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Grasslands and savannas around the world have experienced an increase in woody cover over the past century. However, the mechanisms responsible for the distribution of woody patches in savanna landscapes remain unclear. In the Rio Grande Plain of southern Texas, vegetation is characterized by a two-phase pattern consisting of discrete woody patches embedded within a grassland matrix. The soils on upland portions of this landscape are sandy loams with a laterally continuous subsurface argillic (Bt) horizon with non-argillic inclusions. Prior studies have suggested that large woody patches (> 100 m2) occur only on soils where the argillic horizon is absent. To test this hypothesis, we quantified landscape-scale spatial patterns of soil texture across soil depths by intensively sampling to a depth of 1.2 m in a 160 m ? 100 m georeferenced plot subdivided into 10 m ? 10 m grids. Kriged maps of soil clay content were developed for each soil depth increment throughout the upper 1.2 m of the profile, and the locations of non-argillic inclusions were mapped. Visually comparison of kriged maps to an aerial photograph of the study area showed that large woody patches were present only where the non-argillic inclusions were present. This clear visual relationship was further supported by strong negative correlations between soil clay content and the Normalized Difference Vegetation Index (NDVI) which mapped the locations of the woody patches. Subsurface non-argillic inclusions may favor the establishment and persistence of woody patches by enabling root penetration deeper into the profile, providing greater access to water, nutrients, and other soil resources that are less accessible on those portions of the landscape where the argillic horizon is present