LOCATION AJO AZEstablished Series
TAXONOMIC CLASS: Loamy-skeletal, mixed, superactive, hyperthermic Argic Petrocalcids
TYPICAL PEDON: Ajo very gravelly loam - rangeland. (Colors are for dry soil unless otherwise noted.) 60 to 70 percent of surface covered with gravel.
A--0 to 2 inches; brown (7.5YR 5/4) very gravelly loam, brown (7.5YR 4/4) moist; weak medium platy structure; slightly hard, friable, slightly sticky and moderately plastic; common very fine roots; common very fine vesicular pores; 55 percent gravel; moderately alkaline (pH 8.0); abrupt wavy boundary. (1 to 4 inches thick)
Bt1--2 to 6 inches; reddish brown (5YR 4/4) extremely gravelly clay loam, dark reddish brown (5YR 3/4) moist; weak fine subangular blocky structure; slightly hard, friable, moderately sticky and moderately plastic; common fine roots; common very fine irregular pores; few faint clay films on faces of peds and lining pores; 60 percent gravel; moderately alkaline (pH 8.0); clear wavy boundary. (3 to 10 inches thick)
Bt2--6 to 13 inches; yellowish red (5YR 4/6) extremely gravelly clay loam, yellowish red (5YR 4/6) moist; weak fine subangular blocky structure; hard friable, moderately sticky and moderately plastic; many medium roots; common very fine irregular pores; few faint clay films on faces of peds and lining pores; 65 percent gravel; moderately alkaline (pH 8.2); clear wavy boundary. (6 to 12 inches thick)
Btk--13 to 24 inches; yellowish red (5YR 5/6) extremely gravelly clay loam, yellowish red (5YR 4/6) moist; massive; hard, friable, moderately sticky and moderately plastic; common very fine roots; common very fine irregular pores; few faint clay films lining pores; 70 percent gravel; calcium carbonate coatings on all gravel and few fine masses; strongly effervescent; moderately alkaline (pH 8.2); clear wavy boundary. (10 to 14 inches thick)
2Bkm--24 to 60 inches; indurated calcium carbonate cemented petrocalcic.
TYPE LOCATION: Pima County, Arizona; on Alamo County Road, Organ Pipe Cactus National Monument Survey Area; Northwest 1/4 of the Southeast 1/4 of section 35, T. 15 S., R. 5 W. (projected) lat. 32 degrees 4 minutes 31 seconds N. and long. 112 degrees 45 minutes 64 seconds W., NAD 83.
RANGE IN CHARACTERISTICS:
Soil Moisture - Intermittently moist in some part of the soil moisture control section during July - September and December - February. Driest during May and June. Typic aridic soil moisture regime.
Soil temperature - 72 to 78 degrees F.
Rock Fragments - averages 35 to 90 percent in the control section
Depth to petrocalcic horizon - 20 to 40 inches
Calcium carbonate - Noncalcareous in the upper part; calcareous below
A horizon
Hue: 7.5YR, 10YR
Value: 3 to 6 dry, 3 or 4 moist
Chroma: 3 or 4, dry or moist
Reaction: slightly or moderately alkaline
B horizon
Hue: 2.5YR, 5YR, 7.5YR
Value: 4, 5 or 6 dry, 3, 4 or 5 moist
Chroma: 3 to 8 dry, 4 or 6 moist
Texture: clay loam, sandy clay loam; can include sandy loam in some pedons (averages more than 18 percent clay)
COMPETING SERIES: There are no competing series.
GEOGRAPHIC SETTING: Ajo soils are on fan terraces at elevations of 800 to 2,400 feet. Slopes are dominantly 2 to 5 percent, but range from 1 to 25 percent. These soils formed in stratified fan alluvium from mixed rock sources. The mean annual precipitation ranges from 4 to 10 inches, about half of which falls as summer thunderstorms during the months of July, August and September. The mean annual air temperature ranges from 67 to 76 degrees F. The frost-free period is 240 to 350 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the Cipriano, Gachado and Harqua soils. Cipriano soils have a duripan at shallow depths. Gachado soils have a lithic contact at depths less than 20 inches. Harqua soils do not have a petrocalcic horizon.
DRAINAGE AND PERMEABILITY: Well drained; slow runoff; moderately slow permeability.
USE AND VEGETATION: This soil is used for livestock grazing. Vegetation is a sparse cover of creosotebush, ocotillo, saguaro, cholla cacti, paloverde, triangleleaf bursage, sixweeks grama and fluffgrass.
DISTRIBUTION AND EXTENT: Southern and southwestern Arizona. This soil is moderately extensive. MLRA is 40.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Phoenix, Arizona
SERIES ESTABLISHED: Pima County (Organ Pipe Cactus National Monument Survey Area), Arizona; 1971.
REMARKS: Diagnostic horizons and features recognized in this pedon are:
Ochric epipedon - The zone from 0 to 2 inches (A horizon)
Argillic horizon - The zone from 2 to 24 inches (Bt1, Bt2, Btk horizons)
Petrocalcic horizon - The zone from 24 to 60 inches (2Bkm horizon)
Classified according to Soil Taxonomy, Second Edition, 1999; and Keys to Soil Taxonomy Tenth Edition, 2006.