LOCATION TOBOSA TX+OKEstablished Series
TAXONOMIC CLASS: Fine, smectitic, thermic Aridic Haplusterts
TYPICAL PEDON: Tobosa clay--cropland. At center of microflat. (Colors are for dry soil unless otherwise stated.)
Ap--0 to 12 inches; dark grayish brown (10YR 4/2) clay, very dark grayish brown (10YR 3/2) moist; moderate very fine and fine subangular blocky structure; very hard, very firm, sticky and plastic; common fine roots; few siliceous pebbles; slightly effervescent; moderately alkaline; clear smooth boundary.
A--12 to 16 inches; dark grayish brown (10YR 4/2) clay, very dark grayish brown (10YR 3/2) moist; strong medium angular blocky structure that form wedge-shaped aggregates with long axis tilted 5 to 30 degrees from the horizontal; extremely hard, very firm, sticky and plastic; few fine roots; few siliceous pebbles; strongly effervescent; moderately alkaline; gradual wavy boundary. (Combined thickness of the A horizon is 10 to 40 inches)
Bss1--16 to 44 inches; dark grayish brown (10YR 4/2) clay, grayish brown (10YR 3/2) moist; strong medium and coarse angular blocky structure that forms wedge-shaped aggregates with long axis tilted 5 to 30 degrees from the horizontal; extremely hard, very firm, very sticky and very plastic; few fine roots; few fine pores; common distinct slickensides; few siliceous pebbles; strongly effervescent; moderately alkaline; diffuse wavy boundary. (20 to 35 inches thick)
Bss2--44 to 60 inches; brown (10YR 5/3) clay, dark brown (10YR 4/3) moist; moderate medium angular blocky structure that forms wedge-shaped aggregates with their long axis tilted 5 to 30 degrees from the horizontal; very hard, very firm, very sticky and very plastic; few fine roots; common vertical streaks of darker soil from above; common grooved slickensides; few siliceous pebbles; few soft masses of calcium carbonate in lower 10 inches; violently effervescent; moderately alkaline; gradual wavy boundary. (10 to 32 inches thick)
BCk--60 to 80 inches; pink (7.5YR 7/4) clay, light brown (7.5YR 6/4) moist; weak medium and coarse angular blocky structure; extremely hard, very firm, very sticky and plastic; 15 percent soft masses of calcium carbonate; violently effervescent; moderately alkaline.
TYPE LOCATION: Tom Green County, Texas; From San Angelo, approximately 10 miles northeast to intersection of U.S. Highway 67 and Farm Road 1692, 6.2 miles in a northeasterly direction along Farm Road 1692, 100 feet south in field.
(Latitude: 31N, 36, 35; Longitude: 100W, 14, 54)
RANGE IN CHARACTERISTICS: Solum thickness ranges from 60 to more than 80 inches. When dry these soils have cracks as much as 1 inch wide that extend from the surface to depths of 20 to 60 inches. Cracks remain open for more than 210 cumulative days in most years. Slickensides begin at depths of 14 to 30 inches. Texture is silty clay or clay throughout. Weighted average silicate clay content of the particle size control section ranges from 35 to 55 percent. Carbonate clay content ranges from 1 to about 12 percent. Siliceous pebbles and limestone pebbles, cobbles and stones comprise 0 to 15 percent. Cycles in this soil consist of microflats and microdepressions with the microdepressions occupying 5 to 25 percent of the pedon which is repeated horizontally each 12 to 24 feet.
The A horizon has hue of 7.5YR to 10YR, value of 3 to 5, and chroma of 2 to 4.
The Bss horizon has hue of 7.5YR to 10YR, value of 4 to 6, and chroma of 2 to 4. Visible calcium carbonate content ranges from 0 to 10 percent in the form of masses and concretions.
The BCk horizon has hue of 5YR to 10YR, value of 5 to 8, and chroma of 2 to 6. Calcium carbonate content ranges from 2 to 50 percent. Some pedons are underlain by C horizons of marl, shale, or limestone below 40 inches.
COMPETING SERIES: There are no competing series. Similar soils are the Campwood, Lazbuddie, Leeray, Roscoe, and Stamford series. All of these soils are dry in the moisture control section for less than 210 cumulative days in most years. In addition, Campwood and Leeray soils have P-E indices greater than 36 and Leeray soils have slight to moderate salinity in the lower subsoil. Lazbuddie and Roscoe soils have chroma of less than 2 in the upper part. Stamford soils have hue of 5YR or redder throughout the pedon.
GEOGRAPHIC SETTING: Tobosa soils are on nearly level to gently sloping alluvial plains in wide valleys, wide divides or mesas, and small depressional areas. Slope gradients are mostly less than 1 percent, but range up to as much as 3 percent. The soil formed in brownish and yellowish calcareous clayey alluvium that is several feet thick. Mean annual precipitation ranges from 15 to 25 inches, and mean annual temperature ranges from 64 to 70 degrees F. Frost-free days range from 210 to 240, and elevation ranges from 1,100 to 2,750 feet. Thornthwaite P-E indices range from 23 to 36.
GEOGRAPHICALLY ASSOCIATED SOILS: These are Kavett, Lipan, and Valera series. Kavett and Valera soils have sola less than 40 inches thick. Lipan soils occur in depressions and have chromas of less than 1.5 in the upper 12 inches.
DRAINAGE AND PERMEABILITY: Well drained. Permeability is very slow. Runoff is low on less than 1 percent slopes and medium on 1 to 3 percent slopes. Water enters the soil rapidly when dry and cracked and very slowly when moist.
USE AND VEGETATION: Used for cropland and range. Small grains, sorghums, and cotton are the main crops. Native vegetation is tobosa, buffalograss, curlymesquite, mesquite, and cacti.
DISTRIBUTION AND EXTENT: Mainly in the Edwards Plateau and Rolling Plains of Texas. The series is extensive.
MLRA OFFICE RESPONSIBLE: Temple, Texas
SERIES ESTABLISHED: Crockett County, Texas; 1930.
REMARKS: Classification change from Typic Chromusterts to Aridic Haplusterts based on Amendment 16, SOIL TAXONOMY (2/94). Diagnostic horizons and features recognized in this pedon are:
Ochric epipedon - 0 to 16 inches. (Ap and A horizons)
Vertic feature - Slickensides and wedge-shaped aggregates - 16 to 60 inches. (Bss horizons)
Cambic horizon - 16 to 60 inches. (Bss horizon)
ADDITIONAL DATA: TAMU Laboratory data No. S84TX-105-003 from Crockett County.
Soil Interpretation Record Nos.: TX0092; DEPRESSIONAL -TX1158; LIMESTONE SUBSTRATUM - TX1021