LOCATION THREEFORKS OREstablished Series
TAXONOMIC CLASS: Fine-loamy, isotic, mesic Humic Dystrudepts
TYPICAL PEDON: Threeforks loam-forested, on a southwest-facing slope of 56 percent at an elevation of 3000 feet. (Colors are for moist soil unless otherwise stated.)
Oi--1 inch to 0; slightly decomposed needles, leaves, and twigs.
A--0 to 3 inches; very dark grayish brown (10YR 3/2) loam, grayish brown (10YR 5/2) dry; weak fine and medium granular structure; slightly hard, firm, sticky and slightly plastic, common very fine, fine, medium and coarse roots; few very fine tubular and irregular pores; 10 percent gravel; moderately acid (pH 5.8); clear smooth boundary.
BA--3 to 11 inches; very dark grayish brown (10YR 3/2) loam, brown (10YR 5/3) dry; weak very fine, fine and medium subangular blocky structure; hard, firm, sticky and slightly plastic; common very fine, fine, medium and coarse roots; few very fine tubular and irregular pores; 5 percent gravel; moderately acid (pH 5.6); clear wavy boundary. (combined thickness of the A and BA horizons is 10 to 20 inches thick)
Bw1--11 to 20 inches; very dark grayish brown (10YR 3/2) loam, brown (10YR 5/3) dry; weak fine and medium subangular blocky structure; hard, firm, sticky and slightly plastic; common very fine, fine, medium and coarse roots; common very fine and fine tubular and irregular pores; 5 percent gravel; moderately acid (pH 5.6); clear wavy boundary.
Bw2--20 to 33 inches; dark brown (10YR 3/3) loam, brown (10YR 5/3) dry; weak fine and medium subangular blocky structure; hard, firm, sticky and slightly plastic; common very fine and fine, and few medium and coarse roots; common very fine and fine tubular and irregular pores; 10 percent gravel; moderately acid (pH 5.6); clear wavy boundary.
Bw3--33 to 49 inches; dark brown (10YR 3/3) gravelly clay loam, brown (10YR 5/3) dry; weak medium and coarse subangular blocky structure; hard, firm, sticky and plastic, few very fine and fine roots; common very fine and fine tubular and irregular pores; few distinct clay films on faces of peds and in pores; 30 percent gravel; moderately acid (pH 5.6); clear wavy boundary. (Combined thickness of the Bw horizon is 30 to 50 inches)
BCt--49 to 60 inches; dark brown (10YR 3/3) very gravelly clay loam, brown (10YR 5/3) dry; weak medium and coarse subangular blocky structure; hard, firm, sticky and plastic; few very fine and fine roots; common very fine and fine tubular and irregular pores; few distinct clay films on faces of peds and in pores; 45 percent gravel and 10 percent cobbles; strongly acid (pH 5.4).
TYPE LOCATION: Douglas County, Oregon, 1 1/3 miles north-northwest of Tiller, Oregon; 1365 feet west and 2100 feet north of the southeast corner of section 17, T. 30 S., R. 2 W., W.M. (Latitude 42 degrees, 58 minutes, 1 second N, Longitude 122 degrees, 57 minutes, 58 seconds W.)
RANGE IN CHARACTERISTICS: Solum thickness is greater than 60 inches. The mean annual soil temperature is 49 to 56 degrees F. The soil is usually moist but is dry within all parts of the moist control section for less than 45 consecutive days following the summer solstice. The particle-size control section averages 18 to 35 percent clay and 10 to 25 percent rock fragments. The umbric epipedon is 10 to 20 inches thick. Mollic colors extend to a depth of 60 inches, colors are litho-chromatic and the organic matter content is less than 1 percent below the umbric epipedon.
The A and BA horizon has value of 2 or 3 moist, 4 or 5 dry, and chroma of 1 to 3 moist and 2 or 3 dry. It has 0 to 15 percent gravel.
The Bw horizon has hue of 7.5YR, 10YR, or 2.5Y, value of 3 or 4 moist, 5 or 6 dry, and chroma of 2 to 4 moist and dry. It is loam, clay loam, gravelly loam, or gravelly clay loam with 15 to 30 percent clay increasing with depth. It has 0 to 30 percent gravel. It is moderately acid to very strongly acid.
The BC horizon has hue of 7.5YR, 10YR, or 2.5Y, value of 3 or 4 moist, 5 or 6 dry, and chroma of 3 or 4 moist and dry. It is gravelly clay loam, gravelly loam or very gravelly clay loam with 25 to 35 percent clay. It has 20 to 55 percent gravel and 0 to 10 percent cobbles. It is moderately acid to very strongly acid.
COMPETING SERIES: These are the Barbourville, Crutchfield (T), Colepoint (T), Cutshin, Horeb, Meda, Molalla, Stevenson, and Tuckasegee series. Barbourville soils have a mean annual soil temperature of 54 degrees F or more. Colepoint soils are 40 to 60 inches deep to bedrock, have formed over metavolcanic and metasedimentary bedrock and typically have high chroma and value in the lower part of the subsoil. Crutchfield soils are 20 to 40 inches deep to bedrock. Cutshin soils have channery or flaggy textures. Horeb soils are moderately well drained with mottles having chroma of 2 or less at 24 to 36 inches. Meda soils are on fans and have a solum thickness of 20 to 40 inches. Molalla soils are 40 to 60 inches deep to bedrock, and have hue of 5YR or redder. Stevenson soils have chroma of 4 to 6 moist and dry in the subsoil and substratum and contain 15 to 35 percent soft fragments in the particle-size control section. Tuckasegee soils have hot moist summers, have formed over granite and/or gneiss and the solum is typically fine sandy loam, sandy loam, sandy clay loam, or loam.
GEOGRAPHIC SETTING: Threeforks soils are on mountain side slopes and ridges. They formed in colluvium and residuum derived from mica schist. Slopes range from 30 to 90 percent. Elevation ranges from 1800 to 3000 feet. The climate is characterized by warm, wet winters and hot, moist summers. The mean annual temperature is 45 to 52 degrees F, and the mean annual precipitation is 45 to 60 inches. The frost free period is 100 to 170 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the Beal, Atring, Kanid, Lettia, Sweetbriar, and Zing soils. Beal, Lettia, Sweetbriar and Zing soils all have argillic horizons. Atring and Kanid soils have a loamy-skeletal particle-size control section. In addition, all these soils have a xeric soil moisture regime and occur on adjacent south facing slopes.
DRAINAGE AND PERMEABILITY: Well drained; moderate permeability.
USE AND VEGETATION: These soils are used for timber production, wildlife habitat and watershed. The native vegetation includes Douglas fir, grand fir, western hemlock and bigleaf maple. The understory consists mainly of western swordfern, red huckleberry, western rattlesnake plantain, Pacific rhododendron, white insideout flower, and salal.
DISTRIBUTION AND EXTENT: Southwestern Oregon, MLRA 5. The series is of small extent.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Davis, California
SERIES ESTABLISHED: Douglas County, Oregon, 1994. The name is coined.
REMARKS: Diagnostic horizons and features recognized in this pedon are:
Umbric epipedon - from the surface of the soil to 11 inches. (A and BA horizons)
Cambic horizon - from 11 to 49 inches. (Bw1, Bw2, and Bw3 horizons)
Particle-size control section - from 10 to 40 inches.
Base saturation is assumed to be less than 50 percent in the upper 10 inches.
This soil typically has a uniform sand size (fine and very fine sand) with a high mica content (although not high enough to have a micaceous mineralogy family classification). The soil exhibits weak to moderate dilatency.