EFFECT OF TIME OF APPLICATION AND CONTINUITY OF RAINFALL ON LEACHING OF SURFACE-APPLIED NUTRIENTS

The effect of time of rainfall relative to solute application, and the effect of rainfall continuity, was studied using undisturbed soil lysimeters. Immediate leaching with a surface-applied pulse of nitrate was compared with delayed (24 h) leaching of sulfate in a non-adsorbing soil. Preferential flow of water and solutes through natural soil macropores resulted in: (i) peak solute concentrations occurring before 1.0 pore volume of drainage, and (ii) incomplete leaching of applied solutes after 2.0 pore volumes of drainage. The immediate application of rainfall resulted in faster leaching rates compared with a delayed application. It is considered likely that the delay before rainfall allowed sufficient time for some solute to diffuse into intra- and inter-aggregate pore space and thus be bypassed by water flowing through soil macropores. Under intermittent rainfall, there was a series of time periods during which diffusion could occur into soil micropores and therefore the rate of leaching of surface-applied solute was slower than under continuous rainfall.