A system and method of determining nitrogen levels from a digital image. In particular, a method of determining leaf nitrogen concentration and yield from a digital photograph of a fully developed leaf (collared leaf) of a crop of nonlegumes, such as corn, rice, wheat, cotton, potatoes or sugarcane. The digital image is processed to determine a dark green color index (“DGCI”), which is closely related to leaf nitrogen concentration and yield. Standardized color disks having known DGCI values are included in the digital photograph and serve as an internal standard. The internal standard allows correction of DGCI of samples when using different cameras and/or when lighting conditions change.