LOCATION RIO TXEstablished Series
TAXONOMIC CLASS: Fine, mixed, active, hyperthermic Typic Argiaquolls
TYPICAL PEDON: Rio clay loam--cropland. (Colors are for dry soil unless otherwise stated.)
Ap--0 to 6 inches; dark gray (10YR 4/1) clay loam, very dark gray (10YR 3/1) moist; weak fine granular structure; hard, friable, sticky and plastic; neutral; clear wavy boundary. (4 to 9 inches thick)
A--6 to 12 inches; dark gray (10YR 4/1) clay loam, very dark gray (10YR 3/1) moist; moderate fine subangular blocky structure; hard, friable, sticky and plastic; few fine pores; neutral; clear smooth boundary. (3 to 8 inches thick)
Btg1--12 to 18 inches; dark gray (10YR 4/1) clay, very dark gray (10YR 3/1) moist; few fine distinct reddish brown (5YR 5/4) and strong brown (7.5YR 5/6) mottles; moderate medium blocky structure; very hard, very firm, sticky and plastic; few fine pores; clay on faces of peds; noncalcareous; mildly alkaline; gradual wavy boundary. (4 to 12 inches thick)
Btg2--18 to 38 inches; gray (10YR 5/1) clay, dark gray (10YR 4/1) moist; few fine distinct brownish yellow (10YR 6/8) and strong brown (7.5YR 5/8) mottles; moderate medium and fine blocky structure; very hard, very firm, sticky and plastic; few fine pores; clay on faces of peds; noncalcareous; mildly alkaline; gradual wavy boundary. (8 to 24 inches thick)
Btkg--38 to 58 inches; grayish brown (10YR 5/2) clay loam, dark grayish brown (10YR 4/2) moist; few fine distinct strong brown (7.5YR 5/8) mottles; weak medium blocky structure; very hard, very firm, sticky and plastic; patchy, clay films; few films and threads of calcium carbonate and few calcium carbonate concretions; calcareous; moderately alkaline; gradual wavy boundary. (8 to 20 inches thick)
Ckg--58 to 63 inches; light brownish gray (10YR 6/2) clay loam, grayish brown (10YR 5/2) moist; massive, very hard, very firm, about 3 percent by volume of soft lumps and concretions of calcium carbonate; calcareous; moderately alkaline.
TYPE LOCATION: Hidalgo County, Texas; about 7.5 miles northeast of Edinburg, Texas; 6 miles north of county courthouse on U.S. Highway 281, then 3.25 miles east on Farm Road 2812 and 300 feet north of county road in cultivated field.
RANGE IN CHARACTERISTICS: Solum thickness is 32 to 70 inches. The mollic epipedon is 16 to 32 inches thick and extends into the upper Bt horizon. Soil salinity is 0.5 to 4 mmhos/cm and remains constant or increases with depth. In irrigated areas some pedons have a salinity of 4 to 16 mmhos/cm.
The A horizon is gray (10YR 5/1), dark gray (10YR 4/1), very dark gray (10YR 3/1), grayish brown (10YR 5/2), dark grayish brown (10YR 4/2), or very dark grayish brown (10YR 3/2), loam, fine sandy loam, sandy clay loam, or clay loam. Structure is weak granular or fine subangular blocky. It is neutral or mildly alkaline.
The Bt horizon is very dark gray (10YR 3/1; N 3/0), dark gray (10YR 4/1; N 4/0), gray (10YR 5/1; N 5/0), or grayish brown (10YR 5/2). Few, common, or many prominent or distinct mottles are reddish brown (5YR 5/4), yellowish red (5YR 5/6, 5/8), brownish yellow (10YR 6/6, 6/8), and strong brown (7.5YR 5/6) where matrices have chromas of 2. It is a clay or clay loam with clay content of 35 to 50 percent. Reaction is mildly alkaline or moderately alkaline.
The C horizon is gray (10YR 6/1; N 5/0, 6/0), light brownish gray (10YR 6/2), pale brown (10YR 6/3), or very pale brown (10YR 7/3). It is sandy clay loam, clay loam, or sandy clay. Calcium carbonate content ranges from 1 to about 5 percent by weight.
COMPETING SERIES: There are no series in the same family. Similar soils include Carwile, Clareville, Muldrow, Orelia, and Racombes series. Carwile and Muldrow soils have mean annual soil temperatures less than 72 degrees F. Clareville and Racombes soils as well as Orelia soils have less than 35 percent clay in the upper 20 inches of the Bt horizon. Orelia soils have a surface horizon that is very hard and massive.
GEOGRAPHIC SETTING: Rio soils are on nearly level to depressional areas of coastal uplands. Slope gradients are less than 0.5 percent. The soil formed in fine textured
sediments of deltic or alluvial origin. The climate is dry subhumid to semiarid. The mean annual precipitation ranges from about 18 to 34 inches and the mean annual temperature ranges from 72 to 75 degrees F. Frost-free days range from 310 to 350 days and elevation ranges from 5 to 500 feet. Thornthwaite annual P-E index ranges from 22 to 44.
GEOGRAPHICALLY ASSOCIATED SOILS: These include the competing Racombes series and Brennan, Hidalgo, Pharr, Tiocano, and Willacy series. Racombes soils occur in drainageways at slightly higher elevations. Brennan, Hidalgo, Pharr, Raycombes, and Willacy soils occur on adjacent uplands and have less than 35 percent clay in their control sections and have no mottles due to wetness. Tiocano soils occur in similar positions and are clayey throughout and have intersecting slickensides.
DRAINAGE AND PERMEABILITY: Somewhat poorly drained. Runoff is very slow to ponded. Permeability is slow.
USE AND VEGETATION: Used for native range, tame pasture, and cultivated crops. Cultivated areas are generally drained or leveled. Native grasses are mostly fourflower trichloris, hooded windmillgrass, signalgrass, and buffalograss. Woody vegetation is mostly huisache, retama, mesquite, and lotebush.
DISTRIBUTION AND EXTENT: Mainly in the southern Rio Grande Plain of Texas. The series is inextensive with a total area of about 5,000 acres.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Temple, Texas
SERIES ESTABLISHED: Hidalgo County, Texas 1944. (Emergency Rubber Project, Soil Survey of Hargill-La Sara Area).
REMARKS: Diagnostic horizons and features recognized in this pedon are:
Mollic epipedon - 0 to 18 inches (Ap, A, and Btg1 horizons)
Argillic horizon - 12 to 58 inches (Btg1, Btg2, and Btkg horizons)
Aquic feature - Distinct mottles in the lower part of the mollic epipedon.