Options for in-season adjustment of nitrogen rate for corn

Periods of wetness and dryness extremes can have dramatic effects on available N supply from soil and loss of applied nitrogen (N) from fertilizer and manure. Such effects can happen quickly, as recently occurred with the large profile nitrate levels in the fall following the drought of 2012 and then the losses with excessive precipitation in the spring 2013. Compared to “normal” moisture and temperature conditions, seasonal differences can significantly reduce or increase needed N applications and cause variation in yearly optimal N rates (Fig. 1). Therefore, opportunity exists for seasonal refinement of N application rate. However, based on analysis of the Corn N Rate Calculator (http://extension.agron.iastate.edu/soilfertility/nrate.aspx) database for Iowa, the maximum added profit if the optimum N rate for each trial site could be applied instead of the MRTN rate would only be $22/acre for corn following corn (CC) and $16/acre for corn following soybean (SC); based on $0.50/lb N and $5.00/bu corn. Cost for site-specific N management (ex. information and N application) would subtract from the added profit