Fuente: Wipo "precision farming" A method for in-season fertilizer nitrogen application based on predicted yield potential and a nutrient response index. The inventive method includes the steps of: determining a nutrient response index for a field; determining the normalized difference vegetation index of an area to fertilize; determining a predicted crop yield for the area; determining an attainable crop yield for the area; determining nitrogen uptake for the vegetation within the area; and determining the amount of fertilizer nitrogen required by said vegetation. In one embodiment, the normalized difference vegetation index is determined by scanning an area to fertilize with a spectral reflectance sensor which provides reflectance values for red light and near infrared light. The normalized difference vegetation index (NDVI) is defined by: NDVI=(NIR−red)/(NIR+red) where “NIR” is the reflectance value for near infrared light and “red” is the reflectance value for red light.