The Burns leaching equation

SUMMARY The simplicity and utility of Burns' leaching equation make it worthy of study. The equation may be written as where X is the fraction of initially surface-resident fertilizer leached below depth z by net rainfall I, in soil with a volumetric water content at ‘field capacity’ of θ. The equation is analysed using transfer functions. The analysis shows that Burns' equation is consistent with an ‘independent flow tube’ soil leaching model, rather than the soil solution being well-mixed at each soil depth as Burns suggested. The flux and resident soil solution soil concentration profiles are shown to be quite different. An alternative definition of θ is suggested. The behaviour of ‘a Burns soil’ for different initial and boundary conditions is discussed.