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382 Natural Resource Management in the Andes mineral N present in the soil profile at planting, and leaf N concentration or chlorophyll content at a specific growth stage. The relation between any of these variables and the benefit of added N is usually defined in field calibration studies. Because of site-specific effects, such studies need to be conducted across a broad range of soil-crop-climate conditions. There is, however, a practical limit to the number of soil-crop-climate conditions that can be included in field studies.

An additional tool now available for evaluating N management practices and for making N recommendations is one based on computer simulation. Using dynamic simulation techniques, scientists have been able to construct computer models capable of simulating many of the major processes that control N demand and supply. When assembled within the framework of comprehensive crop-growth models, their dynamic nature makes them valuable for exploring N management options across a theoretically unlimited number of cropping practices, soil types, and weather conditions.