Experiments were conducted with a simulated rainfall on boxes of three different soils (Ruston fine sandy loam, Ruston loam, and Parsons clay), prewetted with a bromide (4,000 ppm Br) solution. The bottom of the soil boxes (100 cm long by 15 cm wide by 10 cm deep) was either an impervious plate that allowed no downward infiltration through the soil during the rainfall, a pervious plate that allowed free infiltration, or a pervious plate covered with a thin layer of soil‐slurry, which allowed infiltration at a reduced level. In all the soils, Br concentrations in runoff from impervious‐base boxes were one to two orders of magnitude higher than in runoff from pervious‐base boxes. The concentrations in slurry‐base boxes were intermediate. This result and the shape of the concentration‐time curves showed that the concept of an effective complete mixing depth of rainwater and soil solution used in modeling the release of chemicals‐to‐runoff is not strictly valid. However, the concept may be applied as an approximation to the data from pervious boxes. Bromide concentrations of the soil in the impervious‐base boxes at the end of 1‐h rainfall events (6.8 cm of rain) showed that the Br was lost to runoff from as great as the 2.0‐cm depth. The amount released, however, decreased with depth. It is suggested that the transfer of chemical from below a thin soil surface layer may be due to pumping action of, or turbulence generated by, the raindrop impacts, and may be considered as an accelerated‐diffusion process. As a simplified approach, the mixing zone concept was modified and tested by incorporating an exponential decrease in the degree of mixing with depth, and a piston displacement of soil solution by infiltration. This approach seemed adequate as a first‐order approximation. The results suggest that clay‐pan soils, wet areas in a watershed, and tillage practices that increase permeability of the soil‐surface layer relative to that of the layer below will greatly increase the loss of soil chemicals to runoff.
Synopsis Field experiments and long term weather data indicate that in the Southern Great Plains plant populations of about 18,000 plants per acre in 40‐inch rows will in most years result in the best grain yields. Forage production was higher with reduced row width and increased seeding rates. Results are explained on the basis of water availability and use during the growing season.
Effects of soil slope length, degree of slope, soil cover, and storm size on soluble P released to runoff were investigated under controlled conditions, in 100‐ by 30‐ by 7.5‐cm soil boxes, under a simulated rainfall. These effects were related to a simplified kinetic model, presented earlier, which was developed further to incorporate the above effects conceptually. An increase in slope length from 33 to 100 cm increased the average P released per unit area of soil. Most of this increase was explained by the effect of slope length on the water to soil ratio, which influences the kinetics of release. However, the effect of overland flow, which influences the mass of soil releasing P to runoff, also increased with the increase in slope length. An increase in percent soil slope also increased the P release, directly proportional to slope between 4 and 16%. This was explained, for a slope length of 100 cm, primarily by the effect of slope on raindrop impact. Soil covers, simulated by different‐mesh screens, decreased the P release. Their effects were explained by the decreases in raindrop impact, which were determined by splash measurements. The effects of soil covers on P release increased with an increase in soil slope. The storm‐average P concentration of runoff decreased logarithmically with an increase in storm size. These experimental results could be explained by terms in the conceptual model, which were based on an analogy with a physical soil erosion mechanism. Further work is needed, however, on longer slopes and under field conditions.
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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