LOCATION OKO SDEstablished Series
TAXONOMIC CLASS: Fine, smectitic, mesic Vertic Argiustolls
TYPICAL PEDON: Oko clay loam - on a 9 percent convex slope in native grass. When described the soil was dry throughout. (Colors are for dry soil unless otherwise noted).
A--0 to 3 inches; dark grayish brown (10YR 4/2) clay loam, black (10YR 2/1), rubbed very dark brown (10YR 2/2) moist; weak fine granular structure; hard, friable, slightly sticky and slightly plastic; cracks 2 inches wide; slightly alkaline; clear wavy boundary. (2 to 6 inches thick)
Bt--3 to 7 inches; grayish brown (10YR 5/2) clay loam, very dark brown (10YR 2/2), rubbed very dark grayish brown (2.5Y 3/2) moist; weak medium prismatic structure parting to moderate fine subangular blocky; hard, friable, slightly sticky and slightly plastic; cracks 1 inch wide; moderately alkaline; clear wavy boundary. (3 to 10 inches thick)
Btk--7 to 15 inches; grayish brown (2.5Y 5/2) clay, very dark grayish brown (10YR 3/2) moist; moderate medium prismatic structure parting to moderate medium and fine subangular blocky; very hard, firm, sticky and plastic; cracks 1/2 inch wide; few organic stains on vertical faces of peds; shiny films on faces of peds; common coarse accumulations of carbonate; strong effervescence; moderately alkaline; clear wavy boundary. (0 to 9 inches thick)
Bk1--15 to 21 inches; light brownish gray (2.5Y 6/2) clay, dark grayish brown (2.5Y 4/2) moist; weak medium prismatic structure parting to weak medium subangular blocky; hard, firm, sticky and plastic; cracks 1/2 inch wide; common fine accumulations of carbonate; strong effervescence; moderately alkaline; gradual wavy boundary.
Bk2--21 to 28 inches; light brownish gray (2.5Y 6/2) clay, grayish brown (2.5Y 5/2) and dark grayish brown (2.5Y 4/2) moist; few fine distinct mottles of yellowish brown (10YR 5/4) and yellowish red (5YR 5/8) moist; weak coarse subangular blocky structure; hard, firm, sticky and plastic; few fine fragments of shale; common fine accumulations of carbonate; strong effervescence; moderately alkaline; gradual wavy boundary.
Bky--28 to 33 inches; gray (5Y 6/1) and light olive gray (5Y 6/2) clay, very dark gray (5Y 3/1) and olive gray (5Y 4/2) moist; weak medium subangular blocky structure; hard, firm, sticky and plastic; common fine accumulations of carbonate; common fine fragments of shale; many fine accumulations of gypsum; strong effervescence; moderately alkaline; diffuse wavy boundary. (Combined Bk horizons is 0 to 35 inches thick.)
C--33 to 60 inches; gray (5Y 6/1) with bands of light olive gray (5Y 6/2) clay, very dark gray (5Y 3/1) with bands of olive gray (5Y 4/2) moist; massive; hard, firm, sticky and plastic; many fine fragments of shale; slight effervescence; moderately alkaline.
TYPE LOCATION: Sully County, South Dakota; about 5 miles east and 1 mile north of Agar; 260 feet south and 40 feet west of the northeast corner of sec. 21, T. 116 N., R. 76 W.
RANGE IN CHARACTERISTICS: Depth to carbonate ranges from 5 to 16 inches. The thickness of the mollic epipedon ranges from 7 to 20 inches and extends into the Bt horizon. The Bt horizon averages 40 to 60 percent clay. When the soil is dry, cracks 1/2 to 2 inches wide and several feet long extend downward through the solum. Pebbles and small stones, 1 to 12 inches in diameter, range from 0 to 15 percent by volume throughout the pedon and up to 60 percent by volume on the surface.
The A horizon has hue of 10YR or 2.5Y, value of 3 to 5 and 2 or 3 moist, and chroma of 1 or 2. It is clay loam or loam with analogs of gravelly to stony in some pedons. It is neutral or slightly alkaline.
The Bt horizon has hue of 10YR or 2.5Y, value of 3 to 5 and 2 to 4 moist, and chroma of 1 or 2. It is clay loam or clay and neutral to moderately alkaline.
The Btk horizon has hue of 10YR or 2.5Y, value of 4 or 5 and 3 or 4 moist, and chroma of 1 or 2. Few or common, fine through coarse accumulations of carbonate are mixed throughout the Btk and Bk horizons. It is slightly alkaline or moderately alkaline.
The Bk horizon has hue of 10YR, 2.5Y, or 5Y; value of 5 or 6 and 3 to 5 moist; and chroma of 1 or 3. It is slightly alkaline or moderately alkaline.
The C horizon has hue of 2.5Y or 5Y, value of 5 or 6 and 3 to 5 moist, and chroma of 1 to 3. It has common fine to coarse accumulations of carbonate in some pedons. It is slightly alkaline or moderately alkaline. Fragments of shale are in the C horizon and become more numerous with depth. Bedded shale is between depths of 40 and 60 inches in some pedons.
COMPETING SERIES: These are the Millboro, Okreek, and Witten series. Millboro soils contain more silt and less sand and formed in clay sediments derived from shale. Okreek soils have bedrock between depths of 20 and 40 inches. Witten soils have a mollic epipedon greater than 20 inches thick.
GEOGRAPHIC SETTING: The Oko soils are on undulating to steep landscapes. Slope gradients typically are 6 to 15 percent but range from 0 to 25 percent. Surfaces are plane or convex. The Oko soils formed in fine-textured, calcareous glacial till that contains many shale fragments. The mean annual air temperature is 45 to 48 degrees F. The mean annual precipitation is 17 to 21 inches.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the Cavo, Demky, Gettys, Hoven, Jerauld, Opal, Peno, and Raber soils. Cavo, Demky, and Jerauld soils are on nearly level landscapes and have a natric horizon. Gettys soils do not have an argillic horizon and are on convex slopes above the Oko soils. The Hoven soils are in upland depressions and are wetter. The Opal soils contain more clay and are on similar positions as the Oko soils. Peno and Raber soils crack less and are on similar positions.
DRAINAGE AND PERMEABILITY: Well drained. Surface runoff is medium or rapid. Permeability is slow.
USE AND VEGETATION: Principal use is native range. Native vegetation is western wheatgrass, green needlegrass, sideoats grama, little bluestem, blue grama, sedges and forbs. The less sloping areas are cultivated with small grain and alfalfa the principal crops grown.
DISTRIBUTION AND EXTENT: Central and east-central South Dakota. The series is of moderate extent.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: St. Paul, Minnesota
SERIES ESTABLISHED: Hughes County, South Dakota, 1970.
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 15 inches (A, Bt & Btk horizons); argillic horizon - the zone from about 3 to 15 inches (Bt & Btk horizons).