Crop simulation models have become important tools for quantifying nitrogen (N) dynamics, with the goal of managing N application with respect to specific soil, crop, and environmental conditions. An advantage of crop simulation models is that they can account for the relationships among environmental conditions, plant growth rate, N demand, and N availability in the soil on a dynamic basis. N uptake in plants involves complex processes that depend on Carbon-assimilation rates, N demand by the plant, transpiration rate, root density and distribution, the form of N, and N concentration and water distribution in the soil. The relationship between C assimilation and N uptake is further complicated by atmospheric CO2 concentrations and the effects on C-assimilation rate and partitioning. Experimentally determined relationships between growth rate and N content have led to functional approaches to modeling N demand and uptake. One approach is to use an allometric relationship between N and biomass.