By James Rogers, Forage crops production specialist

NDSU Extension

A few years ago, I listened to a forage specialist from Auburn University giving a talk at a national meeting. His opening line was “In the South, we love three things: football, NASCAR racing and legumes.” I remember him going on about the benefits of growing legumes, including their ability to fix nitrogen.

In last month’s Forage Matters, we talked about cover crops, grazing cover crops and legume nitrogen fixation. As a quick review, the amount of nitrogen that is fixed by a legume varies depending on several factors: legume species, whether it is grown as a monoculture or mixture, length of the growing season, yield, environmental conditions, soil-available nitrogen and soil pH. 

So, how does nitrogen fixed by the legume move from the legume to plants that are not legumes?

The eighth edition of “Forages: An Introduction to Grassland Agriculture” points out seven pathways through which fixed nitrogen from legumes can be transferred. The first four are major pathways, and the latter three are minor.

1. Nitrogen in urine from grazing. This can account for 70%-75% of the nitrogen consumed, but much (50%-80%) can volatilize and be lost as ammonia.
2. Nitrogen in manure from grazing. The manure must decompose to be eventually available to the plant.
3. Decay of legume roots and nodules. This amount of transfer will vary widely.
4. Decay of leaves, stems and stolons.
5. Nitrogen leaching from leaves.
6. Root exudation of nitrogen from roots to the soil.
7. Direct legume-to-grass transfer.

To maximize the benefits of nitrogen fixation by legumes, we need to consider their long-term benefits and how best to capture them. Consider these things in capturing legume nitrogen fixation benefits:

• The majority of nitrogen is removed when legumes are baled and transferred to another location.
• When legumes are grazed, the majority of the fixed nitrogen will be recycled back to the soil as dung and urine — good grazing management improves the distribution of urine and dung across the grazed area.
• Nitrogen fixation increases with the longevity of the legume — perennial legumes will fix more nitrogen than annual legumes, and annual legumes will fix more nitrogen the longer they are allowed to grow.
• As legume yield increases, nitrogen fixation will increase.
• Legumes will use soil available nitrogen before fixing their own.
• Legumes must form a symbiotic relationship with rhizobia bacteria for fixation to occur.
• If soil pH, phosphorus, and potassium levels are limiting to legume growth, nitrogen fixation will be limited. (The little things are important!)
• Legumes grown as monocultures will fix more nitrogen than when grown as a mixture.
• Major pathways of nitrogen transfer involve the decay of the plant, meaning that the inclusion of a legume may benefit subsequent crops more than the current crop.

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.

Legumes are wonderful plants, and should be utilized if they meet your goals. Be aware that many of the fixed nitrogen rates for legumes are based on a full year of growth from full stands grown as monocultures.

To maximize legume benefits, there needs to be a cycling of above- and below-ground plant parts back to the soil as part of a systems approach. The goal is to capture as much of this fixed legume nitrogen as possible and retain it in our soils, livestock, farms and ranches.

(James Rogers is a North Dakota State University Extension forage crops production specialist at the North Central Research Extension Center near Minot, North Dakota.)

NDSU Agriculture Communication – May 28, 2026

Source: James Rogers, 701-857-7677, james.rogers.1@ndsu.edu

Editor: Dominic Erickson, 701-231-5546, dominic.erickson@ndsu.edu