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.