Bromide and Phosphate in Runoff Water from Shaped and Cloddy Soil Surfaces

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Abstract
Effects of soil surface shaping and clods on the release of applied bromide and phosphate to runoff were investigated experimentally with small pervious boxes and simulated rainfall. The soil material used for the surface shaping experiment was a sample of Ruston loam subsoil, < 4 mm in size. Phosphate as Ca (H2PO4)2 · H2O was mixed with the dry soil (0.008% P by weight) before packing in boxes. Triangular ridges, 3.5 cm in width and 1.7 cm in height, were made either along the slope or across the slope of the soil surfaces. Both types of ridges exposed about 12% more surface area than the control soil boxes without ridges. The soil boxes were wetted with a KBr solution (4000 ppm Br) before application of rainfall. Average Br concentration of runoff from a 1‐h, 6.8 cm/h, rainfall in both lengthwise‐ and cross‐ridged boxes was nearly four times that of the control boxes. The ridging also increased dissolved and reactive P concentration of runoff, but only by about 15%. When the upper 2.5 cm of the packed soil consisted of 4‐ to 20‐mm clods, the average Br concentration of runoff was nearly 48 times that of the control, even though the infiltration was 2.5 times more. Increasing the thickness of the surface cloddy layer to 5.0 cm delayed the start of runoff considerably, and yet the Br concentrations of runoff were higher then those with a 2.5‐cm layer of clods. In another soil (Parsons clay subsoil material), having more stable clods than Ruston loam, the Br concentrations of runoff were two orders of magnitude greater than control.

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