LOCATION THEBO MTEstablished Series
TAXONOMIC CLASS: Very-fine, smectitic, frigid Aridic Leptic Haplusterts
TYPICAL PEDON: Thebo clay, cultivated. (Colors are for dry soil unless otherwise noted.)
Ap--0 to 5 inches; grayish brown (2.5Y 5/2) clay, dark grayish brown (2.5Y 4/2) moist; strong very fine and fine granular structure; very hard, firm, sticky and very plastic; many fine and very fine roots; many fine and very fine pores; slightly effervescent; moderately alkaline (pH 8.2); abrupt wavy boundary. (4 to 6 inches thick)
C1--5 to 20 inches; grayish brown (2.5Y 5/2) clay, dark grayish brown (2.5Y 4/2) moist; massive; extremely hard, very firm, sticky and very plastic; common fine and very fine roots; common very fine pores; many prominent shiny, grooved pressure faces; well expressed slickensides that intersect at 15 to 35 degrees from horizontal forming strong fine and medium sized wedge-shaped aggregates; slightly effervescent; moderately alkaline (pH 8.4); diffuse wavy boundary. (8 to 22 inches thick)
C2--20 to 33 inches; grayish brown (2.5Y 5/2) clay; olive brown (2.5Y 4/3) moist; 10 percent very dark gray (5Y 3/1) moist; massive; extremely hard, firm, sticky and very plastic; common very fine roots; common very fine pores; 5 percent extremely hard shale fragments; slightly effervescent; moderately alkaline (pH 8.4); diffuse wavy boundary. (11 to 15 inches thick)
Cr--33 to 60 inches; gray (5Y 5/1) semiconsolidated shale; very dark gray (5Y 3/1) moist; few seams of gypsum crystals between platy shale fragments.
TYPE LOCATION: Fergus County, Montana; 1,300 feet south and 2,360 feet west of the NE corner of sec. 18, T. 19 N., R. 23 E.
RANGE IN CHARACTERISTICS:
Soil temperature: 41 to 47 degrees F.
Control section: 60 to 75 percent clay
Depth to paralithic contact: 20 to 40 inches
Notes: Surface cracks range from 2 to 4 inches wide when the soil is dry.
Ap and A1 horizons:
Hue: 2.5Y or 5Y
Value, dry: 5 or 6
Value, moist: 4 or 5
Chroma: 1 or 2
Texture, F2 mm: clay or silty clay
Clay content: 50 to 65 percent
Rock fragments: 0 to 15 percent
Pebbles: 0 to 15 percent
Reaction: pH 6.6 to 8.4
C1 and C2 horizons:
Hue: 2.5Y or 5Y
Value, dry: 5, 6, or 7
Value, moist: 4, 5, or 6
Chroma: 1, 2, or 3
Texture, F2 mm: clay
Clay content: 60 to 75 percent
Rock fragments: 0 to 15 percent
Pebbles: 0 to 15 percent
Reaction: pH 7.4 to 9.0
Notes: Slickensides are prominent and extend into the upper part of the C2 horizon.
Cr horizon: This horizon consists of consolidated and semiconsolidated shale or clay lacustrine deposits.
COMPETING SERIES: These are the Bowdoin, Eltsac and Pendroy series. Bowdoin soils are very deep and have more than 15 percent exchangeable sodium throughout the profile. Eltsac soils have a Bky horizon at a depth of 14 inches. Pendroy soils are very deep.
GEOGRAPHIC SETTING: Thebo soils are on uplands. Elevations are 1,900 to 5,500 feet. Slopes are 0 to 45 percent. These soils formed in residuum derived from consolidated or semiconsolidated shale or from clay lacustrine deposits. The climate is cool, with long cold winters, moist springs, and hot dry summers. Mean annual precipitation is 10 to 14 inches. Mean annual temperature is 39 to 45 degrees F. The frost-free season is 115 to 135 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the competing Eltsac and Pendroy soils and the Abor and Lisam soils. Abor soils have less than 60 percent clay in the control section, and Lisam soils have a paralithic contact at a depth of 10 to 20 inches.
DRAINAGE AND PERMEABILITY: Well drained; slow to very rapid runoff; very slow permeability.
USE AND VEGETATION: Thebo soils are used mainly as native rangeland, but some areas are cultivated to nonirrigated small grain crops. Potential native vegetation is mainly western wheatgrass, green needlegrass, bluebunch wheatgrass, forbs, and shrubs.
DISTRIBUTION AND EXTENT: Thebo soils occur mainly east of the mountains in Montana. They are extensive.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Denver, Colorado
SERIES ESTABLISHED: Lewis and Clark County, Montana, 1969.
REMARKS: Thebo soils have an ustic moisture regime that borders on aridic.
Laboratory data: S71MT-69-4, S71MT-27-1. These pedons have since been correlated to Abor and Bascovy, respectively.