LOCATION HOLCOMB OREstablished Series
TAXONOMIC CLASS: Fine, smectitic, mesic Typic Argialbolls
TYPICAL PEDON: Holcomb silt loam, cultivated. (Colors are for moist soil unless otherwise noted.)
Ap--0 to 6 inches; very dark brown (10YR 2/2) silt loam, grayish brown (10YR 5/2) dry; weak coarse subangular blocky structure parting to moderate fine granular; slightly hard, friable, slightly sticky and slightly plastic; many very fine roots; many very fine pores; moderately acid (pH 5.6); clear smooth boundary. (3 to 9 inches thick)
A--6 to 18 inches; very dark grayish brown (10YR 3/2) silt loam, grayish brown (10YR 5/2) dry; weak coarse subangular blocky structure parting to moderate fine subangular blocky; slightly hard, friable, moderately sticky and slightly plastic; many very fine roots; many very fine pores; few fine and medium black and reddish brown concretions; moderately acid (pH 5.8); clear smooth boundary. (5 to 15 inches thick)
E--18 to 24 inches; very dark grayish brown (10YR 3/2) light silty clay loam, light brownish gray (10YR 6/2) dry; moderate fine subangular blocky structure; hard, firm, moderately sticky and moderately plastic; common very fine roots; common very fine pores; common clean silt and sand grains on faces of peds; few fine and medium black and reddish brown concretions; common fine distinct yellowish brown (10YR 5/6) masses of iron accumulation; slightly acid (pH 6.2); abrupt smooth boundary. (3 to 15 inches thick)
2Btg--24 to 34 inches; dark grayish brown (10YR 4/2) clay, grayish brown (2.5Y 5/2) dry; weak coarse angular blocky structure parting to strong fine angular blocky; very hard, very firm, very sticky and very plastic; few very fine roots; few very fine pores; common faint clay films on faces of peds; common fine and medium very dark brown (10YR 2/2) concretions; ; common fine distinct yellowish brown (10YR 5/6) masses of iron accumulation; neutral (pH 6.6); clear smooth boundary. (8 to 36 inches thick)
2BCtg--34 to 50 inches; dark grayish brown (10YR 4/2) silty clay, grayish brown (10YR 5/2) dry; weak fine angular blocky structure; very hard, firm, very sticky and very plastic, common very fine pores; common faint clay films on faces of peds; common medium black concretions; neutral (pH 6.6); clear smooth boundary. (0 to 18 inches thick)
3C--50 to 60 inches; grayish brown (10YR 5/2) dry; silt loam; massive; hard, firm, moderately sticky and slightly plastic; few fine pores; neutral (pH 6.6).
TYPE LOCATION: Marion County, Oregon; about 2,700 feet east and 500 feet north of the SW corner of section 4, T. 9 S., R. 2 W. Willamette Meridian. Turner, Oregon USGS 7.5 topographic quadrangle. Latitude 44 degrees, 48 minutes, 40 seconds N., Longitude 122 degrees, 56 minutes, 15 seconds W. NAD 27.
RANGE IN CHARACTERISTICS: The soils are usually moist and are saturated with water during the winter and early spring. The mean annual soil temperature ranges from 52 to 55 degrees F. Depth to bedrock is 6 feet or more. Redoximorphic features commonly are lacking in the Ap horizon, none to distinct in the A horizons, and distinct or prominent in the E, 2Btg and 3C horizons. Black and reddish brown concretions commonly are in all horizons. Depth to the 2Btg horizon is 15 to 30 inches. Depth to aquic conditions with chroma of 2 or less, with or without redox concentrations is 12 to 20 inches.
The A horizon has value of 2 or 3 moist, 4 or 5 dry and chroma of 2 moist, 2 or 3 dry. It has weak or moderate granular or subangular blocky structure. Texture is silt loam or silty clay loam with 20 to 30 percent clay. It has 0 to 5 percent gravel. Reaction is slightly acid or moderately acid.
The E horizon has value of 3 or 4 moist, 5 to 7 dry and chroma of 2 moist and dry. Texture is silt loam or silty clay loam with 20 to 30 percent clay. It has weak or moderate subangular blocky structure. Reaction is slightly acid or moderately acid. In some pedons the E horizon is replaced in part by a BA horizon.
