To evaluate the temporal patterns of N deficiencies in corn and assess the ability of remote sensing to diagnose N deficiencies during the vegetative growth of corn,three field-scale experiments were conducted with various rates (56,112,and 168 kg N ha~(-1)),timing (early and late applications) and placement (injected into soil and dribbled on soil surface) of N fertilization in a split-plot design.Relationships between canopy reflectancc during the growing season and yield data at the end of growing season were studied for different treatments. Results showed significant variation in both grain yields and canopy reflectance among the three cornfields.The N fertilization made in early June resulted in low canopy reflectance in early July,but the differences disappeared as the season progressed.The effect of N rates on canopy reflectance was not significant in early July but it gradually became detectable in mid-July and thereafter.The fertilizer placement had a significant effect on grain yields only in one field but not on canopy reflectance in all three fields.These observations suggest that the deficiency of N developed under field conditions is a dynamic phenomenon,which adds complexity for accurately defining "N deficiency" and effectively developing management strategies for in-season correction.Remote sensing throughout the season helps collect information about important interactions that have not been given enough attention in the past.