Depending on the legume species and environmental conditions during growth, legumes can produce anywhere from 20 to 300 pounds N/acre.  The majority of fixed N is found in the leaves and stems.  This fixed N is primarily available as protein and is consumed by animals.  Much of the consumed forage is recycled through urine and manure.  As the roots, stems, and leaves decay, small amounts of N become available to other plants.

Harvested Legumes

Alfalfa is the most important monoculture legume to be mechanically harvested; although, peas, beans, soybean and peanut are sometimes use as a hay crop. Several clovers (red, white, rose, arrowleaf, crimson, etc.) as well as alfalfa and lespedeza are grown with grasses and harvested as hay.  When most of the plant growth is removed, little N is available to other crops or accumulates in the soil.

Well-managed alfalfa may yield 8 tons of dry matter per acre per year and may have 20 % protein.  This is equivalent to 1.6 ton of protein per acre.  The protein is approximately 6.25 % N.  This means that 0.1 ton (200 pounds) N/acre was fixed and harvested.  Obviously, considering a 10-ton yield with 25% protein would have required much more nitrogen.

Soybean and peanut represent single-harvest annuals that would normally have much lower yields than alfalfa.  Soybean hay harvested with most of the leaves may yield 1.5 tons/acre with 18% protein (1.12 % N), resulting in harvesting of almost 34 lbs N/acre.

Legume Cover Crops

Using the same process as above we can see that yield is an important consideration when calculating the N contribution of cover crops.  Austrian winter pea and cowpea are used as cover crops that have highly variable yields.  If growing conditions are relatively poor and only yield is only 1 ton/acre that has 15 % protein, less than 20 lbs N/acre would be fixed.  A yield of 3.5 to 4 ton/acre at 15% protein would result in about 70 lbs N/acre.

Legumes in Pastures

Legumes are important in grazed forage systems because they have the potential to extend the grazing season, increase the quantity of grazed forage, and reduce the amount of N fertilizer need to produce that forage.  Nitrogen fixation of well-managed legumes minimizes the need for N fertilizer.

Generally, legumes are more sensitive to nutrient deficiencies and low soil pH than forage grasses.  Successful pasture legume production depends on maintaining adequate levels of P and K with a soil pH of at least 6.0.  Even though N fertilization is not required, adequate levels of P and K are necessary to maintaining productive and persistent stands.  Without adequate P and K or when grown on acid soils, legumes stands will be unproductive and unreliable.  Infertile soils can be improved economically to acceptable levels with annual surface applications of lime, P, and K. Lime should be applied when soil pH values fall below 6.0.  Many of the legumes can tolerate a fairly wide range of soil pH, but they are most productive when the soil pH is near neutral (7.0).   However, this is not a short-term solution since it may take a few years to occur.  An annual soil test should be used to determine the need for P and K.

The amount of N fixed in a grass-legume pasture is more difficult to estimate because the portion of yield that comes from legumes is highly variable and the legumes are growing with grasses that are highly variable in competitiveness.  If a pasture consisting of 20% legume and 80% grass yields 4 tons/acre, the amount of N fixed would be about 20 lbs/acre, assuming the legume component was 15 % to 25 % protein.  In this case of grazed legumes in a mixed pasture, it should be noted that legumes would take up some of the N recycled through the animals.

Nitrogen Pathways 

• Urinary N from grazed legumes.  This path accounts for about 75% of the consumed N; however, much of the N in urine may be lost as ammonia.

• Legume N in Dung.  Dung contains only 2 % to 3 % N in organic forms.

• Decay of nodules and legume roots.  Estimates of the importance of this path is highly variable (0 to 100 lbs N/acre/year).

• Decomposition of leaves and stems.  The N contribution from leaves and stems is highly dependent on management but may account for several pounds per acre.

• Other pathways are considered to be minor and include.

• Leaching of N from living vegetation by water
• Passing of N from legume roots and nodules directly to the soil.
• Direct legume to grass transfer.