Nitrogen Fertilization for Alfalfa Production

Mike Ottman

Extension Agronomist

Nitrogen fertilizer is generally not required for alfalfa production since alfalfa can obtain its own nitrogen from the air. Certain situations exist, however, when nitrogen fertilizer application to alfalfa may be warranted. These situations are discussed below.

What is biological nitrogen fixation?

Nitrogen fixation is the process of obtaining nitrogen from the aire. Alfalfa is in the class of plants called legumes that have the ability to interact with certain bacteria in the soil (Rhizobium) to convert atmospheric nitrogen gas (N₂) to ammonium (NH₄), a form a nitrogen the plant can use. Inoculation is initiated by these bacteria, which invade the root hairs of the plant and cause nodules to develop on the roots. The plants provide the bacteria with food (carbohydrate) and the bacteria provide the alfalfa with nitrogen, and a symbiotic relationship is established. Most desert plants, incidentally, are legumes and obtain their nitrogen from the air in the same way as alfalfa. Nitrogen fixation by Rhizobium bacteria globally equals twice the nitrogen in chemical fertilizers. Certain bacteria, algae, and other organisms in the soil also fix nitrogen, and total biological nitrogen fixation equals three or four times the nitrogen applied in chemical fertilizers.

Why think of applying nitrogen to alfalfa?

Given the prevalence of biological nitrogen fixation, why would we consider applying nitrogen to a legume such as alfalfa? The answer is because of the potential to increase yield and quality. Nitrogen fixation is a costly process in terms of the amount of energy the crop expends. Four pounds of carbohydrate or yield is expended just to break the triple bond that exists between the two nitrogen atoms in N₂ from the air and convert it to ammonium (NH₄). To produce and maintain nitrogen fixing nodules requires an additional 4 to 13 pounds of yield, so that the total energy cost for nitrogen fixation is 8 to 17 pounds of yield for every pound of nitrogen fixed. The energy cost of nitrogen fixation is decreased by the fact that alfalfa obtains roughly half of its nitrogen from nitrogen fixation (reported values range from 33 to 80%) and the remainder from the soil. For an 8 ton per acre alfalfa crop at 20%, the theoretical yield loss due to the energy required for nitrogen fixation equals 1 to 2 tons per acre. Despite the theoretical energy requirement for nitrogen fixation, a well-nodulated, healthy alfalfa crop rarely responds to nitrogen fertilizer.

When is nitrogen fertilization of alfalfa beneficial?

Nitrogen fertilization sometimes appears to increase yield since it can make alfalfa appear darker green and more succulent. These effects are real but often do not translate into increased yield. Nitrogen fertilization has been reported to increase forage quality by increasing protein or non-protein nitrogen. The effect of nitrogen fertilization on alfalfa yields have ranged from moderate decreases to a doubling of production. In most cases, either no yield response is detected or yield increases are small and range from 2 to 3% to 10%. The yield responses may be obtained only in certain cuttings. The amount of nitrogen applied in nitrogen fertilizer studies with alfalfa is usually not intended to equal the total amount of nitrogen that ends up in the forage, but is usually a smaller amount intended to supplement nitrogen fixation by the crop. Nitrogen has been applied in various forms to alfalfa and either in a single application, usually during the dormant season, or in multiple applications after each cutting. There are situations where nitrogen fertilization of alfalfa may be warranted, and these are listed below.

1. Seedling alfalfa - The use of nitrogen with phosphorus as a starter fertilizer is a common practice. Nitrogen fixing nodules may require a period of 2 to 4 weeks to develop and sometimes up to 3 cuttings to be most effective. During this establishment phase, the plant relies on soil and fertilizer nitrogen. If the soil contains more than 15 ppm available nitrogen at planting time, then the soil can usually supply the needs of the crop before nodulation becomes established and nitrogen fertilization is not required. A small amount of nitrate or ammonium can actually stimulate nodule formation but nitrogen fertilizer rates ranging from 20 to 70 pounds of nitrogen per acre have been reported to inhibit nodule formation.

2. Cold soils - Responses to nitrogen fertilizer have been reported in cold soils, particularly those low in nitrate. The soil temperature range for nitrogen fixation is about 40 to 85 F, although the optimum range is between 68 to 78 F.

3. Hot soils - High soil temperatures have not been correlated with decreased nitrogen fixation and response to nitrogen fertilizer in the field. Nevertheless, the possible negative effect of high temperatures on nitrogen fixation has been suggested as a cause for decreased yields during the summer. Surface soil temperature can easily exceed air temperatures, and soil is exposed in alfalfa after each cutting. However, soil temperature moderates greatly 2 to 4 inches below the soil surface and most of the nodules are found between 3 and 12 inches below the soil surface.

4. Spring cuttings - Nitrogen fertilizer often increases alfalfa yield for the first cutting in the spring, and sometimes the second. This response may be related to the cold soil effect mentioned above.

5. Waterlogged or shallow soils - The nitrogen fixing nodules need air to function, and waterlogging, especially combined with a shallow soil, is detrimental to nitrogen fixation.

6. Sandy soils - Nitrogen fertilizer has increased alfalfa yields on sandy soils on the Yuma-Mesa and in Saudi Arabia. These soils may not have an adequate population of nitrogen fixing bacteria. Sandy soils also have less nitrogen supplying ability compared to heavier soils, which would decrease the ability of alfalfa to supplement nitrogen fixation with soil derived nitrogen.

7. Intensive harvesting - Nitrogen fertilizer has been reported to increase yields if repeated harvests occur of immature alfalfa that has not reached the bud to early bloom stage. Some alfalfa nodules are shed after each harvest and 2 to 4 weeks are required to form new nodules. Therefore, short cutting schedules may not allow the nodules to regenerate in some situations.

8. High yield levels - A response to nitrogen fertilizer was obtained for alfalfa in a study conducted at the Yuma-Mesa particularly at yield levels above 14 tons per acre. Biological nitrogen fixation may have a limit that is only evident at high yield levels.

9. Alfalfa-grass mixtures - Nitrogen fertilizer is usually recommended for mixtures of alfalfa with a grass in order to realize the yield potential of the grass.

10. Molybdenum deficiency - Molybdenum deficiency is not known to occur in Arizona but has been reported in the Pacific Northwest. Molybdenum is essential for nodulation and assimilation of biologically fixed nitrogen in the plant. Therefore, molybdenum deficiency in alfalfa is a form of nitrogen deficiency, and molybdenum deficient plants respond to nitrogen fertilizer.

11. Low pH soils - This is not a problem for most soils in the western US. Alfalfa requires a pH of 6.5 or above for optimum nitrogen fixation.

What are the negative aspects of nitrogen fertilization of alfalfa?

Alfalfa response to nitrogen fertilizer is difficult to predict and is generally not recommended. The situations listed above where alfalfa has responded to nitrogen fertilizer can not always be duplicated. Nitrogen fertilizer can have negative effects such as a decrease in yield and stand and an increase in weed problems. Small yield increases have been obtained from application of nitrogen fertilizer, but these yield increases have not usually paid for the cost of the fertilizer. A significant alfalfa yield response to nitrogen fertilizer normally indicates a problem with nitrogen fixation, and the best strategy in this case would be to try to correct the nitrogen fixation problem rather than treat the symptoms with nitrogen fertilizer.