LOCATION ORINOCO MTEstablished Series
TAXONOMIC CLASS: Fine, smectitic, calcareous, frigid Torrertic Ustorthents
TYPICAL PEDON: Orinoco silty clay loam, in grassland (colors are for dry soil unless otherwise noted).
A--0 to 4 inches; grayish brown (2.5Y 5/2) silty clay loam, dark grayish brown (2.5Y 4/2) moist; moderate fine granular structure; soft, very friable, slightly sticky and slightly plastic; common very fine roots; many very fine pores; strongly effervescent; slightly alkaline (pH 7.6); abrupt smooth boundary. (1/2-inch to 4 inches thick)
Bw--4 to 7 inches; grayish brown (2.5Y 5/2) silty clay loam, very dark grayish brown (2.5Y 3/2) moist; moderate medium subangular blocky structure; hard, friable, sticky and plastic; common very fine roots; few very fine tubular pores; strongly effervescent; slightly alkaline (pH 7.8); clear smooth boundary. (3 to 8 inches thick)
Bky--7 to 17 inches; gray (10YR 5/1) silty clay loam, dark gray (10YR 4/1) moist; moderate medium prismatic structure; hard, friable, sticky and plastic; common very fine roots; few very fine irregular pores; few fine masses of lime; few fine masses of gypsum; strongly effervescent; moderately alkaline (pH 8.4); gradual smooth boundary. (5 to 30 inches thick)
BC--17 to 31 inches; gray (10YR 5/1) silty clay loam, dark gray (10YR 4/1) moist; massive; hard, friable, sticky and plastic; few very fine roots; few very fine irregular pores; common yellow (5Y 8/6) bands of iron sulfide; slightly effervescent; slightly alkaline (pH 7.4); clear smooth boundary. (0 to 20 inches thick)
Cr--31 to 60 inches; gray (10YR 5/1) semiconsolidated shale, dark gray (10YR 4/1) moist; neutral (pH 6.8).
TYPE LOCATION: Rosebud County, Montana; north of Vananda; 1,200 feet west and 1,300 feet north of the SE corner of sec. 5, T. 11 N., R. 38 E.
RANGE IN CHARACTERISTICS:
Soil temperature - 41 to 47 degrees F.
Moisture control section - between 4 and 12 inches; dry in all parts between four-tenths and five-tenths of the cumulative days per year when soil temperature at 20 inches is 41 degrees F.
Control section - 35 to 45 percent clay.
Depth to bedrock - 20 to 40 inches.
Depth to Bky horizon - 6 to 10 inches.
A horizon - Value: 5 or 6 dry, 4 or 5 moist
Texture: silty clay loam, clay, or silty clay
Clay content: 18 to 50 percent
Rock fragments: 0 to 5 percent pebbles
Reaction: pH 7.4 to 8.4
Bw horizon - Value: 5 or 6 dry, 4 or 5 moist
Texture: silty clay loam or silty clay
Clay content: 35 to 45 percent
Reaction: pH 7.9 to 8.4
Bky horizon - Hue: 10YR or 2.5Y
Value: 5 or 6 dry, 4 or 5 moist
Chroma: 1 or 2
Texture: silty clay loam, clay, or silty clay
Clay content: 35 to 50 percent
Rock fragments: 0 to 5 percent pebbles
EC: 4 to 16 mmhos/cm
SAR: 5 to 30
Calcium carbonate equivalent: 5 to 15 percent
Gypsum: 1 to 5 percent
Reaction: pH 6.6 to 8.4
BC horizon - Hue: 10YR or 2.5Y
Value: 5 or 6 dry; 3, 4, or 5 moist
Chroma: 1 or 2
Texture: silty clay loam, clay, or silty clay
Clay content: 35 to 50 percent
Rock fragments: 0 to 5 percent pebbles
EC: 8 to 16 mmhos/cm
SAR: 15 to 30
Reaction: pH 6.6 to 8.4
Cr horizon - EC: 8 to 16 mmhos/cm
SAR: 15 to 30
Reaction: pH 6.1 to 7.8
COMPETING SERIES:
Sunburst (MT) - does not have bedrock above a depth of 40 inches.
Vanda (MT) - does not have bedrock above a depth of 40 inches.
GEOGRAPHIC SETTING:
Landform - sedimentary plains; hills.
Elevation - 2,700 to 3,600 feet.
Slope- 2 to 35 percent.
Parent material - residuum derived from semiconsolidated shale.
Climate - cool with long, cold, dry winters; moist springs; hot, dry summers.
Mean annual precipitation - 10 to 14 inches.
Mean annual temperature - 39 to 45 degrees F.
Frost-free season - 100 to 135 days.
DRAINAGE AND PERMEABILITY: Well drained; slow permeability.
USE AND VEGETATION: Orinoco soils are used as rangeland. The potential native vegetation is western wheatgrass, Montana wheatgrass, some forbs, and shrubs.
DISTRIBUTION AND EXTENT: These soils are of limited extent in southeastern Montana.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Denver, Colorado
SERIES ESTABLISHED: Rosebud County, Montana, 1985.
REMARKS: Soil interpretation record: MT0617.
Diagnostic horizons and features recognized in this pedon are: ochric epipedon - from the soil surface to 7 inches (A, Bw horizons); horizon of lime and gypsum accumulation - from 7 to 17 inches (Bky horizon); paralithic contact - at a depth of 31 inches (Cr horizon); particle size control section - from 10 to 31 inches (part of Bky, BC horizons). Orinoco soils have a frigid temperature regime and an ustic moisture regime that borders on aridic.
Classification will be changed to the proposed Torrertic subgroup when it is approved.
ADDITIONAL DATA: Laboratory data--S89MT-041-002; 89T73315-7332S; 89P18585-18615.