LOCATION ST. THOMAS              NV+AZ

Established Series
Rev. LNL/ELS/RLB
06/2011

ST. THOMAS SERIES

The St. Thomas series consists of very shallow and shallow, well drained soils that formed in residuum and colluvium derived from limestone and dolomite. The St. Thomas soils are on hills and mountains. Slope ranges from 2 to 75 percent. The mean annual precipitation is about 5 inches and the mean annual air temperature is about 61 degrees F.

TAXONOMIC CLASS: Loamy-skeletal, carbonatic, thermic Lithic Torriorthents

TYPICAL PEDON: St. Thomas very cobbly loam, rangeland and wildlife habitat. (Colors are for dry soil unless otherwise noted.)

A--0 to 2 inches; very pale brown (10YR 7/3) very cobbly loam, pale brown (10YR 6/3) moist; weak medium platy structure; slightly hard, very friable, slightly sticky and slightly plastic; few very fine roots; many fine and very fine vesicular pores; 25 percent cobbles and 15 pebbles; violently effervescent; moderately alkaline (pH 8.4); abrupt wavy boundary. (2 to 5 inches thick)

Bk--2 to 12 inches; very pale brown (10YR 7/3) extremely cobbly loam, pale brown (10YR 6/3) moist; massive; slightly hard, very friable, nonsticky and slightly plastic; few very fine roots; many very fine and fine, and few medium interstitial pores; 25 percent cobbles and 40 percent pebbles; violently effervescent, few 1/4 to 1/2-inch diameter pockets and veins of very pale brown (10YR 8/2) soft calcium carbonate masses near the lower boundary; moderately alkaline (pH 8.4); abrupt wavy boundary. (6 to 12 inches thick)

R--12 inches; light gray (10YR 7/2) and very pale brown (10YR 8/2) extremely hard limestone with a thin (1/16 to 1/8-inch) layer of secondary calcium carbonate capping bedrock.

TYPE LOCATION: Clark County, Nevada; approximately 1.85 miles south of Interstate 15 on Nevada 169, toward Overton and about 50 feet west of the pavement. About 2,040 feet south and 100 feet east of the northwest corner of section 9 T. 15 S., R. 67 E.

RANGE IN CHARACTERISTICS:

Soil moisture - Usually dry, moist in some part of the moisture control section for short periods during the winter and spring months and for 10 to 20 days, following summer convection storms.

Annual soil temperature - 59 to 67 degrees F.
Depth to bedrock - 4 to 14 inches
Control section - Clay content: 4 to 18 percent.
Rock fragments: 50 to 85 percent, mainly cobbles and gravel.
Reaction: Moderately alkaline or strongly alkaline.

A horizon - Hue: 7.5YR or 10YR.

Value: 5 through 8 dry, 4 through 7 moist.
Bk horizons: Hue: 7.5YR or 10YR.
Value: 6 or 7 dry, 5 or 6 moist.
Texture: Loam, very fine sandy loam, or fine sandy loam.
Consistence: Soft or slightly hard, nonsticky or slightly sticky.

COMPETING SERIES: This is the Birdspring (NV), Dozer (NM), Mariscal (TX), and Saguaro (AZ) series. Birdspring soils have mean winter temperatures of 40 to 45 degrees F. and the upper part of the soil profile is frozen for short periods. Dozer soils have a clay content of 18 to 35 percent and does not receive summer precipitation. Mariscal soils have a mean annual soil temperature of 67 to 72 degrees. Saguaro soils are moist in some part for more than than 20 days cumulative from June through August and are typical of the Sonoran desert and MLRA 40.

GEOGRAPHIC SETTING: St. Thomas soils are on hills and mountains. These soils formed in residuum and colluvium derived from limestone and dolomite. Slope ranges from 2 to 75 percent. Elevations are 1,800 to 5,090 feet. The climate is arid with hot dry summers and mild winters. The mean annual precipitation is 3 to 8 inches. The mean annual temperature is 57 to 65 degrees F., and the mean winter temperature is 47 to 55 degrees F. The upper part of the soil profile does not freeze during the winter. The frost-free season is 180 to 270 days.

GEOGRAPHICALLY ASSOCIATED SOILS: These are the Bard, Moapa and Tonopah soils. Bard soils have petrocalcic horizons. Moapa soils have sandy control sections. Tonopah soils have sandy-skeletal control sections.

DRAINAGE AND PERMEABILITY: Well drained; high runoff; moderate or moderately rapid permeability.

USE AND VEGETATION: These soils are used for rangeland and wildlife habitat. The present vegetation is mainly creosotebush, white bursage, big galleta, and indian ricegrass.

DISTRIBUTION AND EXTENT: Mojave Desert of southern Nevada; MLRA 30 These soils are moderately extensive.

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

SERIES ESTABLISHED: Clark County, Nevada (Virgin River Area), 1970.

REMARKS: Diagnostic horizons and features recognized in this pedon are:

Ochric epipedon - 0 to 7 inches (A and part of the Bk horizon).
Lithic contact - 12 inches (R layer).
Particle-size control section - 0 to 12 inches (A and Bk horizons).

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National Cooperative Soil Survey
U.S.A.