Movement and leaching of residual nitrogen in soil has been studied in some details in a lysimeter experiment. Profile measurements of soil water and of nitrogen concentration in addition to measurements of amount and composition of the drainage water were carried out.
The leaching process could be described by an equation of Day. For a flow rate of approximately 1 cm/day, a dispersion coefficient of 11.5 cm2/day was obtained independent of the concentration level. Using this value, the equation satisfactorily predicted concentration of nitrogen and leached amount of nitrogen in the drainage water. The equation, although the assumptions inherent in the theory are not completely fulfilled, may therefore be used for predictive purposes under field conditions.
An excess of precipitation, approximately equal to field capacity, is predicted necessary for leaching 50 per cent of an amount of dissolved nitrogen initially localized at the surface of the soil. Consequently, under many soil/climate conditions, the composition of drainage water is influenced by agricultural practices from previous years, which makes difficult interpretation of the results. This has been illustrated by relating some available data on drainage water quality from field experiments to the concentration of nitrogen in the soil water, as predicted by the theory.