Soil genesis and soil-geomorphic relationships in an area near Sierra Blanca, Texas

The concepts of soil genesis and soil-geomorphic relationships in desert soils are particularly applicable on the Sierra Blanca Ranch, an approximately 54,000-ha (120,000-ac) area used for the beneficial application ofbiosolids on rangeland operated by MERCO Joint Venture, Inc., near Sierra Blanca, Texas. The study area lies within the southern end of the Basin and Range Province. Mean annual precipitation is approximately 300 mm. Chihuahuan Desert vegetation consists of shrub and grassland communities. Soils were mapped and later described and sampled from 25 backhoe pits. The influences of parent material, topography, and time on soil development and the relationship of soil morphology to landform designations were investigated within a 8,100-ha study area. Major landforms within this area likely formed during the Pleistocene and have been modified by subsequent erosional and depositional episodes. Alluvial fan depositional patterns occur in alluvium/colluvium derived from igneous intrusive rocks (rhyolites) and/or Cretaceous sedimentary rocks (predominantly limestones) with the formation of piedmont surfaces, erosional remnants, and relict surfaces. The basin floor includes an axial drainageway, which grades upslope to alluvial fans. The soils studied have mostly developed in alluvium/colluvium parent material and directly in limestone. Argids, Calcids, Cambids, Ustolls, and Torrerts are present. Surface textures are predominantly sandy loams and loams, and subsoil textures are loams, clay loams, and clays. Gravel percentages vary with proximity to source material. Soils have a predominantly mixed clay mineralogy. Soluble sodium concentrations are highly variable, although increased values are associated with soils developed in predominantly igneous alluvium. This relationship is likely related to the presence of albite in the rhyolite intrusives. Soils developed on stable land surfaces from igneous or mixed alluvium have argillic horizons, which may be buried or associated with the land-surface soil. Soils developed exclusively in limestone alluvium do not have argillic horizons. Calcic horizons, developed in land-surface and buried soils, occur within a range of parent materials. Degrading petrocalcic horizons occur overlying limestone bedrock and in summit and shoulder positions on erosional fan remnants of igneous alluvium. Absolute age dating is lacking, but relative geomorphic land surface age can be correlated with soil profile development.