In simulation models for water movement and nutrient transport, uptake of water and nutrients by roots forms an essential part. As roots are spatially distributed, prediction of root growth and root distribution is crucial for modelling water and nutrient uptake. In a preceding paper, De Willigen et al. (2002; Plant and Soil 240, 225¿234) presented an analytical solution for describing root length density distribution as a diffusion-type process. In the current paper, we present a numerical model that does the same, but which is more flexible with respect to where root input can occur. We show that the diffusion-type root growth model can describe well observed rooting patterns. We used rooting patterns for different types of crops: maize, gladiolus, eastern white cedar, and tomato. For maize, we used data for two different types of fertiliser application: broadcast and row application. In case of row application, roots extend more vertically than horizontally with respect to the broadcast application situation. This is reflected in a larger ratio of diffusion coefficients in vertical versus horizontal direction. For tomato, we considered tomatoes grown on an artificial rooting medium, i.e. rockwool. We have shown that, in principle, the model can be extended by including reduction functions on the diffusion coefficient in order to account for environmental conditions.