Our long-term research goal is to develop practical N application systems for on-the-go remote sensing to assess crop N status and only apply N where needed at times when the crop can most efficiently utilize N. Our preliminary testing of two active sensor systems (Crop Circle and GreenSeeker) has shown considerable promise for rapid and accurate assessment of canopy N status. In this work we use the sensors to estimate corn biomass and N status as affected by canopy depth. Sensor measurements were collected in the greenhouse under variable N availability conditions as well as in the field throughout the season. Different canopy depths were generated artificially by systematically removing layers of leaves from the top, downward, or bottom upward. Biomass and N profiles were determined by destructive sampling and SPAD readings, respectively. Near infrared reflectance was unable to penetrate more than 5 layers of leaves affecting the ability to estimate biomass using traditional vegetation indices. Results are discussed in relation to potential use of these sensors and their limitations.