Nitrogen fertilizer is a crucial input for corn production, and in the U.S. more nitrogen is applied to corn than to all other crops combined.  In wet weather, nitrogen can be lost from soil by leaching and by denitrification.  Which process predominates depends largely on soil drainage.  Nitrogen deficiency in nearly any plant is expressed by a lighter green color of leaves than in nitrogen-sufficient plants.  Nitrogen deficiency in corn can be easily seen from the air.  Fields with yield maps, aerial photos, and appreciable N deficiency were used to develop a calibration between color and yield loss.  Separately, fields with aerial photos and N rate trials were used to develop a calibration between color and optimal N rate.  A new company, NVision Ag, has been formed to use these relationships to help farmers decide whether they need to apply rescue N to corn in wet years, and to provide them with variable-rate N fertilizer control files to put more N where more N was lost.