LOCATION FORBESVILLE CAEstablished Series
TAXONOMIC CLASS: Fine, mixed, active, thermic Ultic Palexeralfs
TYPICAL PEDON: Forbesville loam--on a nearly level north facing slope of about 2 percent under a wine grape vineyard at an elevation of 1,450 feet. (Colors are for dry soil unless otherwise stated. When described on May 25, 1977 the soil was dry to 20 inches and slightly moist below.)
AP--O to 6 inches; brown (7.5YR 5/4) loam, dark reddish brown (5YR 3/4) moist; moderate fine granular structure; hard, friable, slightly sticky and slightly plastic; many very fine and common fine roots; common fine tubular and common very fine interstitial pores; 5 percent pebbles from 2 to 20 mm in diameter; moderately acid (pH 6.0); abrupt smooth boundary. (2 to 7 inches thick)
BA--6 to 16 inches; yellowish red (5YR 5/6) loam, yellowish red (5YR 4/6) moist; weak fine and medium subangular blocky structure; hard, friable, slightly sticky and slightly plastic; common very fine and few fine roots; common very fine, fine and medium tubular pores worm casts present; slightly acid (pH 6.4); abrupt smooth boundary. (5 to 15 inches thick)
Btl--16 to 30 inches; red (2.5YR 5/6) clay, red (2.5YR 4/6) moist; moderate medium and coarse subangular blocky structure; very hard, firm, sticky and plastic; common very fine and few fine roots; few very fine and fine tubular and interstitial pores; many moderately thick clay films on peds; 10 percent pebbles 2 to 30 mm in diameter; common fine black coatings on peds and pebbles; strongly acid (pH 5.4); gradual wavy boundary.
Bt2--30 to 46 inches; red (2.5YR 5/6) very gravelly clay, red (2.5YR 4/6) moist, moderate very fine and fine subangular blocky structure; very hard, friable, sticky and plastic; few very fine roots; few very fine tubular and common very fine interstitial pores; continuous thick clay films on peds and in pores; 35 percent pebbles 2 to 75 mm in diameter and 2 percent cobbles; strongly acid (pH 5.4); clear wavy boundary. (Combined thickness of the B2t horizon is 20 to 40 inches)
BCt--46 to 70 inches; reddish yellow (7.5YR 6/6) very gravelly clay loam, strong brown (7.5YR 5/6) moist; weak moderate subangular blocky structure; very hard, firm, sticky and plastic; few very fine roots; few very fine tubular and interstitial pores; common thick clay films as bridges and on peds; 40 percent weathered rock pebbles 2 to 75 mm in diameter; common fine black coatings on peds and pebbles; moderately acid (pH 5.9).
TYPE LOCATION: Lake County, California; in a vineyard about 250 feet south of Bell Hill Road at a point about 0.6 mile southwest of its intersection with Plunkett Lane; about 750 feet south and 1,150 feet west of the northeast corner of Sec. 21, T. 13 N., R. 9 W .
RANGE IN CHARACTERISTICS: Thickness of the solum and depth to bedrock is greater than 60 inches. The mean annual soil temperature is 59 degrees to 62 degrees F. The soil between the depths of 7 and 21 inches is dry in all parts from June 1 to October 15 and is moist in all parts from January 1 to April 15. Base saturation is 50 to 75 percent throughout (sum of cations).
The A horizon is 5YR, 7.5YR or 10YR 4/4, 5/4 or 5/6. Moist color is 5YR or 10YR 3/3, 3/4, 4/3; 7.5YR 3/4 or 4/4. Moist value and chroma of 3.5 or less occur only in the upper 6 inches. It is loam or gravelly loam with 5 to 30 percent pebbles and is strongly to slightly acid.
The Bt horizon is 2.5YR, 5YR or 7.5YR 5/4, 5/6 or 6/6. Moist color is 2.5YR, 5YR or 7.5YR 4/4, 4/6 or 5/6. It is heavy clay loam or clay or their gravelly or very gravelly equivalents. It is strongly or moderately acid. The upper 20 inches of the argillic horizon averages 35 to 55 percent clay and 0 to 30 percent pebbles. Clay increase by 15 to 20 percent (absolute) or more from the BA horizon to the upper Bt horizon and has a clear or abrupt upper boundary.
COMPETING SERIES: These are the Redvine series in the same family and the Cotati, Manzanita, Red Bluff and Spreckles series in other families. Cotati and Spreckles soil have mesic soil temperatures. Manzanita soils are fine-loamy. Red Bluff soils have kaolinitic mineralogy and lacks an abrupt or clear boundary with 15 to 20 percent clay increase (absolute). Redvine soils lack an abrupt or clear boundary and an absolute clay increase of 20 percent or more between the eluvial horizon and upper Bt horizon.
GEOGRAPHIC SETTING: Forbesville soils are on old dissected terraces. Slopes are 2 to 15 percent. The soils formed in alluvium from mixed rock sources. Elevations are 1,300 to 1,600 feet. The climate is subhumid with warm dry summers and cool moist winters. The average annual precipitation is 25 to 40 inches. Mean January temperature is about 42 degrees F; mean July temperature is about 68 degrees F; mean annual temperature is 56 degrees to 59 degrees F. Frost-free season is 160 to 205 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the competing Manzanita soils and the Wappo soils. Wappo soils have montmorillonitic mineralogy.
DRAINAGE AND PERMEABILITY: Well drained; slow to medium runoff; slow permeability.
USE AND VEGETATION: This soil is used mainly for cropland in orchards, vineyards, hay and pastureland, rangeland and as homesite development. Natural vegetation is annual grasses and forbs with scattered blue oak and scattered stands of tall manzanita, ceanothus, California scrub oak and chamise.
DISTRIBUTION AND EXTENT: Clear Lake Basin in Lake County, California. The series is not extensive.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Davis, California
SERIES ESTABLISHED: Lake County, California 1983.
REMARKS: The activity class was added to the classification in February of 2003. Competing series were not checked at that time. - ET
OSED scanned by SSQA. Last revised by state on 3/84