Nitrogen is a tough nutrient to manage.  Nitrogen can be lost in a number of ways including leaching through the soil, volatilization and denitrification.  In addition to the purchased fertilizer nitrogen applied to corn, soil mineralization, atmospheric deposition, atmospheric fixation and biological fixation by leguminous plants can all supply additional nitrogen.  Corn yield response to nitrogen is curvilinear which means there is a greater yield response and economic return for the first units of nitrogen applied compared the last units of nitrogen applied to get a maximum yield. 

To confound things even further, there is multiple year research in Ohio and other corn belt states that shows there is very little correlation between nitrogen application and corn yield.  For those that have attended fertilizer certification training, you might remember in the nitrogen unit I show research results where 160 to 180 bushel/acre corn crops were obtained with zero nitrogen input and 60 to 90 bushel corn crops with 200 lbs./acre of applied nitrogen.  It all depends upon the year and weather is a huge factor. 

Due to all of this variability and uncertainty, the best method that Ohio State University and other land-grant Universities in the Corn Belt have to provide a nitrogen recommendation is the use of an economic model that calculates a maximum return to nitrogen.  To use the model you need to input location (state), the crop rotation, the price of nitrogen and the price of corn.  Iowa State Extension houses the model, which is available on-line at: http://cnrc.agron.iastate.edu/ .  Recently I looked at a corn-soybean rotation here in Ohio with nitrogen priced at 40, 45 and 50 cents/pound and a corn price of $3.70/bushel.  The recommended nitrogen rates in pounds of nitrogen/acre were 171, 165 and 160 for nitrogen at 40, 45 and 50 cents/pound respectively.  These levels of nitrogen should provide 97 to 98% of a corn maximum yield and provide the best economic return for pounds of nitrogen applied.

Apply nitrogen as close as possible to when the crop needs it to reduce loss potential and increase use efficiency. Split nitrogen and later season applications of nitrogen increase nitrogen efficiency.  The corn plant needs very little nitrogen at early growth stages, maybe only 10% of its total nitrogen need by the V-6 stage.  However, from about the V8 to early tassel stage, the corn plants needs to take up the majority of its total nitrogen.