LOCATION THOMS CA

Established Series
Rev. WBS/TDC/GMK
01/2023

THOMS SERIES

The Thoms series is a member of the loamy, mixed, mesic, shallow family of Xerollic Durargids. Typically, Thoms soils have dark brown, slightly acid, very cobbly loam A horizons and dark brown and reddish brown, neutral, clay loam Bt horizons underlain by an indurated duripan at a depth of 13 inches. Stratified cemented pebbles and cobbles typically occur at depth of 24 inches.

TAXONOMIC CLASS: Loamy, mixed, superactive, mesic, shallow Xeric Argidurids

TYPIFYING PEDON: Thoms very cobbly loam - rangeland (Colors are for dry soil unless otherwise stated.)

A1--0 to 6 inches; brown (10YR 5/3) very cobbly loam, very dark grayish brown (10YR 3/2) moist; moderate medium subangular blocky structure; hard, friable, slightly sticky, slightly plastic; many very fine roots; common very fine interstitial, few very fine tubular pores; 50 percent cobbles by volume on surface; slightly acid (pH 6.3); clear smooth boundary. (5 to 7 inches thick)

B2t--6 to 9 inches; brown (7.5YR 4/4) clay loam, reddish brown (5YR 4/4) moist; moderate coarse angular blocky structure; hard. friable, sticky, plastic; few very fine exped roots; few very fine tubular pores; common moderately thick clay films as bridges, in pores, and on peds; 10 percent gravel and 5 percent cobbles by volume; neutral (pH 6.8); abrupt smooth boundary. (3 to 5 inches thick)

B3t--9 to 13 inches; reddish brown (5YR 4/3) heavy clay loam, reddish brown (5YR 4/4) moist; moderate medium angular blocky structure; hard, friable, sticky, plastic; few very fine tubular and interstitial pores; common moderately thick clay films as bridges in pores and on peds; neutral (pH 6.8); 10 percent gravel and 5 percent cobbles by volume; abrupt smooth boundary. (4 to 6 inches thick)

C1sim--13 to 24 inches; reddish yellow (7.5YR 6/6) massive silica cemented duripan; very hard, very firm; 50 percent cobbles and 40 percent gravel; upper part of duripan has 1 mm thick indurated bands; silica coatings on underside of rock fragments; common fine distinct mottles of very dark gray (N 3/ ); abrupt smooth boundary. (8 to 12 inches thick)

C2sim--24 to 42 inches; reddish yellow (7.5YR 6/6) massive silica cemented duripan, very hard, very firm; silica in interstices and as bridges between sand size particles; 50 percent cobbles and 40 percent gravel; silica coatings on underside of rock fragments; common fine distinct mottles of very dark gray (N 3/ ).

TYPE LOCATION: Modoc County, California; 1.050 feet south and 1,618 feet west of the east 1/4 corner of section 18, R.43N., R.14E.

RANGE IN CHARACTERISTICS: Depth to the duripan is 12 to 18 inches. Depth to stratified cemented gravelly and cobbly alluvium is 20 to 30 inches. The mean annual soil temperature is about 47 degrees to 53 degrees F. The soil temperature usually is above 41 degrees F. from February 20 to January 1. It is above 47 degrees F. from April 1 to November 1. The soil is usually dry in all parts between the depth of 4 to 12 inches or to the duripan from about June 1 to November 15, and is moist in some or all parts of the 4 to 12 inch zone or to duripan from April 1 to June 1 when above 47 degrees F. It is usually dry and is not continuously moist for 90 consecutive days. The soil is slightly acid or neutral throughout the profile. Sand/clay ratios of the upper 15 inches are 1 to 4 and the organic matter content averages more than 0.5 percent in the upper 15 inches. The A horizon is pale brown, light brownish gray, brown, or grayish brown in 10YR hue and dark yellowish brown and dark brown to very dark grayish brown (10YR 4/4, 4/3, 4/2, 3/4, 3/3, 3/2; 7.5YR 4/4, 4/2, 3/4, 3/2) moist. Organic carbon content averages less than 0.6 percent in the upper 7 inches or mixed values are 6. The A horizon is extremely or very cobbly or stony on the surface. It has weak platy or moderate subangular blocky structure or is massive. It is soft, slightly hard or hard but not both massive and hard. The Bt horizon is heavy loam, clay loam, heavy clay loam in the upper part and heavy clay loam or clay in the lower part on top of the duripan. The texture control section averages 30 to 35 percent clay. The Bt horizon has weak subangular blocky or moderate angular blocky structure. It is hard or very hard and friable or firm. The Csim horizon is massive or is platy with thin continuous opal laminar layers in the upper part. The lower part is massive and very firm when moist. Silica coatings occur on the bottom of coarse fragments and opal fills the interstices and forms bridges between sand sized mineral grains.

