Soil Depth Limits Crop Yield in the Palouse

By Haiying Tao,, Washington State University Extension, Washington State University Farmers Network.

The Palouse is known for its capacity for producing highest wheat yield and quality in the world. Its great productivity is in part because of its deep fertile loessial deposited soils. Deep soils are critical for soil water storage and sustain crops throughout the growing season in dryland wheat production. However, in the Palouse, soil depth varies from less than 2 feet to 245 feet deep as a result of uneven deposition and weathering. Some soils in the Palouse contain dense soils at shallow depth. 

Erosion in cultivated fields has further bring the dense soils to shallower depth and intensified the variability in soil depth. Southwesterly prevailing winds erode topsoil from exposed ridge tops. Fast-moving surface water during rain events and snow melts redistribute soil from ridge tops and slopes to lower laying areas. In 1985, it was estimated that approximately 30% of upland areas in the Palouse have dense soils in the root zone and restrict roots from penetrating through.     

When there is no dense soil restricting, the winter wheat roots can penetrate deeper than 7 feet in dryland cropping systems. This allows the roots to access large volume of soil water and nutrients. Dense soils within root zone can restrict roots penetrating through and reduce roots access to water and nutrients located deeper than root restrictive depth. In dryland cropping systems in the Palouse, little rain occurs after April, which makes the amount of stored soil water that a winter wheat plant can access critical for high grain yield.

Soil bulk density, texture, and soil water content are highly correlated with the density (i.e. strength) of a soil and root density. A recent study found that soil bulk density is a good predictor to determine if the soil is a dense soil in the Palouse region. In general, the higher the soil bulk density, the more likely it is a dense soil layer that restrictive root penetration. The critical bulk density at which root growth stops vary and generally lower for soils with high clay content. Natural Resources Conservation Service identified bulk density that restrict root penetration for silt loams and silty clay loams is 1.65 grams/cm3 soil, while for sandy loam is 1.80 grams/cm3 soil. 

Shallow rooting depth caused by dense soil can results in less subsoil water depletion. A recent study conducted in two winter wheat fields in the Palouse region found that comparing with non-restrictive soil profiles, post-harvest soil water content at 42-48 inch depth was 41% higher in profiles where root restrictive layer was found at shallower than 36 inch depth. This suggests that winter wheat roots were not able to take up water deeper than the root restrictive layer. As a result, these locations had lower grain yields. 

Farmers can use a 1¼ inch or larger inner diameter Giddings probe to take an intact soil core to the interested soil depth (e.g. 6 to 7 feet deep), cut the soil core into 6-inch segments, place each segments in a labeled plastic bag, and send it to a soil testing lab for analysis. The best time of the year for soil sampling in low rainfall areas is spring because the surface soils are too dry in fall. The time of the year for soil sampling can be flexible for farm located in high rainfall areas. 

Although it is almost impossible for farmers to rebuild soil depth through changing management practices, there are practices can prevent dense soil layers entering the root zone. This include keeping soil covered by crop residue (tall standing stubble is especially efficient in snow catch and soil water storage efficiency), planting cover crops, and practicing no-till. In addition, growing crops with strong taproots, such as rapeseed, in rotation help improve subsoil structure over time. Crops with strong roots can create paths for subsequent crop roots to follow, a process called biological drilling. Long-term use of rapeseed in rotation can improve subsoil porosity and wheat rooting depth over time.

Brought to you by Washington State University Farmers Network.

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