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Fertiliser nitrogen (N) decisions in Ireland for spring barley are made based on historical information (e.g. previous crop yield and previous crop grown) and no account is taken of the growth or N dynamics of the current crop. Spectral reflectance measurements of crops can indicate the N status of crops and may be useful for guiding N inputs to barley crops in a more site-specific way. This work examined the potential of an active reflectance sensor to indicate biomass and N status characteristics of spring barley crops during the growing season. Results indicate that reflectance measurements were well related to both N uptake and nitrogen nutrition index in spring barley and may be useful to guide N inputs to the crop.