Abstract: A method and apparatus for the real time determination and application of optimum amounts of nitrogen fertilizer to corn and other arable crops such as cotton, sugar beats, wheat, etc. by the use of rapid, non-destructive sensors and various fertilizer application methods. In a first embodiment, optical reflectance measurements of the crop canopy and a reference strip at the fertilizer response plateau are taken by sensors carried by a center pivot irrigation system. A sensor controller receives, stores and analyzes the sensor data as well as position data received from a center pivot controller. The sensor controller then sends commands to the center pivot controller to order fertilizer application, as necessary, through the irrigation system. A second embodiment of the invention utilizes a tractor drawn fertilizer applicator with the sensors mounted on the application booms.