Corn grain yields are known to be impacted by environmental and soil conditions. Air temperature, precipitation, solar radiation, evapotranspiration, along with soil temperature, moisture, and residual inorganic N concentration were evaluated in this study across 15 years and a range of N fertility levels to determine their relationship to corn grain yield. The factor having the greatest impact on grain yield varied from year to year and was related not only to the intensity or extremism of the factor but also more importantly to it's timing.