LOCATION SHONKIN MTEstablished Series
TAXONOMIC CLASS: fine, smectitic, mesic Typic Haplustalfs
TYPICAL PEDON: Shonkin silt loam - grassland. (Colors are for dry soil unless otherwise noted)
E--0 to 9 inches; light brownish gray (10YR 6/2) silt loam, dark grayish brown (10YR 4/2) moist: weak very fine granular structure; slightly hard, very friable, slightly sticky, slightly plastic; few fine and many very fine roots; few fine and many very fine tubular and interstitial pores; many fine and very fine vesicules; moderately alkaline (pH 8.0); clear boundary. (6 to 8 inches thick)
B&E--9 to 12 inches; (Bt) grayish brown (10YR 5/2) silty clay loam, dark brown (10YR 3/3) moist; (E) coatings on peds and threads in the matrix of light brownish gray (10YR 6/2) silt loam, dark grayish brown (10YR 4/2) moist; moderate medium prisms that separate to moderate thin platy structure; hard, firm, sticky, slightly plastic; few fine and many very fine roots; common fine and very fine tubular pores; slightly alkaline (pH 7.4); clear boundary (1 to 10 inches thick)
Bt--12 to 28 inches; dark grayish brown (10YR 4/2) silty clay, dark brown (10YR 3/3) moist; moderate medium prisms that separate to strong fine and medium blocky structure; very hard, firm, very sticky, very plastic, few fine and common very fine roots; common fine and very fine tubular pores; distinct continuous organically-stained clay coatings on all faces of peds; slightly alkaline (pH 7.8); gradual boundary. (15 to 30 inches thick)
Bw--28 to 39 inches; light brownish gray (2.5Y 6/2) clay loam, dark grayish brown (2.5Y 4/2) moist; moderate medium blocky structure; hard, friable, sticky, plastic; few fine and very fine roots; few fine and very fine tubular pores; moderately alkaline (pH 8.4); clear boundary. (10 to 15 inches thick)
Bk--39 to 50 inches; light brownish gray (2.5Y 6/2) clay loam, dark grayish brown (2.5Y 4/2) moist; massive; slightly hard, very friable, sticky, plastic; few very fine roots; common very fine tubular pores; strong effervescence; disseminated carbonates; moderately alkaline (pH 8.2); gradual boundary. (10 to 20 inches thick)
C--50 to 60 inches; grayish brown (10YR 5/2) clay loam; massive; hard, friable, sticky, plastic; weak effervescence; strongly alkaline (pH 8.5)
TYPE LOCATION: Treasure County, Montana, 350 feet east of NW corner of sec. 22, T. 5N, R. 34E.
RANGE IN CHARACTERISTICS: Shonkin soils are usually moist. Mean annual soil temperature is 47 degrees to 50 degrees F. Solum thickness is 32 to 60 inches. Accumulations of carbontes are faint to prominent in the Bw and Bk horizons, and in some pedons gypsum crystals in nests of seams occur in or below the Bk horizon. In some deeply tilled areas, some pedons have clay loam Ap horizons.
The E horizon has hue of 10YR, 2.5Y or 5Y, value of 5 to 7 and 4 to 6 moist, and chroma of 1 or 2. It is loam or silt loam with weak granular to weak vesicular platy structure.
The B&E horizon has lamellae of silt loam or bleached sand and silt 1 to 10 mm thick, and in pedons having columnar Bt horizons tongue into that horizon 5 to 30 mm.
The Bt horizon has hue of 10YR, 2.5Y or 5Y, value of 4 or 5 and 3 or 4 moist, and chroma of 2 or 3. It is silty clay or clay with 40 to 60 percent clay. It has moderate medium prismatic to strong medium columnar structure that separates to weak thin plates in the upper few inches and to moderate to strong blocks in the lower part. Sodium saturation ranges from 5 to 10 percent in the B horizons, and increases with depth into the C horizon.
COMPETING SERIES: This is the Balta series.
GEOGRAPHIC SETTING: Shonkin soils are in nearly level, shallow undrained basins and depressions on high, dry stream terraces and uplands. The soils formed in clay loam, silty clay or clay alluvium. The climate is cool semiarid with mean annual air temperature of 45 degrees to 48 degrees F., mean summer temperature of 65 degrees to 70 degrees F., and frost-free period of 95 to 150 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the Arvada, Cushman, Gilt Edge, Hesper, Keiser, Toluca, Wanetta and Winnett soils, all of which are better drained. In addition, Arvada and Winnett soils have more than 15 percent exchangeable sodium in a major part of the Bt horizon and Gilt Edge soils have E horizons 5 inches or less in thickness resting abruptly on strong columnar Bt horizons.
DRAINAGE AND PERMEABILITY: Somewhat poorly drained; slow permeability. Ponding occurs for periods ranging from a few days to a few weeks following snowmelt and heavy rains.
USE AND VEGETATION: Used mainly as rangeland. Western wheatgrass is the principal vegetation. Some areas are cropped to small grains.
DISTRIBUTION AND EXTENT: Southeastern Montana, Western South Dakota and Eastern Wyoming. Moderately extensive.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Denver, Colorado.
SERIES ESTABLISHED: Treasure County, Montana, 1961.
REMARKS: Diagnostic properties and features recognized in this pedon are: Ochric epipedon - the zone from the surface of the soil to a depth of 9 inches (E horizon); albic horizon - the zone from the surface of the soil to a depth of 9 inches (E horizon); glossic horizon - the zone from 9 to 12 inches (B&E horizon); argillic horizon - the zone from 12 to 28 inches (Bt horizon).