LOCATION POSO ID

Established Series
Rev. KA/MD
09/2018

POSO SERIES

The Poso series consists of soils that are very deep and well drained, they formed in residuum and slope alluvium derived dominantly from calcareous sandstone conglomerate. Poso soils are on nearly level to extremely steep, dissected alluvial fan terrace ridges. Permeability is moderately rapid and slopes range from 1 to 80 percent. The average annual precipitation is about 18 inches, and the average annual air temperature is about 37 degrees F.

TAXONOMIC CLASS: Loamy-skeletal, mixed, superactive Calcic Haplocryepts

TYPICAL PEDON: Poso extremely gravelly loam - on a south southeast-facing convex slope of 27 percent, under sagebrush and grass at 6,640 feet elevation. (When described on October 21, 1981, the soil was moist to 10 inches. Colors are for dry soil unless otherwise noted.)

A1--0 to 2 inches; brown (10YR 5/3) extremely gravelly loam, very dark grayish brown (10YR 3/2) moist; weak fine granular structure; soft, very friable, slightly sticky and slightly plastic; common very fine and few fine roots; common very fine tubular pores; slight effervescence; 50 percent gravel and 10 percent cobbles; slightly alkaline (pH 7.8); clear smooth boundary. (1 to 3 inches thick)

A2--2 to 4 inches; brown (10YR 4/3) very gravelly loam, very dark grayish brown (10YR 3/2) moist; moderate medium and fine subangular blocky structure; soft, very friable, slightly sticky and slightly plastic; common very fine, fine and few medium roots; few very fine tubular pores; slight effervescence; 25 percent gravel and 10 percent cobbles; slightly alkaline (pH 7.8); clear wavy boundary. (2 to 4 inches thick)

Bk1--4 to 12 inches; pale brown (10YR 6/3) very gravelly loam, brown (10YR 4/3) moist; weak fine subangular blocky structure; soft, very friable; slightly sticky and slightly plastic; common very fine, fine and few medium roots; few very fine tubular pores; violent effervescence; 40 percent gravel and 10 percent cobbles; slightly alkaline (pH 7.8); clear wavy boundary. (8 to 10 inches thick)

Bk2--12 to 32 inches; variegated, very pale brown (10YR 7/3) extremely gravelly sandy loam, yellowish brown (10YR 5/4) moist; massive; soft, very friable, slightly sticky and slightly plastic; few very fine and fine roots; violent effervescence; 50 percent gravel and 15 percent cobbles; moderately alkaline (pH 8.0); clear wavy boundary. (20 to 40 inches thick)

C1--32 to 61 inches; light yellowish brown (10YR 6/4) extremely gravelly sandy loam, yellowish brown (10YR 5/4) moist; massive; soft, very friable, slightly sticky and slightly plastic; few very fine roots; violent effervescence; 60 percent gravel and 20 percent cobbles; moderately alkaline (pH 8.0).

TYPE LOCATION: Clark County, Idaho; about 13 miles west and 11 miles north of Small; 750 feet east and 950 feet south of the northwest corner, section. 18, T. 12 N., R. 33 E.

RANGE IN CHARACTERISTICS:

Textural control section (averages)
Clay content - 17 to 24 percent
Coarse fragments - 48 to 68 percent
Profile ranges
Depth to carbonates - 0 to 2 inches
Average annual soil temperature - 37 to 43 degrees F.

A horizon
Color value, dry - 4 through 6
Value, moist - 3 or 4
Chroma, dry and moist 1 through 3

B horizon
Color value, dry - 6 or 7
Value, moist - 4 through 6
Chroma, dry and moist - 2 through 4
Texture - very gravelly loam, extremely gravelly loam, very gravelly sandy loam or extremely gravelly sandy loam
Clay content - 10 to 20 percent

C horizon
Value, dry - 6 or 7
Value, moist - 5 or 6
Chroma, dry and moist - 3 or 4
Texture - very gravelly sandy loam or extremely gravelly sandy loam Clay content - 10 to 18 percent

COMPETING SERIES: These are the Blackleed, Devore, Elve, Firada, Garlet, Ketchum (T), Losee, Nambe, Scout, Taglake, and Targhee (T) series. The Blackleed soils are slightly to strongly acid, are noncalcareous throughout, and receive 35 to 50 inches precipitation. Devore soils have granitic bedrock at less than 40 inches, are noncalcareous throughout, and receive 60 to 70 inches of precipitation. Elve soils are noncalcareous, range from slightly to strongly acid, and have E horizons. Firada soils have limestone bedrock at less than 40 inches and an E at the surface. Garlet soils have O&E and B&A horizons and depth to carbonates is greater than 40 inches. Ketchum soils have base saturation of 60 to 75 in some subhorizons above 75 cm, an A, E, A&B, B&A, C horizon sequence and are noncalcareous throughout. Losee soils have 7.5YR or redder hues in the control section. Nambe soils are extremely to strongly acid, have an A, E, Bir, C horizon sequence, have 38 to 50 inches of precipitation, and have an average annual soil of temperature of about 33 degrees F. Scout soils receive 25 to 40 inches precipitation and have an A1, E, B horizon sequence. Taglake soils have control section textures of cobbly sandy loam only and have an E horizon and lack Ck horizons. Targhee soils receive 30 to 35 inches of precipitation, are less than 40 inches deep to rhyolite tuff bedrock and lack carbonates.

GEOGRAPHIC SETTING: Poso soils are on nearly level to extremely steep, dissected alluvial fan terrace ridges at 6,000 to 8,400 feet elevation. Slopes range from 1 to 80 percent. Poso soils formed in residuum and slope alluvium derived dominantly from calcareous sandstone conglomerate. The average annual precipitation is 15 to 20 inches, the average annual air temperature is 33 to 39 degrees F., the average summer temperature is 52 to 57 degrees F., and the frost-free season is from 45 to 65 days.

GEOGRAPHICALLY ASSOCIATED SOILS: These are the Deadhorse (T), Swampydraw (T), Windicreek (T), and Zeebar (T) soils. The Deadhorse soils are moderately deep to a duripan and found on nearly level alluvial terraces. The Swampydraw sols have a mollic epipedon that is 9 to 16 inches thick, and an argillic horizon with 21 to 34 percent clay, and are found on alluvial fan terrace sideslopes. Windicreek soils have a mollic epipedon that is 9 to 13 inches thick and contains less than 18 percent clay in the control section, and are dominantly on concave ridge slopes. Zeebar soils are neutral throughout the profile and contains greater than 18 percent clay in the control section.

DRAINAGE AND PERMEABILITY: Well drained; slow to rapid runoff; and moderately rapid permeability.

USE AND VEGETATION: Used mainly for range and wildlife habitat. Vegetation is mainly fringed sagebrush, low sagebrush, hood's phlox, and bluebunch wheatgrass.

DISTRIBUTION AND EXTENT: Poso soils are of small extent in southeastern Idaho.

MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Portland, Oregon

SERIES ESTABLISHED: Clark County, Idaho 2018

National Cooperative Soil Survey
U.S.A.