LOCATION OHTWO                   TX

Established Series
Rev. LEL/JLR/ACT
10/2012

OHTWO SERIES

The Ohtwo series consists of very deep, well drained, moderately permeable soils on steep to very steep sideslopes below cliffs and scarps. These soils formed in colluvium derived from the Duff and Pruett Tuffs, the Mitchell Mesa Ignimbrite, the Cottonwood Springs Basalt, and the Crossen Trachyte Formations. Slopes range from 20 to 70 percent.

TAXONOMIC CLASS: Loamy-skeletal, mixed, superactive, thermic Ustic Haplocambids

TYPICAL PEDON: Ohtwo very gravelly clay loam - rangeland. (Colors are for dry soil unless otherwise stated.)

A--0 to 8 inches; brown (7.5YR 4/3) very gravelly clay loam, dark brown (7.5YR 3/3) moist; moderate fine and medium granular structure; slightly hard, friable, moderately sticky and moderately plastic; many fine and common medium roots throughout; common fine interstitial pores; 40 percent subrounded basalt gravel, and 5 percent subrounded basalt cobbles; distinct patchy white (10YR 8/1) calcium carbonate coatings on rock fragments; very slightly effervescent; slightly alkaline; gradual smooth boundary (4 to 10 inches thick)

Bk1--8 to 35 inches; brown (7.5YR 4/4) very gravelly clay loam, brown (7.5YR 4/3) moist; moderate fine and medium subangular blocky parting to moderate fine and medium granular structure; hard, firm, moderately sticky and moderately plastic; common very fine and fine roots throughout; 35 percent subrounded basalt gravel, and 5 percent subrounded basalt cobbles; distinct patchy white (10YR 8/1) calcium carbonate coatings on rock fragments; slightly effervescent; slightly alkaline; clear wavy boundary (11 to 27 inches thick)

Bk2--35 to 42 inches; brown (7.5YR 5/4) very cobbly loam, brown (7.5YR 4/4) moist; weak fine and medium subangular blocky structure; hard, friable, moderately sticky and moderately plastic; 20 percent subrounded basalt gravel, 25 percent subrounded basalt cobbles, and 5 percent subrounded basalt stones; distinct patchy white (10YR 8/1) calcium carbonate coatings on rock fragments; slightly effervescent; slightly alkaline; clear broken boundary (7 to 17 inches thick)

Bk3--42 to 65 inches; brown (7.5YR 5/4) very gravelly loam, brown (7.5YR 4/4) moist; weak fine and medium subangular blocky structure; slightly hard, friable, moderately sticky and moderately plastic; few very fine and fine roots throughout; 35 percent subrounded basalt gravel, and 5 percent subrounded basalt cobbles; distinct patchy white (10YR 8/1) calcium carbonate coatings on rock fragments; slightly effervescent; slightly alkaline.

R--65 to 80 inches; basalt bedrock, indurated; many prominent continuous white (10YR 8/1) coatings of calcium carbonate on upper surfaces of rock; noneffervescent.

TYPE LOCATION: Brewster County, Texas; from the junction of U.S. Highway 90 and Texas Highway 118 in Alpine, 32.3 miles south on Texas Highway 118 to the 02 Ranch gate, 5.3 miles southwest on 02 Ranch headquarters road, 0.4 miles northwest on ranch road to water storage tank, 1.35 miles north and northwest on ranch road, 25 feet north in rangeland. Duff Springs USGS 7.5 minute quadrangle. Latitude: 29 degrees, 55 minutes, 7 seconds North; Longitude: 103 degrees, 39 minutes, 39 seconds West. NAD 1927.

