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Many of the more important processes related to N demand and N supply are described by dynamic simulation techniques. The CERES models, when assembled in the framework of comprehensive crop growth models, are computeraided tools for exploring different N management options across a wide range of varying cropping practices, soil types, and weather conditions. Simulated outcomes provide estimates of economic and environmental impact of different N management options, including the effects of varying N rates, time of application, placement depth, and source.