LOCATION ARROD MTEstablished Series
TAXONOMIC CLASS: Loamy-skeletal, carbonatic, frigid, shallow Petrocalcic Calciustolls
TYPICAL PEDON: Arrod cobbly clay loam, in an area of Arrod-Crago complex, 0 to 4 percent slopes, in rangeland (colors are for dry soil unless otherwise noted).
A1--0 to 4 inches; grayish brown (10YR 5/2) cobbly clay loam, very dark grayish brown (10YR 3/2) moist; moderate fine granular structure; hard, friable, slightly sticky and slightly plastic; many fine roots; many fine pores; 10 percent pebbles and 10 percent cobbles; slightly effervescent; neutral (pH 7.3); abrupt smooth boundary.
A2--4 to 9 inches; grayish brown (10YR 5/2) clay loam, very dark grayish brown (10YR 3/2) moist; moderate fine granular structure; hard, friable, slightly sticky and slightly plastic; many fine roots; many fine pores; 5 percent pebbles; strongly effervescent; slightly alkaline (pH 7.4); clear irregular boundary. (Combined thickness of the A horizons is 8 to 12 inches.)
2Bk--9 to 15 inches; pale brown (10YR 6/3) extremely gravelly clay loam, grayish brown (10YR 5/2) moist; massive; very hard, slightly sticky and slightly plastic; common fine and very fine roots and pores; 65 percent pebbles and 15 percent cobbles; disseminated lime and many prominent lime crusts on surface of coarse fragments, violently effervescent; slightly alkaline (pH 7.6); abrupt wavy boundary. (2 to 10 inches thick)
3Bkm--15 to 24 inches; pale brown (10YR 6/3) calcium-carbonate cemented-rock fragments; grayish brown (10YR 5/2) moist; extremely hard, extremely firm; few fine roots following fractures; violently effervescent; slightly alkaline (pH 7.6); abrupt wavy boundary. (4 to 12 inches thick)
4Bk--24 to 60 inches; very pale brown (10YR 8/3) extremely gravelly loamy sand, pale brown (10YR 6/3) moist; single grain; loose, nonsticky and nonplastic; about 85 percent pebbles and 5 percent cobbles; disseminated lime and many prominent lime crusts on surface of coarse fragments, violently effervescent; slightly alkaline (pH 7.6).
TYPE LOCATION: Fergus County, Montana; 2,400 feet south and 1,000 feet west of the NE corner of sec. 35, T. 12 N., R. 23 E.
RANGE IN CHARACTERISTICS:
Soil temperature - 44 to 47 degrees F.
Moisture control section - between 4 and 12 inches.
Mollic epipedon thickness - 7 to 10 inches.
Depth to petrocalcic horizon - 10 to 20 inches.
Control section - 18 to 35 percent clay.
Soil phases - cobbly and gravelly.
A1 horizon - Hue: 10YR, 2.5Y
Value: 4 or 5 dry, 2 or 3 moist
Chroma: 2 or 3
Texture: loam or clay loam
Clay content: 18 to 35 percent
Rock fragments: 0 to 35 percent--0 to 20 percent stones and cobbles, 0 to 15 percent pebbles
Reaction: pH 6.6 to 7.8
A2 horizon - Hue: 10YR or 2.5Y
Value: 4 or 5 dry; 2, 3, or 4 moist
Chroma: 2 or 3
Texture: loam or clay loam
Clay content: 18 to 35 percent
Rock fragments: 0 to 45 percent--0 to 15 percent stones and cobbles, 0 to 45 percent pebbles
Reaction: pH 6.6 to 7.8
2Bk horizon - Hue: 10YR or 2.5Y
Value: 6, 7, or 8 dry; 4, 5, 6, or 7 moist
Chroma: 2, 3, or 4
Texture: loam or clay loam
Clay content: 18 to 35 percent
Rock fragments: 35 to 90 percent--0 to 15 percent cobbles and stones, 35 to 75 percent pebbles
Calcium carbonate equivalent: 30 to 60 percent
Reaction: pH 7.4 to 8.4
3Bkm horizon - hard massive layer of calcium carbonate cemented rock fragments which is 4 to 12 inches thick
Reaction: 7.4 to 8.4
4Bk horizon - Hue: 10YR or 2.5Y
Value: 6, 7, or 8 dry; 5, 6, or 7 moist
Chroma: 2, 3, or 4
Texture: sand, loamy sand, sandy loam
Clay content: 0 to 10 percent
Rock fragments: 60 to 90 percent--0 to 20 percent stones and cobbles, 60 to 75 percent pebbles
Calcium carbonate equivalent: 25 to 50 percent
Reaction: pH 7.4 to 8.4
COMPETING SERIES: This is the Baconpark series and the similar Ashuelot series series. Baconpark soils have a hue of 7.5YR The similar Ashuelot series is non-skeletal above the petrocalcic horizon.
GEOGRAPHIC SETTING: The Arrod series are on stream terraces and dissected alluvial fans. Elevations are 3,300 to 4,600 feet. Slopes are 0 to 4 percent. These soils formed in calcareous gravelly alluvium. The climate is characterized by long, cold winters; moist springs; and short, dry summers. Mean annual precipitation is 12 to 16 inches, the mean annual temperature is 42 to 45 degrees F., and the frost-free season is 105 to 125 days.
DRAINAGE AND PERMEABILITY: Well drained; slow runoff; moderate permeability to the petrocalcic horizon.
USE AND VEGETATION: Arrod soils are used mainly for rangeland. Small areas are used for cropland and hayland where associated with more productive soils. The potential plant community is mainly green needlegrass, western wheatgrass, prairie junegrass, bluebunch wheatgrass, and some forbs.
DISTRIBUTION AND EXTENT: The Arrod series is moderately extensive in central Montana.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Bozeman, Montana
SERIES ESTABLISHED: Choteau-Conrad Area, parts of Teton and Pondera Counties, Montana, 1991; proposed in Fergus County, Montana, 1977.
REMARKS: Soil interpretations record: MT1180. Diagnostic horizons and features recognized in this pedon are: a mollic epipedon from the soil surface to 9 inches (A1 and A2 horizons); an extremely gravelly clay loam calcic horizon from 9 to 15 inches (2Bk horizon); a petrocalcic horizon from 15 to 24 inches (3Bkm horizon); a particle-size control section from 10 inches to the petrocalcic horizon (2Bk horizon). Arrod soils have an ustic moisture regime and a frigid temperature regime.