COMPETING SERIES: These are the Acana, Ditchcamp, Deven, Exel, Packwood, Puls, and Wellington series. Acana soils are mildly to very strongly alkaline and are strongly calcareous. Their mineralogy is strongly influenced by vitric pyroclastic materials and they have mean annual summer soil temperatures of 74 degrees to 78 degrees F. Ditchcamp and Exel soils have a duripan at depths of more than 20 inches. Deven soils lack a duripan, have a mollic epipedon, and overlie basalt rock at depths of less than 20 inches. Packwood soils are underlain by hard basalt rock below the duripan. Puls soils have abrupt textural A-B boundaries, and have clayey B2t horizons. Wellington soils have lime-silica cemented duripans and have thin A2 horizons.

GEOGRAPHIC SETTING: Thoms soils are nearly level to moderately sloping and occur on nearly level to gently rolling hummocky lake terraces at elevations of 4,700 to 5,200 feet. Thoms soils occur in a complex and typically occupy the intermounds and Exel soils the mounds. They formed in lake terrace alluvium derived from basic igneous and pyroclastic rocks. Stones and cobbles cover most of the surface. In some places they are close enough to form a nearly continuous pavement. A stone line at the base of mounds is common. The climate is semiarid mesothermal with warm dry summers and col moist winters. Mean annual precipitation is 12 to 16 inches. Average January temperature is 28 degrees F.; average July temperature is 67 degrees F.; mean annual temperature is 46 degrees to 49 degrees F. The freeze-free season is 80 to 110 days.

GEOGRAPHICALLY ASSOCIATED SOILS: These are the competing Exel soils and the Bieber, Barnard, Delma, Loveioy, and Modoc soils. Bieber soils have a mollic epipedon and a clayey B2t horizon. Barnard soils have a clayey B2t horizon, a mollic epipedon, and 20 to 40 inches to a duripan. Delma soils have a mollic epipedon and lack a duripan. Lovejoy soils have an abrupt A-B textural boundary and a fine control section. Modoc soils have a mollic epipedon and a fine-loamy control section.

DRAINAGE AND PERMEABILITY: Well-drained; slow or medium runoff; slow permeability.

USE AND VEGETATION: Used for grazing. Vegetation is mainly low sagebrush, Sandberg bluegrass, Idaho fescue and cheatgrass.

DISTRIBUTION AND EXTENT: Valleys of the Modoc Plateau in northeastern California. The soils are inextensive.

MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Davis, California

SERIES ESTABLISHED: Modoc County (Alturas Area). California, 1974.

REMARKS: The rhoms soils would have been classified as Brown soils in the 1938 classification.

The superactive cation exchange activity class was added in 03/2003 to the taxonomic classification by the National Soil Survey Center on request of the Reno MLRA office, without review of the soil series property data. The remainder of this document has not been updated.

ADDITIONAL DATA: Riverside Laboratory sample 572 Calif-25-15; not published to date.

OSED scanned by SSQA. Last revised by state on 6/74.

The superactive cation exchange activity class was added in 03/2003 to the taxonomic classification by the National Soil Survey Center on request of the Reno MLRA office, without review of the soil series property data. The remainder of this document has not been updated.

National Cooperative Soil Survey
U.S.A.