From a number of basic relationships between several crop ecological components (Booij et al., 1996a) a system was developed for giving supplementary nitrogen application in leeks, that was based on the measurement of light interception. A description of the approach is given and a comparison is made with existing systems for supplementary nitrogen application. A comparison was made for four aspects: nitrogen input, yield, quality and residual mineral soil nitrogen. The system based on the measurement of light interception performed in general as well as a system that was based on the determination of soil mineral nitrogen. However, the system based on the measurement of light interception is easier to use and provides a recommendation instantaneously. For both systems yield and quality were the same as for the standard recommendation but nitrogen input was less and residual mineral soil nitrogen was reduced.