Reflectance sensors have been shown to be sensitive to the nitrogen status of crops. Systems using these sensors to control real-time variable-rate N fertilizer applications have been developed. For these systems to accurately diagnose the N rate needed, any shift in sensor value over short time periods (less than a day) must not substantially alter the N rate recommended. Our objective was to evaluate how much reflectance values shifted over time and over changing environmental conditions for corn and cotton. Experiments were conducted with different sensors (Greenseeker, Crop Circle, Cropscan) in a stationary position over corn and cotton. Results for all sensors and both crops suggest that sensor values can shift enough during the course of a day to substantially alter N rates recommended. In some cases, the explanation for changing reflectance values is known: dew evaporation, or occurrence of rain. In other cases, no suitable explanation has been found at the time of abstract submission. We suggest that during fertilization of large fields, reflectance values of high-N reference areas should be re-measured periodically, especially when leaf wetness changes.