Prospects and results for optical systems for site-specific on-the-go control of nitrogen-top-dressing in Germany

Signals for site-specific nitrogen-top-dressing can be obtained by a sensor mounted on a tractor. The plant appearance can serve as a criterion. The question is which plant criteria provide pertinent information and how this can be indicated. Increasing the nitrogen-supply changes leaf colour from yellow-green to blue-green via the chlorophyll-concentration in the leaves and leads to growth of plants. Present sensing systems measure either chlorophyll concentration in the leaves, total area of the leaves or crop resistance against bending. The aim and purpose of this study is to outline prospects for application. Therefore, the emphasis is on results and not on experimental methods, to which references are given. Mainly optical sensing systems relying on reflectance or fluorescence are dealt with. Good signals of the nitrogen-supply can be obtained from the red edge plus the near infrared range of the reflectance. The results with some new spectral indices were better than those with standard spectral indices. Fluorescence sensing instead of reflectance sensing eliminates erroneous signals from bare soil. However, only low supply rates were clearly indicated. The biomass of the crop or the total area of its leaves is a very important criterion. Reflectance indices can take this into account. Fluorescence signals are barely influenced by this parameter.