LOCATION ONAHU COEstablished Series
TAXONOMIC CLASS: Loamy-skeletal, micaceous, acid Aeric Humic Cryaquepts
TYPICAL PEDON: Onahu loam, on a north facing, convex, 23 percent slope in sedges and grasses at an elevation of 11,230 feet. (Colors are for dry soil unless otherwise noted.) When described on August 20, 1997 the soil was moist from 0 to 45 inches. The surface is covered by 1 percent cobbles and 2 percent stones.
A1--0 to 7 inches; brown (7.5YR 4/2) loam, very dark brown (7.5YR 2/2) moist; weak fine granular structure; soft, very friable, slightly sticky and slightly plastic; many very fine and fine roots; very strongly acid (pH 4.5); clear smooth boundary.
A2--7 to 16 inches; brown (7.5YR 4/3) loam, dark brown (7.5YR 3/2) moist; weak medium subangular blocky structure; slightly hard, very friable, slightly sticky and slightly plastic; common very fine and fine roots; 10 percent gravel; very strongly acid (pH 4.5); abrupt smooth boundary. (Combined thickness of the A horizons is 12 to 20 inches)
2Bg--16 to 24 inches; light brownish gray (10YR 6/2) very gravelly sandy loam, dark grayish brown (10YR 4/2) moist; weak medium subangular blocky structure; soft, very friable, slightly sticky and slightly plastic; common very fine and fine roots; common medium dark yellowish brown (10YR 4/6) masses of iron concentrations throughout and common medium light gray (10YR 7/1) iron depletions throughout; 40 percent gravel; very strongly acid (pH 4.6); clear smooth boundary. (8 to 14 inches thick)
2Cg--24 to 45 inches; light gray (10YR 7/1) very gravelly sandy loam, dark gray (10YR 4/1) moist; massive; soft, very friable, slightly sticky and slightly plastic; common fine dark yellowish brown (10YR 4/6) masses of iron concentrations throughout; 50 percent gravel and 5 percent cobbles; very strongly acid (pH 4.6). (20 to 26 inches thick)
3Cr--45 to 55 inches; soft weathered gneiss and schist.
TYPE LOCATION: Larimer County, Colorado; about 2.2 miles southeast of Milner Pass in Rocky Mountain National Park; Fall River USGS quad; lat. 40 degrees 24 minutes 24 seconds N. and long. 105 degrees 46 minutes 31 seconds W., NAD 1927
RANGE IN CHARACTERISTICS:
Soil moisture: aquic moisture regime.
Mean annual soil temperature: 33 to 35 degrees F
Mean summer soil temperature: 36 to 40 degrees F
Depth to paralithic contact: 40 to 60 inches to granitic rocks, gneiss, and schist
Depth to redox concentrations: 7 to 18 inches
Depth to redox depletions: 15 to 24 inches
Depth to episaturation: 6 to 18 inches from June to August
Thickness of the umbric epipedon: 12 to 24 inches
Particle-size control section (weighted average):
Clay content: 8 to 18 percent
Sand content: 45 to 60 percent
Rock fragments: 35 to 45 percent, dominantly gravel and cobbles.
Base saturation: 15 to 45 percent.
Mica content: 35 to 60 percent (by grain count)
A1 horizon:
Hue: 7.5YR or 10YR
Value: 3 or 4 dry, 2 or 3 moist
Chroma: 1 through 3
Clay content: 10 to 18 percent
Rock fragments: 0 to 15 percent
Base saturation: 15 to 35 percent
Reaction: very strongly acid or strongly acid
A2 horizon:
Hue: 7.5YR or 10YR
Value: 3 or 4 dry, 2 or 3 moist
Chroma: 1 through 3
Texture: loam, sandy loam, fine sandy loam
Clay content: 10 to 18 percent
Rock fragments: 0 to 25 percent
Base saturation: 15 to 45 percent
Reaction: very strongly acid or strongly acid
2Bg horizon:
Hue: 7.5YR or 10YR
Value: 4 through 6 dry, 3 through 5 moist
Chroma: 1 or 2
Texture: very gravelly sandy loam, very gravelly fine sandy loam
Clay content: 10 to 18 percent
Rock fragments: 35 to 60 percent
Base saturation: 25 to 45 percent
Reaction: very strongly acid or strongly acid
2Cg horizon:
Hue: 7.5YR through 2.5Y
Value: 5 through 7 dry, 4 through 6 moist
Chroma: 1 or 2
Texture: very gravelly sandy loam, very gravelly coarse sandy loam
Clay content: 8 to 18 percent
Rock fragments: 35 to 60 percent
Base saturation: 25 to 45 percent
Reaction: very strongly acid or strongly acid
COMPETING SERIES: There are presently no competing series this family. The similar
Melton(ID) series is in the fine-loamy over sandy or sandy-skeletal particle-size family.
Melton soils have cobbly till materials with textures of loamy sand and coarser at depths below 24 inches.
GEOGRAPHIC SETTING:
Parent material: Alluvium over till derived from granitic rocks, gneiss, and schist
Landform: mountain slopes and cirque basins
Slopes: 2 to 25 percent
Elevation: 11,000 to 12,500 feet
Mean annual temperature: 32 to 38 degrees F
Mean annual precipitation: 30 to 40 inches
Precipitation pattern:
wettest months: March through August
driest months: October and November
Frost-free period: 10 to 30 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the
Archrock,
Mummy, and
Trailridge series.
Archrock soils have paralithic contact at depths of 20 to 40 inches and are on mountain tops.
Mummy soils do not have aquic conditions and are on mountain flanks.
Trailridge soils have paralithic contact at depths of 10 to 20 inches and are on mountain tops.
DRAINAGE AND PERMEABILITY: Poorly drained, slow to moderate runoff, moderate permeability.
USE AND VEGETATION: Recreation and wildlife habitat. Native vegetation is mainly tufted hairgrass, water sedge, kobresia, purple reedgrass, marshmarigold and planeleaf willow.
DISTRIBUTION AND EXTENT: Northcentral Rocky Mountains, Colorado; LRR E, MLRA 48A; small extent.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Bozeman, Montana.
SERIES ESTABLISHED: Larimer County, Colorado, Rocky Mountain National Park soil survey area, 1999. The name was taken from a prominent drainage in the area. Pronounced ON-uh-hoo.
REMARKS:
Diagnostic horizons and features recognized in this pedon are:
Umbric epipedon: The zone from 0 to 16 inches. (A1 and A2 horizons)
Particle-size control section: The zone from 10 to 40 inches. (parts of the A2, 2Bg and parts of the 2Cg horizons)
Redoximorphic concentrations and depletions: In the zone from 16 to 45 inches. (2Bg and 2Cg horizons)
Episaturation: The zone of saturation at 16 to 45 inches, inferred from the low chroma of the soil matrix. (2Bg and 2Cg horizons). The zone of saturation is perched on the paralithic contact.
Paralithic contact: The contact with gneiss and schist at 45 inches. (3Cr layer)
Taxonomic Version: Eighth Edition
The mineralogy class was changed from paramicaceous to micaceous in 07/2010 by the National Soil Survey Center on request of the responsible MLRA regional office. The change was necessary based on the eleventh edition of the Keys to Soil Taxonomy, 2010.