LOCATION POSANT COEstablished Series
TAXONOMIC CLASS: Clayey-skeletal, smectitic, frigid Lithic Argiustolls
TYPICAL PEDON: Posant gravelly loam - grassland. (Colors are for dry soil unless otherwise noted.)
A--0 to 5 inches; dark grayish brown (10YR 4/2) gravelly loam, very dark grayish brown (10YR 3/2) moist; moderate fine granular structure; soft, very friable, slightly sticky and slightly plastic; 15 percent gravel; neutral; clear smooth boundary. (4 to 6 inches thick)
Bt1--5 to 10 inches; brown (7.5YR 5/3) very gravelly clay loam, dark brown (7.5YR 3/3) moist; strong medium and fine subangular blocky structure; hard, very friable, sticky and plastic; faint nearly continuous clay films on ped faces and in root channels and pores; 55 percent gravel; slightly acid; clear smooth boundary. (2 to 14 inches thick)
Bt2--10 to 16 inches; brown (7.5YR 5/3) extremely gravelly clay loam, brown (7.5YR 4/3) moist; strong medium and fine subangular blocky structure; slightly hard, very friable, sticky and plastic; faint continuous clay films on peds and in root channels and pores; 70 percent gravel; slightly acid; abrupt smooth boundary. (0 to 14 inches thick)
R--16 inches; hard quartz latite.
TYPE LOCATION: Gunnison County, Colorado; SE1/4 of Sec. 3, T. 45 N., R. 4 W.
RANGE IN CHARACTERISTICS:
Soil moisture regime: ustic
Mean annual soil temperature: 44 degrees F.
Mean summer soil temperature: 60 to 62 degrees F.
Thickness of the mollic epipedon: 7 to 15 inches
Base saturation: 80 to 100 percent
Depth to the lithic contact: 10 to 20 inches
Particle-size control section (weighted averages):
Clay content: 35 to 50 percent
Coarse fragments: 35 to 80 percent
A horizon
Hue: 7.5YR through 2.5Y
Value: 4 or 5 dry, 2 or 3 moist
Chroma: 1 to 3, dry or moist
Reaction: moderately acid to slightly alkaline
Bt horizon
Hue: 7.5YR to 2.5Y
Value: 5 through 7 dry, 3 through 6 moist
Chroma: 1 to 6, dry or moist
Texture: very gravelly or extremely gravelly, clay loam or clay
Clay content: 35 to 50 percent
Coarse fragment content: 35 to 80 percent, mainly gravel
Sand content: 15 to 55 percent
Reaction: moderately acid to slightly alkaline
Thin C horizons occur in some pedons.
COMPETING SERIES: There are currently no other series in this family. The Raton series in a closely related family has mixed clay mineralogy.
GEOGRAPHIC SETTING: The Posant soils are on hills, ridges, and mountain slopes. They formed in colluvium from quartz latite, breccia, andesite, rhyolite, or tuff. Slopes range from 5 to 65 percent. Elevation ranges from 8,000 to 10,400 feet; at higher elevations it is on south aspects. Average annual precipitation is 15 to 24 inches. The mean annual temperature is 40 to 44 degrees F., and the mean summer temperature is 63 to 65 degrees F. The frost free period is 65 to 80 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the Nutras and Wetterhorn soils. Nutras soils do not have mollic epipedons, have albic horizons, and do not have bedrock above a depth of 20 inches. Wetterhorn soils do not have mollic epipedons, have albic horizons, and have less than 35 percent coarse fragments.
DRAINAGE AND PERMEABILITY: Well drained; medium to rapid runoff; slow permeability.
USE AND VEGETATION: These soils are used principally for native pastureland. Native vegetation consists of scattered growths of ponderosa pine, big sagebrush, Arizona fescue, pine dropseed, and Indian ricegrass.
DISTRIBUTION AND EXTENT: Mountainous areas of central Colorado. MLRA 48A. The series is of moderate extent.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Bozeman, Montana
SERIES ESTABLISHED: Gunnison County, Colorado; 1974.
REMARKS: Diagnostic horizons and features recognized in this pedon:
Mollic epipedon: from 0 to 10 inches (A and Bt1 horizons).
Argillic horizon: from 10 to 16 inches (Bt1 and Bt2 horizons).
Particle-size control section: the zone from 10 to 16 inches (the entire argillic horizon).
Lithic contact: at 16 inches (R layer).
Ustic soil moisture regime.
Keys to Soil Taxonomy: Series classified according to Seventh Edition, 1996