Nitrogen Management

I attended the University of Illinois Agronomy Day program in Brownstown today.  The last topic of discussion caught my interest the most.  Dennis Bowman demonstrated the Trimble Greenseeker handheld.  Corn was growing side-by-side.  You can see the yellow sickly looking plants in the top photo.  The middle photo shows the healthier looking plants.  The Greenseeker results are shown on the bottom photo.  Bowman used the numbers on his smartphone with the  Connected Farm™ app to get results in seconds.  The app told him that the yellow corn needs 115 pounds per acre of Nitrogen in order to maximize yield.  I am not sure that takes into account the need to use specialized equipment to apply the Nitrogen at this point.  The assumption is that the greener corn has enough nitrogen.  I am not sure how you prove that without a soil test.