Converting Light Reflectance to Nitrogen Requirement: Different Approaches.

Calculating vegetation indices from data extracted from imagery or generated by real-time active optical sensors is straight forward because the procedures are well documented. However, there are several options for translating vegetation index values into a fertilizer N recommendation. It is generally agreed that the magnitude of a vegetation index cannot be used to prescribe a fertilizer N rate due to plant compositional characteristics and canopy architecture variations within a given plant variety or hybrid. Rather, an in-field calibration strategy that normalizes all vegetation index values to the value for plants that are known to have an adequate supply of N works quite well. The predominate algorithms developed for active crop canopy sensors each use some form of normalized vegetation index value, e.g. sufficiency index, as the parameter for changing N application rates. Three basic approaches have been pursued in the development of in-season N algorithms. The simplest characterizes the best and worst areas in a field and then bases the N rates on the vegetation index values for each. Another uses growing degree days to determine the growth rate between planting and the time of sensing to predict yield potential from which crop N need is calculated. The third approach is based on the shape of a typical N fertilizer response function (quadratic plateau) as related to relative crop vigor (sufficiency index) at the time of sensing.