Effects of plot size on size distributions of water stable material at the soil surface under simulated rain

Abstract

When small samples are wetted by simulated rain, a significant proportion of the sample may be lost over the plot boundary as splash, and the relative importance of this edge effect will increase as plot size is reduced. Because there are considerable practical advantages in using plots as small as possible, it is important to determine the smallest plot sizes that will give results unaffected by splash losses. Therefore, this study considered effects of the size of plot exposed to simulated rain on size distributions of particles (aggregates and sand grains, with no distinction made between them) in the surface seal layer. Reducing the areas of square plots from 0.81 to 0.36 m2 had no discernible effect on size distributions in the surface seal layer, but for one of the three soils for which it was tested, a reduction of plot area to 0.09 m2 gave a significant (P < 0.01) increase in the proportion of particles <125 µm in the surface seal layer, and a corresponding significant reduction in the Mean Weight Diameter of particles in the same layer. Therefore, the use of plots of 0.09 m2 is considered to be only marginal in its acceptability, and a minimum plot area of approximately 0.36 m2 is recommended, and is generally preferable as it allows sufficient plot area for both sampling of the surface and any other measurement of surface properties that is required.