LOCATION SWEETGRASS         MT
Established Series
Rev. NRS-WDB-JAL
08/1999

SWEETGRASS SERIES


The Sweetgrass series consists of very deep, well drained soils that formed in alluvium and are moderately deep to very deep over extremely gravelly coarse sands. These soils are on plains, alluvial fans, stream terraces, and relict stream terraces. Slopes are 0 to 15 percent. Mean annual precipitation is about 16 inches, and mean annual temperature is about 45 degrees F.

TAXONOMIC CLASS: Clayey over sandy or sandy-skeletal, smectitic over mixed, frigid Typic Argiustolls

TYPICAL PEDON: Sweetgrass gravelly loam, grassland (colors are for dry soil unless otherwise noted).

A--0 to 7 inches; dark grayish brown (10YR 4/2) gravelly loam, very dark grayish brown (10YR 3/2) moist; weak fine granular structure; soft, very friable, slightly sticky and slightly plastic; common fine and medium roots; many fine and very fine pores; 2 percent cobbles; 15 percent pebbles; slightly alkaline (pH 7.6); abrupt smooth boundary. (4 to 8 inches thick)

Bt--7 to 13 inches; brown (10YR 5/3) clay, dark brown (10YR 4/3) moist; moderate medium subangular blocky structure parting to weak fine subangular blocky; slightly hard, friable, moderately sticky and moderately plastic; continuous distinct dark brown (7.5YR 4/2) clay films in pores and on faces of peds; few fine and medium roots; many very fine pores; 1 percent pebbles; slightly alkaline (pH 7.6); abrupt smooth boundary. (6 to 15 inches thick)

2Bk1--13 to 19 inches; light gray (10YR 7/2) sandy clay loam, pale brown (10YR 6/3) moist; weak coarse prismatic structure; slightly hard, very friable, slightly sticky and slightly plastic; few fine roots; common fine pores; 5 percent pebbles; disseminated lime; common large masses of lime; violent effervescence; moderately alkaline (pH 8.4); clear smooth boundary.

2Bk2--19 to 28 inches; white (10YR 8/3) gravelly sandy loam, light gray (10YR 7/2) moist; weak coarse prismatic structure; soft, very friable, nonsticky and nonplastic; 15 percent pebbles, 5 percent cobbles; disseminated lime; common large masses of lime; continuous distinct lime cast on all sides of coarse fragments; violent effervescence; moderately alkaline (pH 8.4); clear smooth boundary. (Combined thickness of Bk1 and Bk2 horizons is 7 to 21 inches)

3Bk33--28 to 43 inches; light gray (10YR 7/2) very gravelly loamy coarse sand, yellowish brown (10YR 5/4) moist; single grain; loose, nonsticky and nonplastic; 35 percent pebbles, 10 percent cobbles; disseminated lime; continuous distinct lime casts on underside of coarse fragments; violent effervescence; moderately alkaline (pH 8.4); gradual smooth boundary. (10 to 30 inches thick)

3C--43 to 60 inches; pale brown (10YR 6/3) extremely gravelly coarse sand; yellowish brown (10YR 5/4) moist; single grain; loose, nonsticky and nonplastic; 45 percent pebbles, 20 percent cobbles, 10 percent stones; few distinct lime casts on the underside of coarse fragments; strong effervescence; moderately alkaline (pH 8.4).

TYPE LOCATION: Sweet Grass County, Montana; 2,400 feet east and 1,450 feet south of the NW corner of sec. 27, T. 1 N., R. 14 E.

RANGE IN CHARACTERISTICS:

Soil temperature - 42 to 47 degrees F.
Moisture control section - between 4 and 12 inches.
Mollic epipedon thickness - 7 to 15 inches; may extend into Bt horizon.
Control section - 35 to 50 percent clay in the argillic horizon; 35 to 80 percent coarse fragments and 1 to 10 percent clay in the contrasting sandy-skeletal horizon.
Depth to 3Bk horizon - 20 to 40 inches.
Depth to calcic horizon - 10 to 20 inches.

