Nitrogen (N) fertilization recommendations for corn in several states in the Midwest (including WI and IN) are based on the results of many N response trials conducted over a number of years, locations, soil types, and hybrids. The maximum return to N (MRTN) is calculated based on the yield response to applied N derived from the analysis of these trials and the price of grain and N fertilizer (Sawyer and Nafziger, 2006). The recommended fertilizer rate represents the point at which no further profit is realized by the application of additional N. All states using the MRTN approach consider crop rotation an important factor in determining the N recommendation and several include soil type, soil productivity, or region of the state as well (http://extension.agron.iastate.edu/soilfertility/nrate.aspx).
Soil Science Extension
University of Wisconsin Madison