February 2008
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FAQs Regarding Legume Nitrogen

Forage legumes (which include clovers, alfalfa, vetches and lespedezas) are usually grown in combination with grasses, especially in pasture situations. Some of the more important traits a grass/legume, as compared to a grass-only, pasture usually or often offer are higher forage quality, a more favorable distribution of growth and the potential for increasing total forage yield.

However, the trait that makes legumes truly unique and special forage plants is their ability to "fix" nitrogen (N) from the air. This biological process is done in cooperation with Rhizobium bacteria and results in the formation of N-containing nodules on the legume roots. Eventually, much of this N becomes available for use by the legume or other plants growing with, or after, the legume. In light of the recent increases in nitrogen fertilizer costs, interest in growing legumes is higher than it has been in many years, making this a good time to address a few frequently asked questions about the subject.

How long after germination does legume begin fixing nitrogen?
Usually within three or four weeks, however, the N initially fixed is available only to the legume. For example, in the case of a cereal rye and crimson clover pasture there would normally be virtually no N directly available to the rye from the clover in the autumn and winter.

When during the growth period of a legume does the most N fixation generally occur?
A few weeks before, and up to, the full bloom stage is generally the period when the most N is being fixed.

How much N do legumes fix?
Factors that can affect legume N fixation include legume species, strain(s) and populations of Rhizobium bacteria, legume stand, soil fertility and weather conditions. However, in terms of lbs/N/acre/year fixed by good stands, the ranges normally will be: 50 to 150 for annuals, 75 to 200 for white clover and red clover, and 150 to over 200 for alfalfa.

How much of the N fixed by an annual legume will become directly available to an annual grass growing with it?
Very little of the N fixed by an annual legume will go directly to plants growing with it; probably less than 10%. However, if the mixture is grazed, a good portion (perhaps 60-70%) of the N in the legume-forage consumed by the livestock will be returned to the soil in the form of dung and urine. This recycled N is in a highly available form and may contribute substantially to an annual grass late in the growing season. Unfortunately, depending on pasture configuration and the grazing method used, half or more of this N may be deposited in places like under trees or around the water source.

Using the rye/crimson pasture as an example, will there be any significant amount of N from the clover available to crops grown after it?
A substantial quantity of N (maybe 60 to 80 lb/acre or more coming from deteriorating legume and rye top growth and roots, plus some recycled in dung and urine) should be available to a crop growing behind a good annual clover stand. The organic N in top and root growth constitutes a slow release fertilizer source that is environmentally friendly and can help stimulate plant growth throughout the summer. A crop with a high N requirement, like pearl millet, will still need to receive substantial quantities of commercial N fertilizer, but the total amount needed should be lower. As another example, a spring application of N to a bahiagrass pasture should not be necessary when a good stand of overseeded annual clover was obtained.

Should N be applied to a winter grass/legume mixture in spring?
Yes, if early spring forage growth is needed, but the quantity should be limited to 30-60 lb/acre. The reasons are: (1) there is less grass in a grass/legume mixture than in a grass-only pasture, so less N is needed; (2) some fixed N is directly or indirectly going to the grass already; (3) adding large quantities of N to a legume severely retards N fixation; (4) heavy N applications stimulate grasses to compete more against legumes; and (5) N fertilizer is costly. Thus, it makes sense to guard against applying any more N than necessary. One must consider how badly extra pasture growth is needed, how much legume is present in the stand and the cost of N fertilizer before deciding whether to apply N and, if so, how much to apply.

How much (if any) of the N fixed in a given year will be available for crop growth the following year?
In some climates and soils some of the N fixed in a given year may still be coming available as long as four years later. However, in our hot and relatively high rainfall climate, and in most of our soils, soil N doesn’t last long. It is best to assume no N will be available after the first year.

What ultimately happens to legume N?
It can be leached from soil by water moving through it. It can escape into the atmosphere via volatilization (at least 30% of the N forage animals ingest is lost in this manner) or by denitrification (a process that occurs during a cool, wet weather). Forage often contains 2.5% or more N (dry matter basis), so when hay is removed from a field, a lot of N is likewise removed. However, very little is removed from a field when animals are sold.

Conclusion
Biological N fixation by forage legumes can offer ample justification for growing them in situations where they fit well. The amount of N available to crops growing with or after forage legumes can vary widely, but the value of this benefit alone normally more than offsets the cost of growing them. Other benefits forage-legumes offer, especially improved animal performance, can be even more compelling reasons for growing them.

Don Ball is an Extension Forage Crop Agronomist with Auburn University.