Legumes & Nitrogen-Fixing Bacteria

Nitrogen Conversion

• All legumes use nitrogen-fixing bacteria in a similar way. Soon after legume seeds sprout, soil bacteria infect their roots and reproduce within the outer layers, using nutrients supplied by the plant. The bacteria-containing nodules that develop on the roots are usually visible to the naked eye a week after infection. White at first, these nodules grow larger and turn pink or reddish as nitrogen fixation begins, due to the presence of a chemical similar to hemoglobin in blood. As long as the bacteria receive carbohydrates, they supply legumes with nitrogen.

Bacteria and Legumes

• Types of nitrogen-fixing bacteria and the amount of nitrogen supplied vary between different legumes. Rhizobia bacteria fix nitrogen in root nodules on legumes and include a range of species. Rhizobia trifollii fixes nitrogen in white clover (Trifolium repens), which is a perennial legume hardy in U.S. Department of Agriculture plant hardiness zones 3 through 10, and Rhizobia japonicum fixes nitrogen for soybean (Glycine max), which is an annual plant. Clover develops finger-like nodules and its bacteria are efficient at fixing nitrogen. Nodules on soybeans and other annual legumes are pea-shaped, and soybean bacteria fix half as much nitrogen as clover. The bacteria that invade common bean (Phaseolus vulgaris) -- another annual plant -- are poor nitrogen fixers and beans often require additional fertilization.

Nitrogen-Fixing Cycle

• Nitrogen-fixing rates change according to the legume growth stage and soil conditions. Nitrogen is released to the legumes as it's made by the bacteria. When annual legume seedpods are swelling, plants direct nutrients into the seeds and away from the bacteria in their root nodules. During this time, the nodules can stop making nitrogen and turn green or white. Bacteria also don't fix nitrogen if the plant is starved of essential soil minerals, such as potassium, phosphorus, zinc, iron, molybdenum or cobalt, or suffering from drought or other stress. Temperatures above 80 degrees Fahrenheit or below 55 degrees F slow or stop nitrogen fixation in white clover. Once the season is over, most fixed nitrogen is contained throughout the plant and not in its root nodules.

Bacteria Supply

• Nitrogen-fixing bacteria allow legumes to produce their maximum yields when all other conditions are also favorable. Poor growth and low yields are often a result of growing legumes in soil that doesn't contain any nitrogen-fixing bacteria or contains the wrong type. Sow inoculated seed or spray soil inoculant when growing legumes in areas where they haven't grown for the previous three years. Nitrogen-fixing bacteria are living organisms and inoculated seed or soil inoculant should be kept in a cool dry place away from direct sunlight and used before its expiration date. If using soil inoculant, check that the bacteria are the correct type for the legumes you're growing. When legumes receive nitrogen in fertilizers, they use this instead of the nitrogen from their root nodules.