2008b. Crop nitrogen requirement and fertilization

The topic of “crop nitrogen requirement and fertilization ” involves the summary of the major N-cycle processes, described throughout this monograph, into prac-tical “tools ” for managing N effi ciently, profi tably, and with low environmental impact. The foundation to achieve these tasks is a practical understanding of the major soil N-cycle processes within the N management zone, followed by the development of fertilizer N management strategies to achieve high N use effi ciency and profi tability. The main factor aff ecting N use effi ciency is the rate of applied N, as shown by many studies that have found that N losses increase rapidly when N inputs exceed the crop assimilation capacity (e.g., Broadbent and Carlton, 1978; Legg and Meisinger, 1982; Vanott i and Bundy, 1994a; Schle-gel et al., 1996; Dobermann et al., 2006). The eff ects of N timing, N source, and N placement are also important but usually produce smaller improvements in N use effi ciency compared to optimizing the N rate (Power and Schepers, 1989). Effi cient N management is essential for profi table production because N is re-quired in large amounts by cereals and N is the major limiting nutrient in most agricultural soils. Effi cient N management will also minimize excess N and envi-ronmental eff ects, such as nitrate losses to water resources, nitrous oxide eff ects on global warming, and ammonia deposition to neighboring ecosystems. The objectives of this chapter are to describe the fundamental principles for N management, with emphasis on estimating the rate of N to apply; to review and analyze current preplant N recommendation systems; and to discuss oppor-tunities for improving N recommendations through application of within-season technologies. Readers seeking a discussion of the other factors aff ecting N use ef