Quantitative theories have been developed for some of the principal plant processes governing crop response to N fertilizer. One shows that the same simple equation governs the dry matter increase of many U.K. crops during May to September when there are ample supplies of water and nutrients. Another defines the decline in the critical concentration of N in plant material with crop development. Others cover root development in soil and uptake of nitrate by these roots. These theories have been combined to form a dynamic model for estimating day-to-day responses to N fertilizer of crops grown in the field. Simulation studies with the model indicate that failure to take account of variations in subsoil nitrate and rooting depth is a major cause of uncertainty in forecasting fertilizer requirements in the U.K.