MANHATTAN, Kan. – With the price of nitrogen fertilizer skyrocketing over the past year, a Kansas State University scientist suggests growers look to an alternative way of getting nitrogen into the soil – planting legumes in their crop rotations.

Crops such as sweet clover, alfalfa, red and white clover, and soybeans all have the ability to add nitrogen to the soil through a process called symbiotic nitrogen fixation, said K-State Research and Extension soils specialist Ray Lamond.

"Rotating legumes with non-legumes has the double advantage of growing the legume with little or no additional nitrogen fertilizer, plus a nitrogen credit for the subsequent non-legume crop," Lamond said. "An estimated 10 to 20 percent of the annual nitrogen input to soils in the United States comes from symbiotic nitrogen fixation."

Even growers who have already included a legume in their crop rotations are not always taking advantage of the ‘credits’ they’ve gained by the nitrogen left in the soil, and have not reduced nitrogen fertilizer for subsequent crops accordingly, he said.

Rhizobia bacteria interact with legume host plants to change dinitrogen gas in the atmosphere into a form usable by the host plant and subsequent crops, Lamond explained.

To activate the process, growers add an inoculum to the field that is being planted to legumes. The Rhizobia bacteria is plant-specific. In other words, the inoculum for soybeans is not the same strain that would be used for inoculating alfalfa, the agronomist said.

"Inoculum labels indicate the legume plants for which the bacteria are effective. Remember, the bacteria in the inoculum are sensitive living organisms, and should be treated accordingly," he said. "Inoculum should be stored in a cool, dry place away from direct sunlight, and should be used according to label directions before the expiration date.

"Legume seed pre-inoculated more than 30 days prior to seeding should be re-inoculated before seeding," he added.

The amount of nitrogen fixation in any particular field varies, depending on the strain of bacteria, condition of the host plants, and soil and climatic conditions.

The rate has been estimated as high as 450 pounds of nitrogen per acre by a crop of clover in New Zealand, Lamond said. However, such results occur only under optimum conditions. Amounts are typically less – in a range of 40 to 200 pounds per acre.

Conditions that favor nitrogen fixation include neutral soil pH [6.0 to 8.0]; adequate soil moisture and aeration; and an adequate supply of available calcium.

Factors that impede the process include soil pH below 5.5, cool soil temperatures, and either very wet or very dry soils.

"Nitrogen fertilizer rates can and should be adjusted when legumes are used in crop rotations with corn and grain sorghum or other summer annual crops," Lamond said. "The use of credits for legumes in crop rotation could result in cost savings for nitrogen fertilizer ranging from $5 to $40 per acre, depending on the legume. For example, an N credit of as much as 140 pounds per acre could be allowed when corn or grain sorghum follows alfalfa. At today’s fertilizer N prices that could be worth over $40 per acre."

Using legumes in crop rotations bring other benefits as well, he said. Their use may break insect and disease cycles and diminish weed problems in fields continuously planted to the same crop.

Soybeans are the most widely-used legume rotation in Kansas, Lamond said. "The old rule of thumb was to allow 1 pound of N credit for each bushel of soybeans produced, but based on our research, we basically allow a conservative 35 to 40 pounds of N credit for corn, grain sorghum or sunflower grown after soybeans."

Lamond is a coauthor of a K-State Research and Extension Bulletin L-778, Using Legumes in Crop Rotations, which summarizes the credits allowed for various legumes. This bulletin is available at county extension offices.