LOCATION STILLWELL TX
Established Series
Rev. RAC-JLR-ACT-WWJ
10/2012
STILLWELL SERIES
The Stillwell series consists of very deep, well drained, moderately permeable soils on erosional fan and pediment remnants. These soils formed in gravelly alluvium and colluvium, derived from flaggy limestone of the Boquillas Formation. Slope ranges 1 to 30 percent. Mean annual precipitation is about 279 mm (11 in) and the mean annual air temperature is about 21.1 degrees C (70 degrees F).
TAXONOMIC CLASS: Loamy-skeletal, carbonatic, hyperthermic Sodic Ustic Haplocalcids
TYPICAL PEDON: Stillwell very gravelly coarse sandy loam - rangeland. (Colors are for dry soil unless otherwise noted.) Surface has about 75 percent coverage of a gravel desert pavement.
A1--0 to 5 cm (0 to 2 in); pale brown (10YR 6/3) very gravelly coarse sandy loam; yellowish brown (10YR 5/4) moist; moderate medium granular structure; slightly hard, very friable, slightly sticky and nonplastic; common very fine and fine roots; 35 limestone gravel; 2 percent limestone cobbles; violently effervescent; moderately alkaline; abrupt smooth boundary. (Thickness is 2 to 15 cm [1 to 6 in])
A2--5 to 18 cm (2 to 7 in); light yellowish brown (10YR 6/4) very gravelly fine sandy loam; dark yellowish brown (10YR 4/4) moist; moderate medium granular structure; slightly hard, very friable, slightly sticky and nonplastic; common very fine and fine roots; 40 percent limestone gravel; violently effervescent; moderately alkaline; abrupt wavy boundary. (Thickness is 0 to 15 cm [0 to 6 in])
Bk1--18 to 33 cm (7 to 13 in); very pale brown (10YR 8/4) very gravelly fine sandy loam; yellowish brown (10YR 5/4) moist; weak fine subangular blocky structure; hard, friable, slightly sticky and nonplastic; common very fine and fine roots; distinct white (10YR 8/1) calcium carbonate coatings on coarse fragments; 40 percent limestone gravel; violently effervescent; moderately alkaline; abrupt wavy boundary.
Bk2--33 to 63 cm (13 to 25 in); light gray (10YR 7/2) very gravelly fine sandy loam; light yellowish brown (10YR 6/4) moist; weak fine subangular blocky structure; hard, friable, slightly sticky and nonplastic; common very fine and fine roots; distinct white (10YR 8/1) calcium carbonate coatings on rock fragments; few fine irregular masses of calcium carbonate; 35 percent limestone gravel; violently effervescent; moderately alkaline; clear wavy boundary. (Combined thickness of the Bk horizon is 25 to 89 cm [10 to 36 in])
2BCk1--63 to 94 cm (25 to 37 in); pink (7.5YR 7/4) extremely gravelly coarse sandy loam; brown (7.5YR 5/4) moist; weak fine granular structure; soft, very friable, nonsticky and nonplastic; few very fine and fine roots; distinct white (10YR 8/1) calcium carbonate coatings on rock fragments; 65 percent limestone gravel; 5 percent limestone cobbles; violently effervescent; slightly saline; strongly sodic; slightly alkaline; gradual wavy boundary.
2BCk2--94 to 152 cm (37 to 60 in); pink (7.5YR 7/4) extremely gravelly coarse sandy loam; brown (7.5YR 5/4) moist; weak fine granular structure; soft, very friable, nonsticky and nonplastic; few very fine and fine roots; 60 percent limestone gravel; 15 percent limestone cobbles; violently effervescent; slightly saline; moderately sodic; slightly alkaline; diffuse wavy boundary. (Combined thickness of the BCk horizon is 0 to 127 cm [0 to 30 in])
2BCk3--152 to 203 cm (60 to 80 in); pink (7.5YR 7/4) extremely gravelly coarse sandy loam; brown (7.5YR 5/4) moist; weak very fine and fine granular structure; soft, very friable, nonsticky and nonplastic; 65 percent limestone gravel; 15 percent limestone cobbles; violently effervescent; slightly saline, moderately sodic; moderately alkaline.
TYPE LOCATION: Brewster County, Texas; from the junction of Texas Highways 170 and 118 at Study Butte; 4.0 miles west on Texas Highway 170 to intersection with South County Road; 1.6 miles north on South County Road; 0.55 miles west on road to Sawmill Mountain; and 40 feet south in rangeland. Terlingua USGS 7.5 minute topographic quadrangle Latitude: degrees, minutes, seconds, N; Longitude: degrees, minutes, seconds W; NAD83. UTM coordinates: 634593 meters E, 3245726 meters N, Zone 13.
RANGE IN CHARACTERISTICS:
Soil moisture: The moisture control section is dry in all parts more than three fourths of the time the soil temperature exceeds 5 degrees C (41 degrees F). Intermittently moist in some part of the soil moisture control section during June to September. More than 60 percent of the annual rainfall occurs during that period. The soil does not receive significant amounts of moisture during winter months. Ustic aridic soil moisture regime.
