By Richard Ferguson and Guillermo Balboa et.al
When deciding how much nitrogen (N) should be applied for a corn crop, growers utilize a variety of resources: their own experience, advice from retailers and crop advisors, and university extension recommendations. These decisions often rely on quantitative data: soil test values, last year’s yield level, irrigation water tests, rates and timing of manure application, and fertilizer and crop prices. The rate may also be influenced by factors such as application timing, use of inhibitors (urease or nitrification), or use of biological products. These factors may be used in prediction equations that attempt to develop an Economic Optimum Nitrogen Rate ahead of the growing season.
Yet some — perhaps most, if not all — of the N needed by a corn crop can come from sources other than fertilizer. The challenge for farmers is knowing how much N credit to assign for these sources in order to determine the most profitable fertilizer rate. Guidance comes primarily from universities and industry research, as well as the farmer’s personal experience.
Legume Credits
Soybean is the most common legume in Nebraska crop rotations. Soybean meets most of its N needs from symbiotic N fixation from the atmosphere but is also efficient at utilizing inorganic N from soil. Consequently, residual soil N after soybean is usually quite low, and the crop does not add significant N to the soil, as is sometimes assumed. Instead, soybean residue has a low carbon to nitrogen (C:N) ratio, meaning that decomposition will immobilize less mineralized soil N than corn residue, leaving more mineralized soil N available for the following crop.
Source : unl.edu