Nitrogen-fixing bacteria combine elemental nitrogen with hydrogen or oxygen to create compounds plants require for growth, including ammonia and nitrates. Some nitrogen-fixing bacteria work independently, while others work with legumes to complete the transformation. Nitrogen-fixing bacteria convert over 90 percent of the nitrogen plants use.
Blue-green algae are an example of nitrogen-fixing bacterial that work independently. Independent nitrogen fixers transform less than 5 pounds of nitrogen per acre but are a significant source of nitrogen for rice paddies.
In legumes, the nitrogen-fixing bacteria enter the root system of the plant. The infection causes the formation of a node for nitrogen storage. The plant supplies the food necessary to keep the bacteria alive in this symbiotic relationship. In alfalfa, the nodes form into hand-like structures with a palm and fingers approximately 1/2 inch in diameter. As long as growing conditions remain favorable, the bacteria continue converting nitrogen throughout the remainder of the growing season.
On beans, peas and peanuts, the nodes are round in shape and approximately the size of a large pea. A single peanut plant forms up to 1,000 of these nodes. However, stress on the plant causes it to stop fixing nitrogen. While farmers can alter some of the conditions responsible for stress due to soil conditions, they have little control over temperature or a lack of moisture. Legumes produce up to 25 pounds of fixed nitrogen per acre.