LOCATION RAHWORTH MTEstablished Series
TAXONOMIC CLASS: Fine-silty, mixed, superactive, frigid Aridic Haplustepts
TYPICAL PEDON: Rahworth loam, in grassland (colors are for dry soil unless otherwise noted).
E--0 to 2 inches; light brownish gray (2.5Y 6/2) loam, dark grayish brown (2.5Y 4/2) moist; moderate thin platy structure; slightly hard, very friable, slightly sticky and slightly plastic; many very fine roots; mildly alkaline (pH 7.9); abrupt smooth boundary. (1/2 inch to 3 inches thick)
Bw1--2 to 4 inches; grayish brown (2.5Y 5/2) clay loam, dark grayish brown (2.5Y 4/2) moist; moderate medium prismatic structure parting to moderate very thin platy; hard, friable, sticky and plastic; many very fine roots; many very fine pores; moderately alkaline (pH 8.0); abrupt smooth boundary. (2 to 4 inches thick)
Bw2--4 to 11 inches; grayish brown (2.5Y 5/2) clay loam, dark grayish brown (2.5Y 4/2) moist; moderate medium prismatic structure parting to moderate medium angular blocky; hard, friable, sticky and plastic; common very fine roots; many very fine pores; strongly effervescent; moderately alkaline (pH 8.2); clear smooth boundary. (7 to 10 inches thick)
Bk--11 to 23 inches; grayish brown (2.5Y 5/2) silty clay loam, dark grayish brown (2.5Y 4/2) moist; moderate coarse prismatic structure; very hard, firm, sticky and plastic; common very fine roots; common very fine pores; strongly effervescent; few fine masses of lime; moderately alkaline (pH 8.2); gradual smooth boundary. (6 to 15 inches thick)
Byz--23 to 36 inches; variegated grayish brown (2.5Y 5/2) and light yellowish brown (2.5Y 6/4) silty clay loam, dark grayish brown (2.5Y 4/2) and light olive brown (2.5Y 5/4) moist; massive; very hard, firm, sticky and plastic; common very fine roots; slightly effervescent; common fine masses and seams of gypsum; few fine masses of salt; moderately alkaline (pH 8.4); gradual smooth boundary. (12 to 25 inches thick)
Bz--36 to 43 inches; variegated grayish brown (2.5Y 5/2) and light olive brown (2.5Y 5/6) silty clay loam; dark grayish brown (2.5Y 4/2) and light olive brown (2.5Y 5/4) moist; massive; very hard, firm, sticky and plastic; few very fine roots; few fine masses of salt; moderately alkaline (pH 8.2); gradual smooth boundary. (0 to 15 inches thick)
BC--43 to 60 inches; variegated light yellowish brown (2.5Y 6/4) and grayish brown (2.5Y 5/2) silty clay loam, light olive brown (2.5Y 5/4) and dark grayish brown (2.5Y 4/2) moist; massive; very hard, friable, sticky and plastic; few very fine roots; few crystals of primary gypsum; moderately alkaline (pH 8.2).
TYPE LOCATION: Rosebud County, Montana; 800 feet west and 300 feet south of the NE corner of sec. 24, T. 9 N., R. 37 E.
RANGE IN CHARACTERISTICS:
Soil temperature - 42 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 when the soil temperature at a depth of 20 inches is 41 degrees F or higher.
Clay content - 27 to 35 percent
Sand content - 10 to 30 percent, mainly very fine sand
Depth to Bk horizon - 10 to 17 inches
Depth to Bz horizon - 16 to 28 inches
E horizon - Clay content: 20 to 25 percent
Reaction: pH 7.4 to 7.8
Bw1 horizon - Hue: 2.5Y
Value: 5 or 6 dry
Texture: loam or clay loam
Clay content: 25 to 30 percent
Reaction: pH 7.4 to 8.4
Bw2 horizon - Texture: loam or clay loam
Clay content: 25 to 32 percent
Reaction: pH 7.9 to 8.4
Bk horizon - Value: 5 or 6 dry
Texture: silty clay loam or clay loam
Clay content: 30 to 35 percent
Calcium carbonate equivalent: 5 to 15 percent
EC: 2 to 4 mmhos/cm
SAR: 10 to 25
Reaction: pH 7.9 to 8.4
Byz, Bz, BC horizons - These horizons are always variegated. Very thin sandy laminations are common. These horizons remain moist in some part throughout the growing season.
Hue: 10YR or 2.5Y
Value: 5, 6, or 7 dry; 3, 4, or 5 moist
Chroma: 1, 2, and 4
Texture: silty clay loam or clay loam
Clay content: 30 to 35 percent
Gypsum: 0 to 2 percent
EC: 16 to 30 mmhos/cm
SAR: 25 to 45
Reaction: pH 7.9 to 8.4
COMPETING SERIES:
Alona (MT) - very strongly alkaline in the Bkz and Bz horizons; does not have an E horizon.
Cambeth (MT) - has a paralithic contact at a depth of 20 to 40 inches.
Lonna (MT) - does not have a horizon with gypsum and salt accumulation that is moist throughout the year when not frozen.
GEOGRAPHIC SETTING:
Landform - alluvial fans; sedimentary plains.
Elevation - 2,600 to 3,000 feet.
Slope- 2 to 8 percent.
Parent material - semiconsolidated shale.
Climate - cool with long, cold, dry winters; moist springs; warm summers.
Mean annual precipitation - 10 to 14 inches.
Mean annual temperature - 40 to 45 degrees F.
Frost-free period - 115 to 135.
DRAINAGE AND PERMEABILITY: Well drained; moderately slow permeability.
USE AND VEGETATION: Rahworth soils are used for rangeland. The potential native vegetation is western wheatgrass, thickspike wheatgrass, alkali sacaton, forbs, and shrubs.
DISTRIBUTION AND EXTENT: Southeastern Montana. Porcupine Dome area of Rosebud County. Rahworth soils are of small extent.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Denver, Colorado
SERIES ESTABLISHED: Rosebud County, Montana, 1985.
REMARKS: Soil interpretation record: MT0586. Diagnostic horizons and features recognized in this pedon are: an ochric epipedon from the soil surface to 7 inches (E, Bw1, Bw2 horizons); a cambic horizon from 2 to 11 inches (Bw1, Bw2 horizons); a particle-size control section from 10 to 40 inches (Bw2, Bk, Byz, Bz horizons); a BC horizon that is moist when not frozen. Rahworth soils have a frigid temperature regime and an ustic moisture regime bordering on aridic.
Classification was changed from Borollic Camborthids to Aridic Ustochrepts in 1993.
ADDITIONAL DATA: Laboratory sample S79MT-087-523.