James Rogers, a forage crops production specialist at North Dakota State University Extension, discussed the benefits of legumes and their role in nitrogen fixation in a May 28 column. Rogers recounted hearing a forage specialist from Auburn University emphasize the importance of legumes in southern agriculture due to their ability to fix nitrogen.
Rogers outlined several factors that affect how much nitrogen is fixed by legumes, including species type, whether they are grown alone or mixed with other plants, growing season length, yield, environmental conditions, soil-available nitrogen and pH. He said there are seven pathways through which fixed nitrogen from legumes can be transferred to non-legume plants. These include “nitrogen in urine from grazing,” “nitrogen in manure from grazing,” “decay of legume roots and nodules,” “decay of leaves, stems and stolons,” as well as minor pathways such as leaching from leaves and root exudation.
Rogers wrote that maximizing the benefits of legume nitrogen fixation requires attention to long-term management practices. He said most fixed nitrogen is removed when baled legumes are moved off-site but is largely recycled back into the soil when grazed properly. Other considerations include perennial versus annual legume longevity, yield levels, symbiotic relationships with rhizobia bacteria for effective fixation, adequate soil nutrients such as phosphorus and potassium, monoculture versus mixture plantings, and the impact on subsequent crops.
“Be patient. In annual systems, legume nitrogen fixation may benefit subsequent crops more than the current crop. In perennial grass-legume mixtures, maintaining a sufficient percentage of the plant population as legumes and grazing them can create a sustainable system,” Rogers said.
He concluded that using legumes should align with producers’ goals but cautioned that many estimates for fixed nitrogen rates assume full stands grown over an entire year as monocultures. To maximize benefits for soils and livestock operations, he advised cycling above- and below-ground plant parts back into the soil within a systems approach.
North Dakota State University Extension and Ag Research News supports rural communities through education programs focused on sustainable agriculture practices across North Dakota; it functions within North Dakota State University offering undergraduate and graduate degrees related to agriculture, according to the official website.
