Mancos shale and plant community relationships: Analysis of shale, soil, and vegetation transects

Analyses of an extensive collection of Mancos shale and derived soils from the western United States showed pH values from 3.4 to 9.4, sulfate concentrations high and widely distributed, and soil solution equilibria of gypsum and sodium-dominated smectite important to soil salinity. In situ soils show little change in pH, conductivity, sulfate, GSI (gypsum saturation index), or sodium values with respect to the shales from which they were derived. On slopes, vegetative sequences from big sagebrush or blackbrush to shadscale to mat saltbush to greasewood are related to increases of sulfate. Galleta grass is also restricted by high sulfate values. The blackbush to shadscale transition is related to sodium increase. Mat saltbush was found in soils with 17,685 ppm sodium (saturated soil solution) and 4350 ppm sulfate (1:5 extract).