Leaching patterns in soil under pasture and crop

Abstract

The distribution of surface-applied bromide, after leaching with 50 mm of ponded water, was measured in soil profiles under long-term pasture and under an oat crop. Also measured was the bromide distribution under pasture after leaching with natural rainfall. The method of water application, the vegetative cover, and the soil structure interacted to produce quite different leaching patterns. However, in all experiments the highest bromide concentrations after leaching were in the top 20 mm of soil. It is suggested that the vegetation, by inducing preferential flow pathways, retarded the leaching of bromide from the soil near the surface. Also rainfall on pasture apparently was subject to interception and stem-flow, which caused less effective leaching from the topsoil than would have occurred under fallow. Rainfall did, however, leach more efficiently than ponded water, probably as it induced largely unsaturated flow. But, even after 182 mm of rain in excess of evapotranspiration, 10% of the applied bromide was still recoverable from the top 50 mm of soil under pasture. Different soil structures under cropping and pasture affected the leaching patterns with ponded water. A compacted layer at 100-140 mm depth in the cropped soil apparently throttled infiltration, resulting in unsaturated flow, and hence more uniform miscible displacement below than above it. In all cases the bromide concentration at any soil depth was highly variable, with replicates tending toward a log-normal rather than normal frequency distribution.