The 2Btg horizon has hue of 10YR to 5Y, value of 3 to 5 moist, 4 to 6 dry and chroma of 1 or 2 moist and dry. Texture is silty clay or clay with 40 to 50 percent clay. It has weak or strong subangular or angular blocky structure. Reaction is neutral or slightly acid. In some pedons it has weak or moderate prismatic secondary structure.
The 2BCt horizon, when present, has hue of 10YR to 5Y, value of 4 or 5 moist, 5 or 6 dry and chroma of 2 to 4 moist and dry. Texture is silty clay or clay with 40 to 50 percent clay. It has 0 to 15 percent gravel. Reaction is neutral or slightly acid.
The 3C horizon, when, present has hue of 10YR to 5Y, value of 4 or 5 moist, 5 or 6 dry chroma of 2 to 4 moist and dry. Texture is silt loam, silty clay loam, or clay loam with 25 to 40 percent clay. It has 0 to 15 percent gravel. Reaction is neutral or slightly acid.
COMPETING SERIES: These are the Chauncey, Plankinton, Rolfe, Sperry, and Willowfork series. The Chauncey soils lack a discontinuity, have a Bt horizon that is moderately acid to very strongly acid, and a particle-size control section with 35 to 42 percent clay. The Plankinton soils have carbonates at depths of 24 to 50 inches and a mollic epipedon greater than 24 inches thick. The Rolfe soils have carbonates at a depth of 42 to 80 inches, and a 2BCg or 2Cg horizon at a depth of 24 to 60 inches of glacial drift origin and having 18 to 35 percent clay. The Sperry soils have an A horizon with chroma of 1, have a Btg horizon that is moderately acid or strongly acid, and lack a discontinuity. The Willowfork soils have a Btg horizon that is moderately acid or strongly acid, has 10 percent chert gravel in the lower part of the solum, and has a mean annual soil temperature of more than 55 degrees F.
GEOGRAPHIC SETTING: The Holcomb soils are in nearly level to slightly convex areas on broad valley terraces. Elevations are 125 to 650 feet. The soils formed in silty alluvium over silty and clayey glaciolacustrine deposits. Slope is 0 to 3 percent. Summers are warm and dry and winters are cool and moist. The mean annual precipitation is 40 to 50 inches. The mean January temperature is 39 to 40 degrees F. and the mean July temperature is 65 to 67 degrees F. The mean annual temperature is 50 to 54 degrees F. The frost-free period is 165 to 210 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the Amity, Concord, Dayton, Willamette, and Woodburn soils. Amity soils are fine-silty, do not have an abrupt textured change and occur on adjacent terraces. Concord and Dayton soils have an ochric epipedon. In addition, Concord soils do not have an abrupt textural change. Concord and Dayton soils occur on lower terraces. Willamette and Woodburn soils are fine-silty. In addition, Willamette and Woodburn soils lack aquic conditions with chroma of 2 or less above 20 inches from the surface. They occur on broad valley terraces.
DRAINAGE AND PERMEABILITY: Somewhat poorly drained; slow runoff; very slow permeability. A perched water table is at its uppermost limit from November to April. This soil also has an apparent water table.
USE AND VEGETATION: Soils are under cultivation with small grains, hay, pasture, and grass seed as the principal crops. The vegetation is largely annual and perennial grasses, wild blackberry, wild rose, and oak.
DISTRIBUTION AND EXTENT: These soils are distributed on terraces in the Willamette Valley in Oregon; MLRA 2. Holcomb soils are moderately extensive.
MLRA OFFICE RESPONSIBLE: Portland, Oregon
SERIES ESTABLISHED: Clackamas County, Oregon, 1921.
REMARKS: Diagnostic horizons and features include:
Mollic epipedon - the zone from 0 to 18 inches (Ap and A horizons)
Albic horizon - the zone from 18 to 24 inches (E horizon)
Argillic horizon - the zone from 24 to 50 inches (2Btg and 2BCtg horizons)
Albolls feature - an albic horizon that has concretions or masses of iron accumulation.
Particle-size control section - the zone from 24 to 44 inches.
The Holcomb soils are found on the Senecal geomorphic surface in the Willamette Valley, Oregon.
ADDITIONAL DATA: Laboratory data available for this series. Sample #S96OR-043-002, NSSL, Lincoln, NE; 2/99.