Dense Subsoils Limit Winter Wheat Rooting Depth And Soil Water Depletion

Dense subsoils pose a challenge to dryland winter wheat (Triticum aestivum L.) production in the Palouse region of the semiarid northwestern United States. Subsoils, commonly fragipans and argillic horizons, may restrict root growth and limit crop access to critical stored soil water, but this phenomenon is not well characterized in the Palouse. During the 2017-2018 growing season, an on-farm study of soil profiles in two commercial winter wheat fields in eastern Washington and northern Idaho was established to observe the effects of soil bulk density on winter wheat root system depth and subsoil water depletion. At harvest, grain yield, root density, soil bulk density, and postharvest soil water and nitrogen content were measured to 120 cm at 15-cm segments in the profile. Root-restrictive depth (RRD) was defined as the depth where root density was less than or equal to 0.25 intersections cm(-2) cross-sectional area. Soil bulk density negatively affected root density and consequently increased the probability of shallow RRD. Shallow RRD was common, with 38% of profiles having RRD less than or equal to 105 cm. Postharvest soil-water content at the bottom of the measured profile (105-120 cm) was 41% higher in profiles with RRD of 90 cm than nonrestricted profiles. Yield declined with increase in postharvest soil water content at 105-120 cm depth. Findings indicate that dense subsoils can reduce stored soil water depletion by limiting root density and root system depth that, in turn, limits yield.