Nitrogen Fertilization in Alfalfa Production

Nitrogen fertilization of alfalfa ( Medicago sativa L.) at establishment has been a controversial practice. A review of the literature was conducted to determine conditions when preplant N fertilization of alfalfa resulted in increased yields in the seeding year. Field and lab studies demonstrated that when the soil nitrate levels were greater than 15 ppm and conditions were favorable for effective nodulation (soil pH from 6.2 to 7.5 and high populations of appropriate Rhizobium bacteria present), using preplant N fertilizer in alfalfa establishment did not result in economically important yield increases. Under field conditions that inhibit effective nodulation, alfalfa yield in the seeding year increased when a small amount of N fertilizer (10–50 lb N/acre) was applied. Application of fertilizer N at establishment has been demonstrated to increase yields when soils are low in N (<15 ppm soil nitrate) or organic matter (< 1.5%). Greenhouse and growth chamber research suggests that when alfalfa is established in low N soils that are also relatively cool less than 60°F) for several weeks after planting, a yield increase due to fertilizer N is even more likely. Before deciding to use N in alfalfa establishment, it is necessary to determine levels of soil nitrate and percent organic matter, and to obtain records of soil temperature for the period following the expected planting date. Research Question Using N fertilizer in alfalfa establishment has been a controversial practice. A review of the scientific literature was conducted to determine if applying N fertilizer at establishment results in higher yields in the seeding year. Combining the literature search with our research experience, we provide guidelines to the appropriate use of fertilizer N in alfalfa production. Literature Summary Applying N fertilizer during alfalfa establishment results in economically beneficial yield increases in the seeding year under certain field conditions. In most alfalfa production conditions, however, with favorable soil pH (6.2–7.5) and soil N levels (>15 ppm soil nitrate), applying fertilizer N at establishment did not result in yield increases. When field conditions were not favorable for effective nodulation, first‐cut alfalfa yield increased when fertilizer N was applied. This was observed when soil pH was <6.0 or an ineffective inoculant was used. Applying 10 to 50 lb N/acre at establishment increased yields when soils were low in N (<15 ppm soil nitrate) or organic matter (less than 1.5%). The response of alfalfa to N was increased when the average daily soil temperature remained below about 60°F for several weeks after planting. Applied Question Under what conditions is using N fertilizer in aflalfa establishment most likely to result in increased yield? The key to determining the usefulness of fertilizer N in alfalfa establishment is for alfalfa producers to know their soil conditions and to provide favorable conditions for nodulation of alfalfa. Before deciding to use N in alfalfa establishment, it is necessary to determine levels of soil nitrate and percent organic matter, and to obtain records of soil temperature for the period following the expected planting date. The application of 20 to 50 lb N/acre is recommended only when soil nitrate levels are less than 15 ppm, soil organic matter levels are less than 1.5%, and weed control is assured. The likelihood of a yield increase is greater if the average daily soil temperature is less than 60°F for several weeks after planting. Regardless of what decision is made about the use of N in alfalfa production, it is important for the grower to provide favorable conditions for nodulation. This includes adjusting the pH to a minimum of 6.2 and treating seed with an appropriate rhizobial inoculum.