concepts that were discussed in this chapter.



Crop Diversity: A Virtual Crop Science Field Tour Copyright © 2021 by Patrick F. Byrne; Meagan Schipanski; and Deana Namuth-Covert. All Rights Reserved.

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