LOCATION COOMBS CAEstablished Series
TAXONOMIC CLASS: Fine-loamy, mixed, active, thermic Ultic Haploxeralfs
TYPICAL PEDON: Coombs gravelly loam - cultivated. (Colors are for dry soil unless otherwise noted).
Ap--0 to 4 inches; brown (10YR 5/3) gravelly loam, dark brown (10YR 3/3) moist; fine to coarse subangular blocky structure; hard, friable, nonsticky, slightly plastic; very fine roots; 15 to 25 percent gravel; moderately acid (pH 6.0); abrupt wavy boundary. (3 to 8 inches thick)
A3--4 to 13 inches; pale brown (10YR 6/3) clay loam, very dark grayish brown (10YR 3/2) moist; massive; hard, friable, slightly sticky, slightly plastic; common very fine roots; many very fine tubular pores moderately acid (pH 6.0); clear smooth boundary. (7 to 11 inches thick)
B1t--13 to 25 inches; brown (10YR 5/3) clay loam, dark brown (10YR 3/3) moist; weak fine subangular blocky structure; hard, friable, slightly sticky, slightly plastic; common very fine roots; many very fine, few fine tubular pores; common this clay films on peds and in pores; strongly acid (pH 5.5); clear smooth boundary. (8 to 12 inches thick)
B21t--25 to 35 inches; light brown (7.5YR 6/4) clay loam, dark brown (7.5YR 3/4) moist; weak fine subangular blocky structure; slightly hard, friable, slightly sticky, plastic; few very fine roots; many very fine, few fine tubular pores; many thin clay films on peds and in pores; strongly acid (ph 5.5); clear smooth boundary. (8 to 12 inches thick)
B22t--35 to 43 inches; light brown (7.5YR 6/4) heavy clay loam, dark brown (7.5YR 4/4) moist; massive; slightly hard, friable, slightly sticky, plastic; few very fine roots; many very fine, few fine tubular pores; many thin clay films in pores; strongly acid (pH 5.5); clear smooth boundary. (7 to 11 inches thick)
B3t--43 to 54 inches; pink (7.5YR 7/4) clay loam, dark brown (7.5YR 4/4) moist; massive; slightly hard, friable, slightly sticky, plastic; few fine tubular pores; many thin clay films in pores; strongly acid (pH 5.3); clear smooth boundary. (6 to 14 inches thick)
IIC--54 to 58 inches; very gravelly loamy sand, few very fine roots; strongly acid (pH 5.2); about 85 percent gravel.
TYPE LOCATION: Napa County, California; 100 feet south of Tanita Ranch entrance road and 200 feet northeast of Big Ranch Road in prune orchard; in the NW 1/4 of sec. 27 (projected) T. 6 N., R. 4 W.
RANGE IN CHARACTERISTICS: The mean annual soil temperature is about 59 to 62 degrees F, and the soil temperature usually is not below 47 degrees F at any time. The soil between depths of 4 to 15 inches is usually dry from June 15 until October 15 and is moist in some or all parts the rest of the year.
Rock fragments make up about 5 to 25 percent of the A and Bt horizons and 50 to 90 percent of the very gravelly C horizons. Fragments consist mostly of 1/4 to 1 inches rounded pebbles with some ranging up to 3 inches. The A horizon is massive and hard, has less than 1 percent organic matter in the surface 8 to 10 inches, or has value of 6 dry or 4 moist.
The Coombs soils have brown and pale brown, medium acid gravelly loam and clay loam A horizons, light brown strongly acid clay loam B2t horizons underlain by a strongly acid very gravelly substratum at a depth of 54 inches.
The A horizon has 10YR or 7.5YR hue and has value of 5 to 6 dry, 3 or 4 moist and chroma of 2 or 3. It is usually gravelly and is heavy loam, loam or clay loam and ha subangular blocky structure or is massive. This horizon is slightly hard or hard and is medium or slightly acid.
The Bt horizon has hue of 10YR or 7.5YR, value of 5 through 7, 3 ro 4 moist and chroma of 3 or 4 moist and dry. It is clay loam in the upper part and heavy clay loam or clay in the lower part. The upper 20 inches has about 28 to 35 percent clay.
The B2t horizon has subangular blocky structure or it is massive. It is slightly hard or hard and is moderately to strongly acid.
The C horizon varies considerably in texture, thickness of horizons, and coarse fragments over short distances and some pedons are underlain by very weakly consolidated tuff.
COMPETING SERIES: These are the Arbuckle, Auberry, Butte, Churn, Sierra, and Wisheylu series. Arbuckle and Auberry soils have over 75 percent base saturation in the argillic horizon. Butte and Wisheylu soils have a paralithic contact at depths of 20 to 40 inches. Churn soils have 18 to 28 percent clay in the upper 20 inches of the Bt horizon and have moist hue of 5YR in the B2t horizon. Sierra soils have a paralithic contact at depths of 40 to 80 inches.
GEOGRAPHIC SETTING: The Coombs soils are nearly level to gently sloping. They are on gravelly terraces at elevations of 100 to 500 feet. They formed in gravelly alluvium from mixed sources. The climate is subhumid mesothermal with warm dry summers and cool moist winters. Mean annual precipitation is about 30 inches. Mean annual temperature is 59 to 62 degrees F, average January temperature about 47 degrees F and average July temperature about 67 degrees F. The frost-free season is 220 to 260 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the clear Lake, Cole, Haire, and Yolo soils. Clear Lake soils are clayey throughout and have intersecting slickensides. Cole soils have thick mollic epipedons and fine particle-size class. Haire soils have less than 35 percent base saturation in the lower horizons and clayey particle-size class. Yolo soils lack argillic horizons.
DRAINAGE AND PERMEABILITY: Well-drained; medium runoff; moderately slow permeability.
USE AND VEGETATION: Most soils are cultivated in orchards, vineyards, irrigated pasture and dryland grain. Natural vegetation was annual grasses and forbs with scattered oak trees.
DISTRIBUTION AND EXTENT: Napa County, California. The soils are not extensive.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Davis, California
SERIES ESTABLISHED: Napa Area, California, 1938.
REMARKS: The activity class was added to the classification in February of 2003. Competing series were not checked at that time. - ET
Last revised by the state on 10/74.