Mancos shale and plant community relationships: Field observations

Field observations were made of associated vegetational communities during an extensive programme of collection of Mancos shale samples and derived soils. Vegetative responses in the same climate regime were related to physical and chemical variations of the shale. Sulfuric acid, formed by the oxidation of sulfur compounds, either lowers pH or forms calcium sulfate by alteration of calcium carbonate. Partial dissolution of calcium sulfate results in the formation of sodium sulfate and an ion exchange equilibrium with sodium-dominated smectite is attained. The most mesic vegetation occurs where sandstone fragments cap, or are mixed with, Mancos shale soils. Leaching, and transport of weathered shale also removes inhibitive aspects of the Mancos shale materials and favours good vegetational development.