Crust cover preparation
After soil samples were obtained from the field, they were transported to a laboratory located at the Palouse Conservation Field Station (PCFS), Washington State University, Pullman, WA. The samples were processed to remove plant residue by hand. Samples were then placed in a
greenhouse to dry for a few weeks; dry soil samples were then sieved using a 2 mm screen to remove large non-erodible materials.
Dry crust or aggregate stability increases with the clay content or organic matter within certain limits (Skidmore and Layton, 1992). In this study, we added Wyoming bentonite clay to the four soil series. Wyoming bentonite clay is an industrial product from Wyoming where 70 percent of the world’s known deposits are located and exploited for industrial clay for more than 125 years. Wyoming bentonite clay has been added to Tivoli sand from Kansas to reduce wind erosion erodibility and found to be several times more effective than kaolinite in reducing wind erosion (Diouf et al., 1990). Wyoming bentonite clay was mixed into our four soils to achieve 2, 4, 8%, and 16% higher clay content compared with the soil without the clay amendment. Mixing of the clay into the soil was accomplished by hand after which the mixed soil was placed in trays. The trays (0.015 m deep, 0.2 m wide, and 1.0 m long) were filled with the mixed soil layer-by-layer until overfilled. The irregular surface was then leveled with a metallic screed to create a flat and uniform surface. This method of filling trays resulted in a bulk density of about 1.1 kg m-3. A backpack sprayer was used to wet the soil surface of each tray. Approximately 1 L of water was applied to the surface to create a uniform 10-mm thick crust. The sprayer was equipped with a nozzle 1 cm in diameter to evenly spray water. The nozzle applied 0.5-mm diameter water drops which is representative of the largest natural raindrops in the region (McCool et al., 2009). After applying water drops to the soil surface of each tray, the tray was placed in an oven and dried at 60°C for >24 h to achieve a 10-mm thick complete crust cover. The presence of a soil surface crust is typically disturbed by tillage on agricultural lands (Usón and Poch, 2000). In our study, we created a soil tillage simulator to mimic tillage in the field. A tandem disk plough with blades spaced 25 cm apart is typically used for tillage of fallow lands in the iPNW. The blades are typically inserted into the soil to a depth of 10 cm. The tillage simulator was created based on a tillage depth to spacing ratio of 1:3 in the field. The tillage simulator was made by uniformly spacing nails along a board which was mounted on a frame above the soil tray. As the nails were manually pulled through the soil at a depth of 1 cm in the tray, ridges were created that were 0.8 cm high at 3 cm spacing. The tillage simulator maintained consistent disturbance for the soil surface. The orientation of tillage was parallel to the long axis of the trays or wind direction. Hagen and Armbrust (1992) demonstrated that ridge orientation and wind direction affected soil erosion. In our experiment, tillage was performed with our simulator to avoid any overlap. Four replications of each treatment were prepared for assessing soil loss using the wind tunnel.