Established Series


The Los Osos series consists of moderately deep, well drained soils that formed in material weathered from sandstone and shale. Los Osos soils are on uplands and have slopes of 5 to 75 percent. The mean annual precipitation is about 25 inches and the mean annual air temperature is about 60 degrees F.

TAXONOMIC CLASS: Fine, smectitic, thermic Typic Argixerolls

TYPICAL PEDON: Los Osos loam - grazed range grass. (Colors are for dry soil unless otherwise noted.)

A--0 to 14 inches; brown (10YR 5/3) loam, very dark grayish brown (10YR 3/2) moist; weak fine subangular blocky structure; slightly hard, friable, slightly sticky and slightly plastic; common very fine roots; many very fine tubular pores; moderately acid (pH 6.0); clear smooth boundary. (10 to 16 inches thick)

Btss1--14 to 24 inches; yellowish brown (10YR 5/4) clay, dark yellowish brown (10YR 4/4) moist; weak medium prismatic structure; very hard, very firm, sticky and plastic; few very fine roots; few very fine tubular pores; many moderately thick clay films on faces of peds and lining pores; few slickensides; moderately acid (pH 6.0); gradual wavy boundary. (6 to 12 inches thick)

Btss2--24 to 32 inches; light yellowish brown (10YR 6/4) clay loam, dark yellowish brown (10YR 4/4) moist; massive; very hard, very firm, sticky and plastic; few very fine roots, common very fine tubular pores; many moderately thick clay films lining pores; few slickensides; few manganese concretions; slightly acid (pH 6.5); gradual wavy boundary. (4 to 8 inches thick)

C--32 to 39 inches; pale yellow (2.5Y 7/4) sandy loam, light olive brown (2.5Y 5/4) moist; massive; hard, friable, nonsticky and nonplastic; few manganese stains; neutral (pH 7.0); gradual wavy boundary. (0 to 7 inches thick)

Cr--39 to 43 inches; yellowish brown (10YR 5/4) sandstone, brown (10YR 4/3) moist; many moderately thick clay films and few manganese stains coat fracture faces that are less than 10cm apart and less than 1mm in width.

TYPE LOCATION: San Luis Obispo County, California; on the Cal-Poly University campus, approximately 2,000 feet west and 1,500 feet south of the northeast corner of section 23, T. 30 S., R. 12 E., MDB&M; Latitude 35 degrees, 18 minutes, 18 seconds north and Longitude 120 degrees, 39 minutes, 23 seconds west; San Luis Obispo Quadrangle.

RANGE IN CHARACTERISTICS: Depth to a paralithic contact of sandstone or shale is 20 to 40 inches. The mean annual soil temperature at a depth of 20 inches is 60 degrees to 67 degrees F. and the coldest temperature is warmer than 41 degrees F. All of the soil between depths of about 4 and 12 inches is continuously dry after some time in May until some time in October. Some or all of the soil between these depths is moist all the rest of the time. There is 6 to 15 percent clay increase (absolute) from the A horizon to the B2t horizon and the soil lacks an abrupt A/B horizon boundary.

The A1 horizon is grayish brown, dark grayish brown, brown, or dark brown in 10YR or 7.5YR hue and has moist value of 3 or 2. It is loam, silt loam, clay loam, or silty clay loam and it has weak to strong structure. This horizon has 2 to 4 percent organic matter. It is medium acid to neutral.

The B2t horizon is brown, dark brown, grayish brown, dark grayish brown, light yellowish brown, yellowish brown, dark yellowish brown, brownish yellow, pale brown, light olive brown, light brown, or light brownish gray in 10YR, 7.5YR, or 2.5Y hue. In most pedons it has one unit higher value and one or two units brighter chroma than the A horizon. This horizon is heavy clay loam, clay, or silty clay and averages 35 to 50 percent clay. It has weak to strong angular or subangular blocky structure or is prismatic in the upper part and is massive in the lower part in some pedons. The B2t horizon is moderately acid to neutral. Some pedons have a C horizon consisting of a weathering front. The C horizon is sandy loam, loam, or clay loam.

COMPETING SERIES: These are the Fagan, Gabino, Gridley, Marcum, Sespe, Sween, and Todos series. Fagan and Todos are underlain by soft sandstone or shale at depths of 40 to 60 inches. Gabino soils have 5YR hue in the argillic horizon and have 10 to 30 percent pebbles and cobbles. Gridley soils are moderately well drained and are neutral to moderately alkaline. Marcum soils have paralithic horizons at 40 to 80 inches and are neutral to moderately alkaline. Sespe soils have hue of 5YR or 2.5YR in the argillic horizon. Sween soils have 10 to 15 percent (absolute) increase in clay from the A horizon to the B horizon, and have a lithic contact at a depth of 20 to 40 inches.

GEOGRAPHIC SETTING: Los Osos soils are at elevations of 100 to 3,500 feet. They formed in material weathered from firm to hard sandstone and shale. Slopes are 5 to 75 percent. The climate is dry subhumid mesothermal with warm, dry, somewhat foggy summers and cool, moist winters. Mean annual precipitation is 14 to 40 inches. Average annual temperature is 56 degrees to 63 degrees F., average January temperature is about 51 degrees F., and average July temperature is about 65 degrees F. The average freeze-free season is 200 to 320 days.

GEOGRAPHICALLY ASSOCIATED SOILS: These are the Altamont, Arnold, Diablo, Gaviota, Millsap, San Benito, Santa Lucia, and Vallecitos soils. Altamont and Diablo soils are of clay texture throughout. Gaviota, Millsap, and Vallecitos soils have a lithic contact at a depth of less than 20 inches. San Benito and Santa Lucia soils lack an argillic horizon.

DRAINAGE AND PERMEABILITY: Well drained; very high runoff; slow permeability.

USE AND VEGETATION: Used mostly for range, limited areas are cropped to grain and sudan grass pasture. Vegetation is mostly annual grasses and forbs with some perennial grasses, coastal sagebrush, and live oak.

DISTRIBUTION AND EXTENT: Central part of the Coast Range in California. The soils are of moderate extent.


SERIES ESTABLISHED: San Luis Obispo County (San Luis Obispo Area), California, 1928.

National Cooperative Soil Survey