Established Series


The Sobrante series consists of moderately deep, well drained soils that formed in material weathered from basic igneous and metamorphic rocks. These soils are on foothills and have slopes of 2 to 75 percent. The mean annual precipitation is about 32 inches and the mean annual temperature is about 60 degrees F.

TAXONOMIC CLASS: Fine-loamy, mixed, active, thermic Mollic Haploxeralfs

TYPICAL PEDON: Sobrante silt loam, rangeland. (Colors are for dry soil unless otherwise noted.)

A--0 to 5 inches; reddish brown (5YR 5/4) silt loam, dark reddish brown (5YR 3/4) moist; massive; slightly hard and hard, friable, slightly sticky and slightly plastic; many very fine roots; common very fine and medium tubular and interstitial pores; moderately acid (pH 6.0); clear smooth boundary. (4 to 9 inches thick)

Bt1--5 to 11 inches; yellowish red (5YR 4/6) silt loam, yellowish red (5YR 3/6) moist; weak medium subangular blocky structure; hard, friable, sticky and slightly plastic; common very fine roots; many very fine and fine tubular pores; few thin clay films in pores; slightly acid (pH 6.3); clear smooth boundary. (5 to 10 inches thick)

Bt2--11 to 24 inches; yellowish red (5YR 5/6) light clay loam, dark red (2.5YR 3/6) moist; moderate medium subangular blocky structure; hard, friable, sticky and slightly plastic; common very fine roots; common very fine, many fine and medium tubular pores; many thin clay films in pores and on peds; slightly acid (pH 6.3); clear wavy boundary. (11 to 18 inches thick)

Cr--24 to 30 inches; soft well-weathered basic schist, slightly acid (pH 6.5). (0 to 6 inches thick)

R--30 to 34 inches; hard basic schist with pockets of slightly weathered schist.

TYPE LOCATION: El Dorado County, California; 2.5 miles south of Shingle Springs, 200 feet west of French Creek Road, 200 feet south of the N 1/4 corner of sec. 19, T. 9 N., R. 10 E.

RANGE IN CHARACTERISTICS: Depth to a lithic contact ranges from 20 to 40 inches and is variable over short distances. The soils between depths of about 5 to 15 inches are usually moist but become dry in all parts in May or early June and remain dry until October to mid-November. Coarse fragments throughout the profile range from about 3 to 30 percent with the largest amount usually in the uppermost and lowermost horizons. The mean annual soil temperature is 59 degrees to 66 degrees F. The soil is slightly or moderately acid throughout.

The A horizon has dry colors of 7.5YR 5/2, 5/4, 5/6, 5/8, 4/4; 5YR 5/3, 5/4 and moist colors of 7.5YR 2/4, 3/2, 3/4; 5YR 3/3 or 3/4. Moist colors of 2/4, 3/2, and 3/3 are usually to a depth of 5 inches or less. It is silt loam, loam or gravelly loam with 10 to 25 percent clay. Organic matter ranges from 3 to 7 percent in the upper few inches and from 1.5 to 3 percent at 5 inches.

The Bt horizon has dry colors of 2.5YR 5/6, 5/8; 5YR 5/8, 5/6, 4/6, 4/4, 3/4, 3/6; or 7.5YR 5/6 and moist colors of 2.5YR 3/6, 4/6, 4/8; 5YR 3/4, 4/4, 3/6; or 7.5YR 3/4. It is loam, silt loam, clay loam, silty clay loam, gravelly loam or gravelly clay loam with 25 to 35 percent clay and with a 4 to 15 percent (absolute) increase over the A horizon. Base saturation ranges from 70 to 90 percent.

Some pedons lack a Cr horizon above the lithic contact.

COMPETING SERIES: These are the Academy, Burchell, Cajalco, Coarsegold, Honn, Jacinto, Modesto, Olashes, Perkins, Pleasanton, Rescue, Sobay, Trimmer and Whitney series. Academy, Burchell, Coarsegold, Honn, Jacinto, Modesto, Perkins, Pleasanton and Rescue and Sobay soils lack a lithic contact above depth of 40 inches. Cajalco, Trimmer and Whitney soils have a paralithic rather than a lithic contact.

GEOGRAPHIC SETTING: The Sobrante soils are on foothills and have slopes of 2 to 75 percent. Elevation ranges from 125 to 3,500 feet. They formed in material weathered from basic igneous and metamorphic rocks, mainly amphibolite schist, diabase, andesite, or basalt. Rock outcrops are common. The climate is moist subhumid with warm dry summers and cool moist winters. Mean annual precipitation is 15 to 50 inches. Mean annual temperature is about 56 to 60 degrees F; average January temperature about 45 degrees F; and average July temperature about 76 degrees F. Frost-free season is about 200 to 270 days, but may be as few as 140 in Lake Co.

GEOGRAPHICALLY ASSOCIATED SOILS: These are the Argonaut, Auburn and Boomer soils and the competing Rescue soils. Argonaut soils have more than 35 percent clay in the argillic horizon. Auburn soils have a lithic contact within a depth of 20 inches in part of each pedon. Boomer soils are in cooler sites with a mean soil temperature of less than 59 degrees F.

DRAINAGE AND PERMEABILITY: Well drained; low to very high runoff; moderate permeability.

USE AND VEGETATION: Used mainly for range, some areas are used for irrigated hay and pasture and dry land crops. The native vegetation is oak-grass and forbs with some scattered perennial grasses.

DISTRIBUTION AND EXTENT: Coast Range mountains and foothills of the Sierra Nevada Range in California. The series is of moderate extent.


SERIES ESTABLISHED: Contra Costa County, California, 1935.

REMARKS: The activity class was added to the classification in January of 2003. Competing series were not checked at that time. - ET

National Cooperative Soil Survey