LOCATION RIGGS AZInactive Series
TAXONOMIC CLASS: Fine, mixed, superactive, thermic Torrertic Haplustolls
TYPICAL PEDON: Riggs clay-rangeland. (Colors are for dry soil unless otherwise noted.)
A1--0 to 6 inches; brown (7.5YR 5/3) clay, dark brown (7.5YR 3/3) moist; moderate medium and fine granular structure; slightly hard, friable, very sticky and very plastic; common very fine, fine and medium roots; many fine interstitial pores; slightly effervescent;; moderately alkaline (pH 8.2); clear smooth boundary. (5 to 10 inches thick)
A2--6 to 38 inches; brown (7.5YR 5/2) clay, dark brown (7.5YR 3/2) moist; moderate medium and coarse subangular blocky structure; very hard, firm, very sticky and very plastic; common very fine, fine and medium roots; common fine and very fine tubular pores; common small pressure faces; common 1/4 to 1/2 inch vertical cracks; slightly effervescent; moderately alkaline (pH 8.2); gradual smooth boundary. (25 to 40 inches thick)
2Bk--38 to 60 inches; brown (7.5YR 5/2) and pink (7.5YR 7/4) heavy clay dark brown (7.5YR 4/2) and light brown (7.5YR 6/4) moist; massive; very hard, firm; very sticky and very plastic; few very fine and fine roots; many very fine and fine tubular pores; many soft masses; violently effervescent; 12 percent calcium carbonate equivalent; strongly alkaline (pH 8.6).
TYPE LOCATION: Cochise County, Arizona, about 7 miles southwest of McNeal; 1400 feet west, 1300 feet north of the southeast corner of Section 17, T. 22 S., R. 26 E.
RANGE IN CHARACTERISTICS:
Soil moisture - Intermittently moist in some part of the soil moisture control section during July through September and December through April. Moist in some part for more than 90 consecutive days.
Soil temperature - 62 to 71 degrees F.
Rock fragments - Less than 5 percent pebbles in the particle-size control section.
Thickness of mollic epipedon - Greater than 20 inches.
A horizon - Hue: 7.5YR or 10YR.
Value: 4 or 5 dry, 2 or 3 moist.
Chroma: 2 or 3 dry and moist.
Texture: Clay, silty clay, clay loam, silty clay loam (35 to 60 percent clay), common small pressure faces below 24 inches. Organic matter: 1 to 4 percent. Reaction: Mildly to very strongly alkaline. Conductivity: Less than 10 mmhos/cm. Cracks: Greater than 1 cm wide above 10 cm that remain open more than 180 days most years.
B horizon - Hue: 7.5YR or 10YR. Value: 4, 5, or 6 dry, 3, 4, or 5 moist. Chroma: 2 or 3 dry and moist. Texture: Clay, silty clay (40 to 65 percent clay). Drainage mottles: Iron oxide stains in some pedons below 30 inches. Carbonates: Soft masses; less than 15 percent calcium carbonate. Reaction: Moderately to very strongly alkaline. Conductivity: Less than 10 mmhos/cm.
In some areas a buried soil is a depths of 27 inches or more.
COMPETING SERIES: This is the Pecos series. Pecos soils have gypsum crystals. The Arno, Cogswell, Frio, Guest, and Nipsum series are in similar families. Arno soils lack a mollic epipedon. Cogswell and Frio soils have a calcic horizon. Guest soils are moist for less than 45 consecutive days when the soil temperature at a depth of 50 cm is above 5 degrees C. Nipsum soils have a cambic horizon.
GEOGRAPHIC SETTING: Riggs soils are on level to nearly level alluvial fans and flood plains at elevations of 2400 to 5000 feet. Slopes range from 0 to 2 percent. These soils formed in stream or fan alluvium from volcanic, sedimentary and granitic rock. The climate is semiarid continental. The average annual precipitation ranges from 10 to 16 inches occurring as summer thunderstorms and as gentle winter rains. The mean annual temperature ranges from 59 to 69 degrees F. The frost- free period is 180 to 280 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the Cogswell, Elfrida, Gothard, Hooks and Swisshelm soils. Cogswell and Elfrida soils have a calcic horizon and are fine-loamy. Gothard soils lack a mollic epipedon and have a natric horizon. Hook soils are fine-loamy. Swisshelm soils lack a mollic epipedon and are coarse-loamy.
DRAINAGE AND PERMEABILITY: Moderately well drained; slow runoff; very slow permeability.
USE AND VEGETATION: Used for grazing and irrigated cropland. Native vegetation is tobosa, vine-mesquite, sacaton, sideoats grama, cane bluestem and alkali sacaton. Irrigated crops are cotton and pasture.
DISTRIBUTION AND EXTENT: Southeastern Arizona. Riggs soils are moderately extensive.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Phoenix, Arizona
SERIES ESTABLISHED: Sulpher Springs Valley Area, Arizona, 1940.
REMARKS: Diagnostic horizons and features recognized in this pedon are:
Mollic epipedon - the zone from 0 to 38 inches (A1 and A2 horizons)
Cracking feature - soil cracks to depths of 20 inches or more, are at least 1 cm wide and are open for 6 months or more in most years.
COLE feature - is greater than 0.07