LOCATION ORINOCO            MT
Established Series
Rev. GFB-DRS-JAL
02/1999

ORINOCO SERIES


The Orinoco series consists of moderately deep, well drained soils that formed in residuum weathered from semiconsolidated shale. These soils are on sedimentary plains and hills. Slopes are 2 to 35 percent. Mean annual precipitation is about 12 inches, and mean annual air temperature is about 42 degrees F.

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.


National Cooperative Soil Survey
U.S.A.