In site-specific management for precision farming there is a demand for sensors that can easily monitor crop nitrogen requirements throughout the growing season with a high resolution. Currently used optical measurement platforms such as satellites, airplanes, and hand-held sensors do not meet the requirements for good site-specific nitrogen management possibilities. An automated sensor system mounted on a tractor is developed and used to detect crop nitrogen status optically. A line spectrograph is used to detect the amount of nitrogen (kg N/ha) and chlorophyll (kg/ha) in a wheat crop (Triticum aestivum L.). By calculating the red edge inflection point of the plant spectra in the images, wheat crop nitrogen stress within small areas in the field can be detected. The spectrograph red edge is highly correlated with nitrogen applied to the wheat crop (in kilograms of N per hectare) (0.90), with the crop nitrogen amount (in kilograms of N per hectare) (0.89), and with the chlorophyll amount in the crop (in kilograms per hectare) (0.86). The average errors when estimating these variables with the red edge inflection point are 0.4% (24.15 kg N/ha), 3% (17.31 kg N/ha), and 8% (14.72 kg/ha), respectively. The average standard deviation of the calculated crop nitrogen inside a plot is 7.78 kg N/ha (6%) and for a nitrogen application rate of 9.22 kg N/ha (6%). The average standard deviation of the calculated chlorophyll inside a plot was 5.87 kg/ha (8%) and for a nitrogen application rate of 6.79 kg/ha (9%). This means that spectrograph red edge measurements of the wheat crop during the growing season are good estimators of crop nitrogen uptake. In this way, it can be an indirect predictor of topdress nitrogen needs.