RANGE IN CHARACTERISTICS:

Soil moisture: Ustic aridic moisture regime

Clay content in the particle-size control section: 20 to 35 percent

Calcium carbonate equivalent: less than 15 percent throughout the pedon

Ignimbrite, basalt, and trachyte rock fragments: 45 to 75 percent in the control section

Ignimbrite, basalt, and trachyte rock fragments cover 35 to 95 percent of the soil surface

Reaction: slightly alkaline to moderately alkaline

A horizon
Hue: 7.5YR, 10YR
Value: 3 to 5, dry or moist
Chroma: 2 to 4, dry or moist
Texture: loam, clay loam
Silicate clay content: 20 to 35 percent
Rock fragments: 35 to 75 percent by volume

Bk horizons
Hue: 5YR, 7.5YR, 10YR
Value: 4 to 6, dry or moist
Chroma: 3 to 5, dry or moist
Texture: loam, clay loam
Silicate clay content: 20 to 35 percent
Rock fragments: 35 to 75 percent

2R horizon
Unweathered basalt bedrock with calcium carbonate coatings on upper surfaces and in fractures. Tuffaceous paralithic material underlies some pedons.

COMPETING SERIES: These are the Altar (AZ) and Pilabo (NM) series. Altar and Pilabo soils do not have bedrock at very deep depths (60-80 inches).

GEOGRAPHIC SETTING: Ohtwo soils are on steep and very steep sideslopes immediately below scarps. These soils formed in colluvium derived mainly from tuffs of the Duff and Pruett Formations. Lesser amounts of rock fragments are from the Mitchell Mesa Ignimbrite, the Cottonwood Springs Basalt, and the Crossen Trachyte. Slopes range from 20 to 70 percent. Mean annual precipitation ranges from 10 to 15 inches, with most occurring July through September. Mean annual temperature ranges from 62 to 68 degrees F. Frost-free period is 210 to 260 days, and elevation ranges from 3,500 to 5,000 feet.

GEOGRAPHICALLY ASSOCIATED SOILS: These are the Borunda, Gemelo, Lingua, Reduff, and Scotal series. Borunda soils are moderately deep to weathered tuff bedrock, have calcic and gypsic horizons, and are in the fine-loamy family. They are on lower gently undulating uplands. Gemelo soils are very deep, have cambic horizons, and are in the coarse-loamy family. They are on lower alluvial fan aprons. Lingua soils are shallow and very shallow to basalt bedrock, and are on higher erosional slopes, hills and scarps. Reduff soils are shallow and very shallow to red tuff and are on adjacent erosional slopes, hills, and scarps. Scotal soils are shallow and very shallow to white tuff and are on adjacent erosional slopes, hills, and scarps.

DRAINAGE AND PERMEABILITY: Well drained. Permeability is moderate. Runoff is high.

USE AND VEGETATION: This soil is used for livestock grazing and wildlife habitat. Woody species in the overstory include mesquite, creosotebush, whitethorn acacia, range ratany, feather dalea, allthorn, lotebush, ocotillo, pitaya, prickly pear, cholla, Spanish dagger, and narrowleaf forsteria. Grasses in the understory are black grama, green sprangletop, sideoats grama, cane bluestem, and tanglehead.

DISTRIBUTION AND EXTENT: West Texas. MLRA 42. The series is moderately extensive.

MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Phoenix, Arizona

SERIES ESTABLISHED: Brewster County, Texas, 1997. The name is from the Ohtwo triangulation station about 2 miles south of the type location.

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

Ochric epipedon - the zone from 0 to 8 inches (A horizon) The soil meets all requirements for a mollic epipedon, except the soil is moist less than 3 cumulative months in 8 out of 10 years.

Cambic horizon - the zone from 8 to 60 inches (Bk horizons).

Superactive cation exchange activity class - CEC to percent clay ratio 0.60 or greater. This is assumed from the zeolitic minerals present in the Duff and Pruett Tuffs, and from laboratory data from similar soils from Presidio Count, Texas.

Classified according to Soil Taxonomy, Second Edition, 1999; Keys to Soil Taxonomy, Eleventh Edition, 2010

Update and revisions for Recorrelation of Brewster County, Texas, Main Part, 2/12/08, CEM

Revised for the correlation of Presidio County, Texas ; Oct, 2012, WWJ

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National Cooperative Soil Survey
U.S.A.