How Reliable Are N Credits From Alfalfa to Corn?.

The first Century farmer and writer, Columella, wrote that alfalfa (Medicago sativa L.) ‘dungs the land,’ and it is likely that most of the benefit he saw was derived from improved nitrogen (N) supply. Today, there is widespread skepticism among growers and farm advisors about how much fertilizer N is needed by corn (Zea mays L.) grown after alfalfa. This presentation will summarize the literature (> 130 site-yr) and historical University recommendations in the Midwest and Northeastern USA. It appears that corn responded to additional N in only 5% of the cases after a good stand of alfalfa was terminated. Thus, the legume N credit (also called the fertilizer N replacement value) essentially equals the fertilizer N recommendation for corn after corn under most conditions, even when corn yields are high.