LOCATION OBURN MT+NDEstablished Series
TAXONOMIC CLASS: Fine, smectitic, frigid Aridic Natrustalfs
TYPICAL PEDON: Oburn silt loam, native grass (colors are for dry soil unless otherwise noted).
A1--0 to 1 inch; grayish brown (10YR 5/2) silt loam, very dark grayish brown (10YR 3/2) moist; weak thin platy structure; soft, very friable, nonsticky and nonplastic; many fine roots; many fine pores; slightly acid (pH 6.1). (0 to 3 inches thick)
E--1 to 7 inches; light brownish gray (10YR 6/2) silt loam, dark grayish brown (10YR 4/2) moist; moderate thin platy structure; soft, very friable, nonsticky and nonplastic; many very fine and micro roots and tubular pores; sand and silt is clear and unstained; dark colored silty clay plates in lower inch are coated with light brownish gray (10YR 6/2) skeletans; slightly acid (pH 6.1); abrupt wavy boundary. (5 to 7 inches thick)
Bt1--7 to 14 inches; dark grayish brown (10YR 4/2) clay, dark brown (10YR 3/3) moist uncrushed peds; strong medium and fine columnar structure; extremely hard, firm, sticky and very plastic; many very fine roots concentrated between columns and very few on peds; continuous dark stained continuous clay skins on all ped faces below columnar caps; tops of columns have thin (1/8-inch thick) white (10YR 8/2) skeletans; neutral (pH 6.6); clear wavy boundary. (6 to 10 inches thick)
Btn--14 to 19 inches; grayish brown (2.5Y 5/2) clay, dark grayish brown (2.5Y 4/2) moist; moderate coarse prismatic structure parting to moderate medium blocky; extremely hard, firm, sticky and very plastic; very few roots; continuous distinct clay skins on all faces of peds; slightly effervescent; moderately alkaline (pH 8.4); gradual boundary. (4 to 6 inches thick)
Bkny--19 to 36 inches; light brownish gray (2.5Y 6/2) clay, grayish brown (2.5Y 5/2) moist; weak medium prismatic structure parting to weak medium blocky; extremely hard, firm, sticky and plastic; very few roots; few to common nodules and threads of lime; few faint lime crust on pebbles; many fine nests of gypsum in lower part of horizon; strongly effervescent, moderately alkaline (pH 8.4); clear irregular boundary. (10 to 14 inches thick)
2Bkn--36 to 60 inches; light brownish gray (2.5Y 6/2) very gravelly sandy loam, grayish brown (2.5Y 5/2) moist; 70 percent pebbles; continuous distinct lime crust on gravels mainly in upper part of horizon; strongly effervescent; strongly alkaline (pH 8.5)
TYPE LOCATION: Yellowstone County, Montana; 150 feet south and 125 feet east of the N 1/4 corner of sec. 9, T. 2 S., R. 27 E.
RANGE IN CHARACTERISTICS:
Soil temperature - 45 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 that the soil temperature is 41 degrees F or above.
Depth to Bk horizon - 15 to 20 inches.
Depth to 2Bk horizon - 25 to 35 inches.
Ap horizon - Hue: 10YR or 7.5YR
Chroma: 2 or 3
Texture: loam or silt loam
Clay content: 15 to 27 percent
EC: 2 to 4 mmhos/cm
Reaction: pH 6.6 to 7.3
E horizon - Hue: 2.5Y, 10YR, 7.5YR
Value: 5, 6, or 7 dry; 4 or 5 moist
Chroma: 2 or 3
Texture: loam or silt loam
Clay content: 15 to 27 percent
EC: 2 to 4 mmhos/cm
Reaction: pH 6.1 to 7.3
Bt horizon - Hue: 2.5Y, 10YR, or 7.5YR
Value: 4 or 5 dry, 3 or 4 moist
Chroma: 2, 3, or 4
Clay content: 50 to 60 percent
EC: 4 to 16 mmhos/cm
SAR: 3 to 13
Reaction: pH 6.6 to 7.8
Btn horizon - Hue: 2.5Y, 10YR, or 7.5YR
Value: 4, 5, or 6 dry; 4 or 5 moist
Clay content: 40 to 55 percent
EC: 8 to 16 mmhos/cm
SAR: 13 to 30
Reaction: pH 7.4 to 8.4
Bkny horizon - Value: 5, 6, or 7 dry; 4, 5, or 6 moist
Chroma: 2, 3, or 4
Clay content: 40 to 55 percent
EC: 8 to 16 mmhos/cm
SAR: 13 to 30
Reaction: pH 7.4 to 8.4
2Bkn horizon - Hue: 2.5Y, 10YR, or 7.5YR
Value: 4, 5, or 6 dry; 4 or 5 moist
Chroma: 2, 3, or 4
Clay content: 5 to 10 percent
Rock fragments: 35 to 50 percent pebbles
EC: 16 to 25
SAR: 13 to 30
Reaction: pH 7.9 to 9.0
COMPETING SERIES:
Absher (MT) - does not have a loamy-skeletal discontinuity above a depth of 40 inches.
Creed (MT) - does not have a loamy-skeletal discontinuity above a depth of 40 inches.
Elloam (MT) - does not have a loamy-skeletal discontinuity above a depth of 40 inches.
Gerdrum (MT) - does not have a loamy-skeletal discontinuity above a depth of 40 inches.
Thoeny (MT) - does not have a loamy-skeletal discontinuity above a depth of 40 inches.
Waltham (MT) - does not have a loamy-skeletal discontinuity above a depth of 40 inches.
Weingart (MT) - does not have a loamy-skeletal discontinuity above a depth of 40 inches.
GEOGRAPHIC SETTING:
Landform - alluvial fans; stream terraces.
Elevation - 2,000 to 3,500 feet.
Slope- 0 to 4 percent.
Parent material - calcareous clay alluvium from clay shale that contains alkali salts and gypsum.
Climate - long, cold winters; moist springs; warm summers.
Mean annual precipitation - 12 to 16 inches.
Mean annual temperature - 40 to 45 degrees F.
Frost-free period - 90 to 135 days.
DRAINAGE AND PERMEABILITY: Well drained; very slow permeability.
USE AND VEGETATION: Oburn soils are used mainly for native range. Potential native vegetation is bluebunch wheatgrass, western wheatgrass, green needlegrass, and blue grama.
DISTRIBUTION AND EXTENT: Oburn soils are of moderate extent in eastern Montana and western North and South Dakota.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Denver, Colorado
SERIES ESTABLISHED: Yellowstone County, Montana, 1969.
REMARKS: Soil Interpretations Record: MT0206. Diagnostic horizons and features recognized in this pedon are: an ochric epipedon from the soil surface to 7 inches, mixed (A, E horizons); an albic horizon from 1 to 7 inches (E horizon); an argillic horizon from 7 to 19 inches (Bt, Btn horizon); horizons of sodium accumulation from 14 to 36 inches (Btn, Bny horizons); a discontinuity at 36 to 60 inches; a particle-size control section that is the argillic horizon 7 to 19 inches (Bt, Btn horizons). Oburn soils have a frigid temperature regime and an ustic moisture regime that borders on aridic.
Classification changed from Borollic Natrargids to Typic Natriboralfs in 1993. A preferred classification is Glossic Aridic Natriboralfs. An amendment to Soil Taxonomy is being proposed to provide for these subgroups.