Forage Matters: More About Cover Crops, Grazing And Nitrogen Fixation

By James Rogers

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.