Soil temperature: 20.0 to 22.2 degrees C (68 to 72 degrees F). Hyperthermic soil temperature regime.
Depth to calcic horizon: 10 to 30 cm (4 to 12 in)
Reaction: slightly alkaline or moderately alkaline
Particle-size control section (weighted average):
Clay content: 6 to 18 percent
Rock fragments: 40 to 80 percent limestone gravel and cobbles
A Horizon
Hue: 7.5YR or 10YR
Value: 5 to 7, dry or moist
Chroma: 2 to 4
Texture of the fine-earth fraction: coarse sandy loam, sandy loam, fine sandy loam, or loam
Rock fragments: 35 to 60 percent, primarily gravel
Calcium carbonate equivalent: 35 to 70 percent
Effervescence: violent
Bk Horizon
Hue: 7.5YR or 10YR
Value: 5 to 8, dry or moist
Chroma: 2 to 4
Texture of the fine-earth fraction: coarse sandy loam, sandy loam, fine sandy loam, or loam
Rock fragments: 35 to 60 percent mainly gravel
Calcium Carbonate equivalent: 40 to 70 percent
Effervescence: violent
2BCk Horizon
Hue: 7.5YR or 10YR
Value: 5 to 7, dry or moist
Chroma: 2 to 4
Texture of the fine-earth fraction: loamy coarse sand, coarse sandy loam, sandy loam, or loam
Rock fragment content: 35 to 80 percent
Calcium carbonate equivalent: 40 to 80 percent
Sodium Adsorption Ratio (SAR): 20 to 35
Salinity: 2 to 8 dS/m
Effervescence: violent
COMPETING SERIES: There are no competing series in the same family. Similar series are the
Bankston (NM), and Cienaga (TX) series. These series have thermic soil temperature. In addition, Bankston soils are moderately deep to limestone bedrock. Cienaga soils occur at elevations of 1,070 to 1,370 m (3,500 to 4,500 ft), receive 305 to 380 mm (12 to 15 in) of annual precipitation, and support desert grassland vegetation.
GEOGRAPHIC SETTING:
Parent material: Pleistocene-age gravelly alluvium and pedisediment derived from limestone.
Landscape: piedmont slope
Landform: erosional remnants of mid-Pleistocene fans and pediments
Slopes: 1 to 30 percent
Mean annual air temperature: 20.0 to 22.2 degrees C (68 to 72 degrees F)
Mean annual precipitation: 254 to 330 mm (10 to 13 in)
Precipitation pattern: Most precipitation falls as intense rain during convectional afternoon thunderstorms from June to September. Precipitation during the months of January, February, and March is less than 13 percent of the total.
Frost-free period: 240 to 280 days
Elevation: 460 to 1,219 m (1,500 to 3,995 ft)
GEOGRAPHICALLY ASSOCIATED SOILS: These are the
Mariscal (TX), and the proposed
Blackgap (TX),
Studybutte (TX), and
Strawhouse (TX) soils. Blackgap and Mariscal soils are very shallow and shallow to limestone bedrock and are on hillsides and mountainsides. Studybutte soils are very shallow and shallow to igneous bedrock and are on hillsides and mountainsides. Strawhouse soils are very shallow and shallow to a petrocalcic horizon and are in higher, older pediments.
DRAINAGE AND PERMEABILITY: Well drained. Permeability is moderate. Runoff is low on 1 to 5 percent slopes, medium on 5 to 20 percent slopes, and high on 20 to 30 percent slopes.
USE AND VEGETATION: Rangeland used for livestock grazing, wildlife habitat, and recreation. Vegetation physiognomy is desert shrubland. Dominant woody plants are creosotebush, lechuguilla, catclaw acacia. Grasses include fluffgrass, red grama, slim tridens and perennial threeawns.
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 a mountain in southern Brewster County.
REMARKS: Diagnostic horizons and features recognized in this pedon are:
Ochric epipedon: 0 to 18 cm (0 to 7 in). (A horizon)
Calcic horizon: 18 to 203 cm (7 to 80 in). (Bk and 2BCk horizons)
Sodic feature: SAR is greater than 13 in all horizons below 50 cm (25 in). (2BCk horizons)
Soil temperature regime classification changed from thermic to hyperthermic based on the summary of a 6-year soil temperature study in Far West Texas. Taxonomic class and moisture regime classification changed from typic aridic to ustic aridic based on the recommendation by Phil Camp, MO8 Leader, Phoenix.
Classified according to Soil Taxonomy, Second Edition, 1999; Keys to Soil Taxonomy, Eleventh Edition, 2010
Revised for the correlation of Presidio County, Texas ; Oct, 2012, WWJ
ADDITIONAL DATA: NSSL data are available for 2 pedons from Brewster County, Texas, S91-TX-043-011, and S91-TX043-009.
National Cooperative Soil Survey
U.S.A.