This chapter describes the merits of simulating a three-dimensional (3-D) root system for improving estimates of plant nitrogen (N) uptake, and the effect of replacing the 3-D root system with a one-dimensional (1-D) root system with few parameters. It provides the SPACSYS model first with its detailed 3-D root component and then with a new 1-D root-system simulation. The chapter compares the results from the two options of the model with the same data set produced in our field experiment. Understanding the processes that govern plant uptake is of major importance with respect to both crop production and environmental protection. The dynamics of plant N uptake during the growing season in both arable land and grassland is a major factor influencing the fate of inorganic N within the soil profile. The process of nutrient uptake from the soil by a root depends on the concentration of nutrient at the root surface and the kinetics of uptake.