Alfalfa is one of the few crops that leaves a gift for the next crop — especially if that next crop is corn. Agronomists are learning more about how to quantify the value of that gift, which is nitrogen, and how to incorporate it into planning the nutrient budget for first-year corn after alfalfa.

Plant density, or the number of alfalfa plants per square foot in the remaining stand before termination, was long considered the key to determining how much nitrogen you can factor in for the next corn crop. However, more recent research indicates that alfalfa plant density is not always a good predictor for nitrogen credits.

“We are learning that older stands tend to provide more nitrogen for the next corn crop, and that soil texture is also important,” says Marta Moura Kohmann, Extension specialist in forage systems agroecology at the University of Wisconsin. “The impact of soil type and of age of the alfalfa stand shows up in some of the more recent research carried out to determine how much nitrogen credit a farmer can take for nitrogen following alfalfa.”

Take-home observations

Here are key findings about nitrogen for corn following alfalfa based on recent research results:

Biological nitrogen fixation varies. Everyone knows alfalfa, as a legume, fixes nitrogen from the air, thanks to bacteria at work on alfalfa roots. However, how much fixation occurs in any one year can vary by a wide margin, Kohmann says. In fact, research indicates the amount of nitrogen fixation by alfalfa can vary from 45 to 344 pounds per acre per year.

Age of stand matters. Kohmann points to trials at the University of Minnesota, which indicated that more nitrogen was recovered in alfalfa biomass from older stands, even if the stands weren’t as dense as first-year alfalfa stands when terminated. Third-year stands tended to produce more nitrogen than second-year stands.

Soil texture matters. The Minnesota trials were repeated on medium- and fine-textured soils. There was a trend toward more nitrogen produced in the forage biomass in alfalfa grown on medium-textured vs. fine-textured soils.

“Based on this study, there will be less nitrogen produced from alfalfa grown on fine-textured soils,” Kohmann says. “If you are growing corn after alfalfa on fine-textured soils, assume there will be less available nitrogen than if they were medium-textured soils. You will likely need to add more nitrogen for corn.”

Response on medium soils favors older stands. Corn planted after alfalfa in Minnesota trials received either 0, 25, 50, 75, 100 or 150 pounds of nitrogen per acre. “There was no yield response to nitrogen fertilizer at any N rate if the preceding alfalfa stand was 3 years old,” Kohmann says.

Based on this trial, the economic optimum nitrogen rate for corn following alfalfa was 30 to 100 pounds per acre. By comparison, on fine-textured soils, it was 60 to 115 pounds per acre.