Some plants—chiefly those belonging to the group called legumes—associate with nitrogen-fixing bacteria to get their nitrogen from the air.
Nitrogen fixation process in plants
The process involves a symbiotic relationship between the leguminous plants and various strains of Rhizobium bacteria found in the soil. The bacteria “infect” the plant roots, causing small nodules to grow. Within these nodules, the bacteria extract nitrogen from the air and fix it into forms the plants can use to make protein. In return, the bacteria receive sugars from the plant for their own nutrition.
Why is nitrogen fixation important
Nitrogen absorbed and fixed into plant-usable forms by bacteria is a very important source of organic nitrogen. For centuries, the bacteria that live on the roots of legumes such as clovers and alfalfa were the principal way (except for manures) to add nitrogen to soil. In parts of New Zealand where clovers grow the year round, bacteria living on the roots will supply all the nitrogen needs of some crops for a full season.
Best nitrogen fixing plants
The bacteria on the roots of alfalfa are usually the most nitrogen-productive. Following alfalfa, in descending order of nitrogen production, are ladino clover, sweet clover, red clover, and white clover. Although these are the most productive nitrogen fixers we know, many other bacteria that live in the soil or on plant roots have the ability to capture and use nitrogen from air.
Using legumes as a nitrogen source
Legumes can be used as a nitrogen source in several ways. Clover, mixed with grass in a lawn, supplies much of the nitrogen needed by the lawn. (However, the clover blossoms attract bees, which pose a stinging hazard.) Legumes can be grown in a rotation with other crops, leaving more nitrogen in the soil than before they were planted. Or they can be grown as a cover crop, to be turned in when a new crop is to be planted.