Charles Shapiro, Extension Soils Scientist
Bruce Anderson, Extension Forage Specialist
Assessing and providing for the nutrient needs of your alfalfa crop starts with collecting soil and irrigation water samples to determine available nutrients. Nebraska research has shown that alfalfa production may benefit from nitrogen, phosphorus, potassium, sulfur, and boron applications. Lime additions also can benefit alfalfa on low pH soils. Nutrient removal from alfalfa fields is high because most of the aboveground plant material is harvested and removed several times during the growing season.
To determine nutrient needs of your alfalfa, collect soil samples from a depth of 0 to 8 inches every three to five years to determine soil pH, soil buffer pH, and phosphorus and potassium needs. If you’re seeding a new crop, soil testing and fertilizer applications are recommended at least six months before seeding to allow for incorporation during tillage. Irrigation water may contain significant amounts of lime, potassium, and sulfur, which can reduce the cost of the fertilizer program. Irrigation water samples should also be taken every three to five years. The amount of nutrient supplied by the soil and the irrigation water should be subtracted from the recommended need to determine the actual application rates.
On established alfalfa stands, nitrogen fertilization is not required because Rhizobium meliloti bacteria convert nitrogen gas from the air into a plant-usable form of nitrogen. To ensure establishment of these bacteria in the alfalfa roots, alfalfa seed needs to be inoculated with the bacteria prior to planting. Most seed is pre-inoculated.
Newly seeded alfalfa can benefit from 10-15 lb of nitrogen fertilizer per acre to ensure a good start, particularly on sandy or low organic matter sites and with early spring plantings into cold soils. Past fertilizer and cropping history and/or a soil analysis for nitrate in the top 8 inches of soil can help determine if there is sufficient soil nitrogen. Excessive nitrogen will reduce the effectiveness of the nodules. When this occurs, the alfalfa will use fertilizer nitrogen.
Table 2. Phosphorus fertilizer recommendations for alfalfa in Nebraska based on Bray-1 or Olsen phosphorus tests.
Phosphorus Soil Test
Bray-1 or Mehlich-3
Every 2 Yrs
|Phosphorus Soil Test||Irrigated||Non-irrigated|
|Bray-1 or Mehlich-3||Olsen||Relative Level||Annually||Annually||Every 2 Yrs|
|> 25||> 14||High||0||0||0|
Phosphorus applications will be need to achieve top alfalfa yields on many soils in Nebraska. When establishing alfalfa, apply a two- to three-year supply of phosphorus, preferably before tillage, in order to incorporate the phosphorus. Yearly applications of phosphorus are suggested for irrigated alfalfa and dryland alfalfa with a pH greater than 7.0. On calcareous soils in northeast Nebraska (Crofton and Nora soil types), application ahead of seeding followed by topdressing every two years was found to be most profitable. Knifing phosphorus in bands before planting may be especially effective on higher pH soils since less soil-phosphorus contact reduces phosphorus fixation. Phosphorus should be applied early in the spring or in the fall after the last cuttings to get maximum benefit from broadcast applications.
Alfalfa responds better to incorporated applications than to top-dressed applications. Under drought conditions, surface phosphorus may not be available; therefore, providing adequate phosphorus deeper in the soil is an important risk management strategy, especially in rainfed production systems.
Lime needs to be incorporated into the soil for maximum benefit for a newly seeded alfalfa crop. Unincorporated lime will only change the pH at the soil surface and will offer little benefit to newly seeded alfalfa. In a no-till system incorporation is not need if the lime is applied two or more years before sowing. Soil pH will gradually increase during the first 6 to 18 months after application.
Table 3. Potassium fertilizer recommendations for alfalfa in Nebraska based on Bray-1 or Olsen potassium tests.
Potassium Soil Test Potassium to Apply
(ppm) (lb K2O/acre)
|Potassium Soil Test||Potassium to Apply|
Most Nebraska soils, especially non-sandy soils, can provide sufficient native potassium for alfalfa production; however, potassium fertilizer may be needed on some coarse-textured, sandy soils, many of which are irrigated. As with phosphorus, annual broadcast applications are recommended for irrigated alfalfa while large applications of two- to three-year supplies applied at seeding are suggested for non-irrigated production. In established fields apply potassium fertilizers after final harvest to prevent damage to the crowns of growing plants and to increase the winter hardiness of the crop.
Alfalfa often needs sulfur on sandy, low organic matter soils. In eastern and central Nebraska, sulfur is most likely needed in sandy soils where the organic matter content is less than 1%. In western Nebraska it’s most likely needed in sandy soils with an organic matter content less than 0.6%. Sulfur may increase protein content in alfalfa but my not increase yield. When needed, annual applications of 30-40 lb of sulfur are suggested for irrigated alfalfa. For dryland alfalfa, an application of 100 lb sulfur every three years should be adequate.
Except for boron, micronutrient deficiencies have not been found in alfalfa in Nebraska. Boron deficiencies are rare. If soil and water testing indicates a boron deficiency, apply 1 lb of boron per acre, usually with other fertilizers. Do not apply boron near the seed. It is easy to overapply boron and it may become toxic to the alfalfa and subsequent crops.
Applying Fertilizer to Establish Alfalfa
When establishing an alfalfa stand and based on soil test results, fertilizer can be broadcast and incorporated before establishment, applied with a drill at planting time, or applied by a combination of these two methods. When low rates are to be applied, application when seeding with a drill would be appropriate. Fertilizer with high levels of nitrogen and potassium should not be placed in direct contact with the seed. These nutrients create a salt effect that can reduce germination and stand. This should not occur with fertilizers containing only phosphorus. The sum of nitrogen and potassium applied with the drill should be limited to less than 20 lb per acre. If soil is drier than normal, limit application rates to half this amount. If not all the required fertilizer can be placed with the seed, broadcast the balance and incorporate prior to seeding.