Nitrogen Fixation of Legumes

Nitrogen Fixation

• The roots of leguminous plants harbor beneficial microbes that carry out the process of nitrogen fixation. Nitrogen is available in limited quantities in the soil, but is the primary component of the atmosphere. These bacteria (Rhizobium spp.) have the ability to convert atmospheric nitrogen into a form of soluble nitrogen that plants are able to absorb through their roots.

Inoculant

• Nitrogen-fixing bacteria exist naturally in the soil environment, but it is also possible to introduce them directly to the seeds of leguminous plants. This process, called inoculation, ensures complete colonization of the roots as the plant grows, which maximizes the total amount of nitrogen produced. There are particular strains of bacteria associated with different types of legumes, so it is important to verify that the seed is matched with the appropriate inoculant.

Types of Nitrogen-Fixing Plants

• There are thousands of species of legumes, most of which are capable of fixing nitrogen. However, gardeners use only a few of these for this purpose. Annual and perennial clovers (Trifolium spp.) are some of the most common. These can grow in U.S. Department of Agriculture plant hardiness zones 3 to 11. Low-growing clovers can mix with grass seed to provide for the nitrogen needs of a lawn. There are also nitrogen-fixing shrubs and trees that you can plant in proximity to other plants for the benefits of the extra available nitrogen.

Fertilizing With Legumes

• Nitrogen accumulates in tiny nodules on the roots of legumes at the locations where they are invaded by the nitrogen-fixing bacteria. When any portion of the roots die, the nitrogen releases into the soil where other plants can use it as fertilizer. Gardeners can plant annual legumes prior to or interspersed with the main crop and then incorporate the plant matter into the soil once the life cycle of the legumes is complete. In using perennial, tree or shrub legumes for fertilizer, it is possible to cut back the plant, which will cause a portion of the root system to die and release the nitrogen contained in the root nodules.