LOCATION OKREEK             SD
Established Series
Rev. RFS-LDZ
02/97

OKREEK SERIES


The Okreek series consists of moderately deep, well drained soils formed in clayey residuum weathered from shale on uplands. Permeability is slow. Slopes range from 2 to 15 percent. Mean annual precipitation is 17 inches, and mean annual temperature is about 47 degrees F.

TAXONOMIC CLASS: Fine, smectitic, mesic Vertic Argiustolls

TYPICAL PEDON: Okreek silty clay - on a west-facing convex slope of 6 percent under native grass. When described the soil was moist throughout. (Colors are for dry soil unless otherwise stated.)

A--0 to 5 inches; very dark gray (10YR 3/1) silty clay, very dark brown (10YR 2/2) moist; moderate medium and fine granular structure; hard, friable, slightly sticky, slightly plastic; many fine roots; neutral; clear smooth boundary. (4 to 6 inches thick)

Bt1--5 to 10 inches; dark gray (10YR 4/1) silty clay, very dark brown (10YR 2/2) moist; moderate medium prismatic structure parting to strong fine and medium subangular blocky; very hard, very firm, sticky, plastic; many fine and medium roots; slightly alkaline; clear smooth boundary.

Bt2--10 to 16 inches; dark gray (10YR 4/1) silty clay, very dark brown (10YR 2/2) moist; moderate medium prismatic structure parting to strong medium and fine subangular blocky and blocky; hard, firm, sticky, plastic; common fine roots; slightly alkaline; clear smooth boundary. (Combined Bt horizon is 6 to 15 inches thick.)

Btk--16 to 22 inches; light brownish gray (2.5Y 6/2) clay, grayish brown (2.5Y 5/2) moist; strong coarse prismatic structure parting to moderate medium and coarse blocky; extremely hard, extremely firm, sticky, plastic; few fine accumulations of carbonate; strong effervescence; slightly alkaline; clear smooth boundary. (0 to 10 inches thick)

Bk--22 to 30 inches; light olive gray (5Y 6/2) clay, grayish brown (2.5Y 5/2) moist; strong coarse prismatic structure parting to weak coarse blocky; extremely hard, extremely firm; very sticky, very plastic; common fine and medium accumulations of carbonate; strong effervescence; slightly alkaline; gradual wavy boundary. (6 to 12 inches thick)

C--30 to 38 inches; light olive gray (5Y 6/2) clay, light brownish gray (2.5Y 6/2) moist; massive; extremely hard, extremely firm, very sticky, very plastic; few to common fine and medium accumulations of carbonate; strong effervescence; moderately alkaline; clear smooth boundary.

Cr--38 to 60 inches; light olive gray (5Y 6/2) clay shale, olive gray (5Y 5/2) moist; strong effervescence; moderately alkaline.

TYPE LOCATION: Todd County, South Dakota; about 3/4 mile east of Okreek; 900 feet east and 200 feet north of the southeast corner of sec. 10, T. 39 N., R. 26 W.

RANGE IN CHARACTERISTICS: The depth to shale bedrock ranges from 30 to 40 inches. The thickness of the mollic epipedon ranges from 12 to 18 inches and extends into the Bt horizon. When the soil is dry, cracks 1/2 to 2 inches wide and several feet long extend through the solum.

The A horizon has 10YR hue, value of 3 or 4 and 2 or 3 moist and chroma of 1 or 2. It is silty clay or silty clay loam and neutral or slightly alkaline.

The Bt horizon has 10YR hue, value of 3 or 4 and 2 or 3 moist, and chroma of 1 or 2. It is silty clay or clay averaging between 40 and 55 percent clay. It is neutral or slightly alkaline.

The Btk and Bk horizons have hue of 2.5Y or 5Y, value of 5 to 7 and 4 to 6 moist, and chroma of 1 or 2. They are silty clay or clay averaging between 45 and 60 percent clay. They are slightly alkaline or moderately alkaline.

The C and Cr horizons have hue of 5Y or 2.5Y, value of 5 to 7 and 4 to 6 moist and chroma of 1 or 2. The C horizon is clay averaging between 50 and 60 percent clay. It is slightly alkaline or moderately alkaline. Bands of yellowish brown, brown and redder colors are typical of the bedrock.

COMPETING SERIES: These are Millboro, Oko, and Witten soils. Millboro, Oko and Witten soils do not have shale within a depth of 40 inches. In addition, Oko soils formed in shaly glacial till.

GEOGRAPHIC SETTING: Okreek soils are on gently sloping to moderately steep uplands. Slope gradients range from 2 to 15 percent. These soils formed in clayey residuum over clayey shale bedrock. Mean annual temperature ranges from 45 to 48 degrees F, and mean annual precipitation from 15 to 18 inches.

GEOGRAPHICALLY ASSOCIATED SOILS: These are the competing Millboro soils and the Boyd, Mosher, Norrest, Opal, Orella, Promise and Sansarc soils. Boyd, Millboro, and Promise soils are on similar or slightly lower parts of the landscape and do not have shale within a depth of 40 inches. Mosher soils have a natric horizon and are on slightly lower parts of the landscape. Norrest soils do not have a mollic epipedon. Opal soils are on similar landscapes and do not have an argillic horizon. Orella and Sansarc soils have bedrock at depths of less than 20 inches and do not have an argillic horizon. Norrest, Orella, and Sansarc soils are on higher parts of the landscape.

DRAINAGE AND PERMEABILITY: Well drained. Runoff is medium to very high. Permeability is slow.

USE AND VEGETATION: Mostly native rangeland. Western wheatgrass, buffalograss, blue grama, green needlegrass, sideoats grama, and forbs are the dominant native vegetation.

DISTRIBUTION AND EXTENT: South central South Dakota. The soil is inextensive.

MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Denver, Colorado

SERIES ESTABLISHED: Todd County, South Dakota, 1966.

REMARKS: Diagnostic horizons and features recognized in this pedon are: mollic epipedon - the zone from the surface of the soil to a depth of about 16 inches (A, Bt1, and Bt2 horizons); argillic horizon - the zone from a depth of about 5 to 22 inches (Bt1, Bt2, and Btk horizons).


National Cooperative Soil Survey
U.S.A.