A horizon - Hue: 10YR or 7.5YR
Value: 4 or 5 dry; 2 or 3 moist
Chroma: 2 or 3
Texture: loam, clay loam or clay
Clay content: 20 to 50 percent
Rock fragments: 0 to 25 percent--0 to 5 percent cobbles, 0 to 25 percent pebbles
Reaction: pH 6.6 to 7.8

Bt horizon - Hue: 10YR or 7.5YR
Value: 4, 5, or 6 dry; 3, 4, or 5 moist
Chroma: 2, 3, or 4
Texture: clay loam or clay
Clay content: 35 to 50 percent
Rock fragments: 0 to 25 percent--0 to 5 percent cobbles, 0 to 25 percent pebbles
Reaction: pH 6.6 to 7.8

2Bk horizon - Hue: 10YR or 2.5Y
Value: 5, 6, 7, or 8 dry; 4, 5, 6, or 7 moist
Chroma: 2, 3, or 4
Texture: clay loam, sandy clay loam, loam, sandy loam
Clay content: 10 to 30 percent
Rock fragments: 5 to 35 percent--0 to 10 percent cobbles, 5 to 35 percent pebbles
Calcium carbonate equivalent: 15 to 50 percent
Reaction: pH 7.4 to 8.4

3Bk horizon - Hue: 10YR or 2.5Y
Value: 5, 6, or 7 dry; 4, 5, or 6 moist
Chroma: 2, 3, or 4
Texture: loamy sand, loamy coarse sand, coarse sand, sand
Clay content: 1 to 10 percent
Rock fragments: 35 to 80 percent--0 to 15 percent stones, 5 to 25 percent cobbles, 30 to 50 percent pebbles
Calcium carbonate equivalent: 10 to 20 percent
Reaction: pH 7.4 to 8.4

3C horizon - Hue: 10YR or 2.5Y
Value: 5, 6, or 7 dry; 4, 5, or 6 moist
Chroma: 2, 3, or 4
Texture: loamy sand, loamy coarse sand, coarse sand, sand
Clay content: 1 to 10 percent
Rock fragments: 35 to 80 percent--0 to 15 percent stones, 5 to 25 percent cobbles; 30 to 50 percent pebbles
Calcium carbonate equivalent: 5 to 10 percent
Reaction: pH 7.4 to 8.4

COMPETING SERIES: There are no competing series.

GEOGRAPHIC SETTING:

Landform - plains, alluvial fans, stream terraces, relict stream terraces.
Elevation - 3,400 to 6,000 feet.
Slope - 0 to 15 percent.
Parent material - alluvium derived as outwash from the mountains or from major streams.
Climate - long, cold winters; moist springs; warm summers.
Mean annual precipitation - 14 to 19 inches.
Mean annual air temperature - 39 to 45 degreed F.
Frost-free period - 70 to 130 days.

DRAINAGE AND PERMEABILITY: Well drained; moderately slow permeability above the sandy-skeletal material and rapid in the sandy-skeletal material.

USE AND VEGETATION: Sweetgrass soils are used mainly for native grassland. Potential native vegetation is mainly Idaho fescue, bluebunch wheatgrass, bluegrass, prairie junegrass.

DISTRIBUTION AND EXTENT: Sweetgrass soils are moderately extensive along the mountain front in central Montana.

MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Bozeman, Montana.

SERIES ESTABLISHED: Chouteau County, Montana, 1992.

REMARKS: Soil interpretations record: MT0436. Diagnostic horizons and features recognized in this pedon are: a mollic epipedon from the soil surface to 7 inches (A horizon); an argillic horizon from 7 to 13 inches (Bt horizon); a calcic horizon from 13 to 28 inches (2Bk1, 2Bk2 horizons); and a particle-size control section from 7 to 40 inches (Bt, 2Bk1, 2Bk2, 2Bk3 horizons). Sweetgrass soils have an ustic moisture regime and a frigid temperature regime.

ADDITIONAL DATA: Laboratory sample S89MT-097-001.


National Cooperative Soil Survey
U.S.A.