LOCATION SIWASH AK
Established Series
Rev. MHC/JPM/DM
02/2022
SIWASH SERIES
The Siwash series consists of shallow, well drained soils formed in a thin mantle of ash influenced loess overlying friable to firm glacial till underlain by igneous or metamorphic bedrock. Siwash soils are on hummocks on mountain crests and backslopes. Slopes range from 0 to 70 percent. The mean annual precipitation is about 35 inches and the mean annual temperature is about 33 degrees F.
TAXONOMIC CLASS: Medial over loamy-skeletal, amorphic over mixed, superactive Lithic Humicryods
TYPICAL PEDON: Siwash silt loam - on a hummocky north facing slope of 4 percent at 2200 feet elevation under bog blueberry and crowberry shrub vegetation. (All colors are for moist soil)
Oi-0 to 4 inches; dark reddish brown (2.5YR 2.5/3) slightly decomposed plant material; very strongly acid (pH 4.6); abrupt smooth boundary (3 to 9 inches thick)
E--4 to 5 inch; light grayish brown (10YR 5/2) silt loam with occasional patches of very dusky red (2.5YR 2.5/2) Bhs material; weak fine subangular blocky and weak fine platy structure; very friable, nonsticky and nonplastic; common very fine, fine and medium and few coarse roots; extremely acid (pH 4.0); abrupt broken boundary. (1 to 3 inches thick)
Bhs--5 to 9 inches; very dusky red (2.5YR 2.5/2) very fine sandy loam with occasional patches of reddish brown (5YR 4/4); moderate medium subangular blocky structure parting to strong very fine granular; friable with firm lenses and pockets, nonsticky and nonplastic; common very fine and fine roots; extremely acid (pH 4.0); abrupt irregular boundary. (3 to 6 inches thick)
Bs/Eb--9 to 13 inches; 75 percent yellowish red (5YR 4/6) and 25 percent brown (7.5YR 5/4) silt loam; moderate medium subangular blocky structure parting to strong very fine granular; friable with firm lenses and pockets, nonsticky and nonplastic; few very fine and fine roots; extremely acid (pH 4.2); abrupt wavy boundary. (4 to 8 inches thick)
Bhsb--13 to 15 inches; very dusky red (2.5YR 2.5/2) sandy loam; weak medium subangular blocky structure; friable with very firm pockets and lenses, nonsticky and nonplastic; very strongly acid (pH 4.8); clear wavy boundary. (0 to 5 inches thick)
2BC--15 to 21 inches; dark yellowish brown (10YR 3/4) very gravelly sandy loam; weak coarse subangular blocky structure; friable, nonsticky and nonplastic; 35 percent subrounded and angular gravel and 15 percent angular cobble; moderately acid (pH 5.8); abrupt wavy boundary. (0 to 6 inches thick)
3R--21 inches; hard granite bedrock.
TYPE LOCATION: Matanuska-Susitna Valley Area, Alaska; about 10 miles north of Talkeetna, Alaska; in the SE 1/4 of the SW 1/4, section 5, T. 28N., R. 3W., Seward Meridian.
RANGE IN CHARACTERISTICS: Depth to very gravelly and very cobbly till material ranges from 12 to 18 inches. Depth to bedrock ranges from 13 to 20 inches below mineral surface. The silty solum ranges in thickness from 8 to 14 inches and maybe bisequal. The upper ten centimeters of the spodic horizon has more than 6 percent organic carbon. The upper part of the control section formed in loess dominated by amorphous material. Horizon topography in the upper part is irregular or broken due to cryoturbation. Reaction throughout the profile ranges from extremely acid to strongly acid.
The A horizon (when present) has hue of 5YR to 10YR, value of 2 or 3, and chroma of 1 to 3.
The E and Eb horizon has hue of 5YR to 10YR, value of 4 to 6, and chroma of 2 or 3.
The Bhs horizon has hue of 10R to 5YR, value of 2 or 3, and chroma of 2 or 3. Texture is silt loam, fine sandy loam, sandy loam and loam.
The Bs/Eb horizon has hue of 2.5YR to 7.5YR, value of 3 to 5 and chroma of 4 to 8. The Eb portion has hue of 5YR to 10YR, value of 4 or 5 and chroma of 2 to 4. Texture is silt loam, fine sandy loam and sandy loam.
The 2BC horizon has hue of 10YR or 2.5Y, value of 3 or 4 and chroma of 4 to 6. Texture is loam or sandy loam, modified by 35 to 50 percent coarse fragments; 30 to 45 percent gravel, and 5 to 25 percent cobbles.
COMPETING SERIES: This is the
Deneka series. Deneka soils are not churned by cryoturbation.
GEOGRAPHIC SETTING: The Siwash soils occur on hummocks on mountain crest and backslopes above tree line and have horizons that are broken or irregular due to cryoturbation. Slopes range from 0 to 70 percent. The soils formed in loess, mixed with ash, overlying friable to firm glacial till and underlain by igneous or metamorphic bedrock. Mean annual temperature is about 34 degrees F. Mean annual precipitation is about 30 to 50 inches.
GEOGRAPHICALLY ASSOCIATED SOILS:
DRAINAGE AND PERMEABILITY: Well drained. Low to very high runoff. Moderate permeability in the loess mantle and till.
USE AND VEGETATION: Wildlife habitat and recreation. Vegetation consists of bog blueberry and crowberry shrub.
DISTRIBUTION AND EXTENT: Southcentral Alaska Mountains. The series is of moderate extent.
SOIL SURVEY REGIONAL OFFICE (SSRO) RESPONSIBLE: WASILLA, ALASKA
SERIES ESTABLISHED: Matanuska-Susitna Valley Area, Alaska, 1996.
REMARKS: Diagnostic features and horizons recognized in this profile include: albic horizons from 4 to 5 inches and intermittently from 9 to 13 inches; spodic horizon from 5 to 15 inches; more than 6 percent organic carbon in the upper 4 inches of the spodic; amorphous material dominant from 4 to 15 inches; consolidated bedrock at 17 inches below the mineral surface.
This soil was originally defined as shallow to bedrock; the O horizon was 4 to 0 inches, and bedrock at 17 inches. When soil depth was defined as starting at the top of O horizons (when present) that changed the depth of the soil to moderately deep. This changes the description class but not the Taxonomic Class. Eight Edition of Soil Taxonomy defines Lithic subgroups in this classification starting from the top of the mineral soil surface (page 708).
National Cooperative Soil Survey
U.S.A.