LOCATION MAXWELL CAEstablished Series
TAXONOMIC CLASS: Fine, smectitic, thermic Typic Haploxererts
TYPICAL PEDON: Maxwell clay - rangeland. (Colors are for dry soil unless otherwise stated.)
A1--0 to 10 inches; dark gray (N 4/) clay, very dark gray (N 3/) when moist; moderate, medium, prismatic structure; extremely hard, very firm, sticky and very plastic; common very fine roots; common very fine tubular pores; many slickensides; mildly alkaline (pH 7.5); gradual smooth boundary. (10 to 12 inches thick)
A2--10 to 26 inches; dark gray (N 4/) clay, very dark gray (N 3/) when moist; strong, medium, prismatic structure with cracks 1 to 2 cm wide; very hard, very firm, sticky and very plastic; common very fine roots; common very fine tubular pores; many slickensides; moderately alkaline (pH 8.0); gradual smooth boundary. (14 to 18 inches thick)
AC--26 to 38 inches; dark gray (N 4/) clay, dark gray (N 4/) when moist; moderate, medium, prismatic structure; hard, very firm, sticky and very plastic; few very fine roots; few very fine tubular pores; few slickensides; moderately alkaline (pH 8.0); gradual smooth boundary. (12 to 14 inches thick)
C1--38 to 48 inches; gray (N 5/) clay, dark gray (5Y 4/1) when moist; massive; very hard, very firm, sticky and very plastic; few very fine roots; few very fine tubular pores; moderately alkaline (pH 8.0); gradual smooth boundary. (10 to 12 inches thick)
C2--48 to 62 inches; gray (5Y 5/1) clay, dark gray (5Y 4/1) when moist; massive; very hard, very firm, sticky and very plastic; few very fine roots; few very fine tubular pores; strongly effervescent; moderately alkaline (pH 8.0); lime in medium size soft masses.
TYPE LOCATION: Pope Valley, Napa County, California; 250 feet east of Pope Canyon Road; 700 feet west and 250 feet south of center of sec. 14, T. 9 N., R. 5 W.
RANGE IN CHARACTERISTICS: Depth to carbonates is 24 to 56 inches. The mean annual soil temperature is about 59 to 64 degrees F and the soil temperature usually is not below 47 degrees F at anytime. Crack open and close once each year. They remain open during the period from July to October and are closed the rest of the year. Common to many intersecting slickensides occur in all or part of the lower A horizon. The soils have Ca/Mg ratio of less than 2:1. The A horizon is neutral, 5Y, 2.5Y, or 10YR hue with value of 2, 3, 4, or 5 dry and 3 moist. Chroma is less than 1.5 dry or moist. The A horizon is heavy clay loam or clay and usually has prismatic or blocky structure. In some pedons the surface few inches have strong granular structure. This horizon ranges from neutral to moderately alkaline and is noneffervescent.
The C horizon is neutral, 10YR, 2.5Y, or 5Y hue and has value of 4, 5, 6, or 7 dry and 3 or 4 moist. Chroma is less than 1.5 dry or moist to depths of more than 40 inches. In some pedons the C horizon below a depth of 40 inches has chroma of 2. It is silty clay or clay and is very weak to strongly effervescent. Carbonates commonly occur as medium-size soft masses.
COMPETING SERIES: These are the Clear Lake, Hildreth, Subaco, and Willows series. Clear Lake, Hildreth, and Willows soils have Ca/Mg ratio of 2 or more. Also, Clear Lake soils are poorly or very poorly drained; Hildreth soils are over unrelated material (see Remarks); and Willows soils have more than 15 percent exchangeable sodium and commonly have gypsum or salt crystals, or both, in the C horizon. Subaco soils are 20 to 40 inches deep to a paralithic contact.
GEOGRAPHIC SETTING: Maxwell soils are on basin rims or fans. Slopes are 0 to 9 percent. Elevations are 200 to 1,700 feet. These soils formed in alluvium derived mostly from serpentinitic rocks. The climate is subhumid with hot dry summers and cool moist winters. Mean annual precipitation is 20 to 50 inches. Average January temperature is 45 degrees F; average July temperature is 75 degrees F; mean annual temperature is 58 degrees to 62 degrees F. Frost-free season is 150 to 275 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the Climara, Henneke, Lodo, Maymen, and Montara soils. All these soils except Climara are less than 20 inches deep to a lithic contact. Also, Henneke soils have a clayey argillic horizon and Lodo, Maymen and Montara soils have loamy control sections. Climara soils have chroma of more than 1.5 within a depth of 10 inches.
DRAINAGE AND PERMEABILITY: Somewhat poorly drained; slow runoff; very slow permeability.
USE AND VEGETATION: Used mostly for dryland range and pasture. A few areas are in irrigated pasture or prunes. Vegetation is annual grasses and forbs. In some areas the soil is naturally barren and lacks vegetation.
DISTRIBUTION AND EXTENT: Central California, coast mountain valleys. The soils are inextensive.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Davis, California
SERIES ESTABLISHED: Napa County, California 1933.
REMARKS: Maxwell soils formerly were classified as Grumusols. Hildreth soils are currently listed in the same family. Differences are not clearly defined and more study is needed. A part of the Hildreth soils should be classified as Entic Pelloxererts because moist value in the upper 12 inches is higher than 3.5.