LOCATION ROBBS IL+KYEstablished Series
TAXONOMIC CLASS: Fine-silty, mixed, active, mesic Aquic Fragiudalfs
TYPICAL PEDON: Robbs silt loam (cultivated). (Colors are for moist soil unless otherwise stated.)
Ap--0 to 8 inches; dark grayish brown (10YR 4/2) silt loam; weak medium granular structure; friable; common black (iron-manganese) concretions; moderately acid; abrupt smooth boundary. (6 to 10 inches thick)
E--8 to 13 inches; yellowish brown (10YR 5/4) silt loam; weak medium platy structure parting to weak medium granular; friable; common black (iron-manganese) concretions; very strongly acid; clear smooth boundary. (2 to 12 inches thick)
Bt1--13 to 21 inches; yellowish brown (10YR 5/4) silty clay loam; weak medium subangular blocky structure; firm; few thin clay films on faces of peds; common medium distinct grayish brown (10YR 5/2) and light brownish gray (10YR 6/2) iron depletions; common medium distinct brownish yellow (10YR 6/6) masses of iron and manganese accumulations; few black (iron-manganese) concretions; very strongly acid; abrupt smooth boundary.
Bt2--21 to 26 inches; variegated light brownish gray (10YR 6/2), yellowish brown (10YR 5/4), and brownish yellow (10YR 6/6) silty clay loam; moderate medium prismatic structure parting to moderate medium angular blocky; firm; thick light gray (10YR 7/2) silt coatings on peds; very strongly acid; abrupt wavy boundary. (Combined thickness of the Bt horizon is 8 to 15 inches.)
Btx1--26 to 43 inches; variegated light yellowish brown (10YR 6/4), grayish brown (10YR 5/2), and light brownish gray (10YR 6/2) silty clay loam; moderate medium angular blocky structure within larger polygons surrounded by gray material; firm, brittle; thick continuous grayish brown (10YR 5/2) clay films; many black (iron-manganese) concretions and reddish brown (5YR 4/4) iron stains; very strongly acid; clear smooth boundary.
Btx2--43 to 49 inches; variegated light yellowish brown (10YR 6/4), grayish brown (10YR 5/2), and brown (10YR 5/3) silty clay loam; weak medium and coarse angular blocky structure within large polygons surrounded by gray material; firm, brittle; discontinuous thin dark grayish brown (10YR 4/2) clay films; common black (iron and manganese) concretions and reddish brown (5YR 4/4) iron stains; strongly acid; gradual smooth boundary. (Combined thickness of the Btx horizon is 15 to 30 inches)
C--49 to 60 inches; yellowish brown (10YR 5/6) silt loam; massive; friable; many medium distinct light gray (10YR 7/2) iron depletions; strongly acid.
TYPE LOCATION: Johnson County, Illinois; about 3 miles southeast of Bloomfield; 915 feet east and 1,350 feet south of northeast corner of sec. 36, T. 12 S., R. 3 E.
RANGE IN CHARACTERISTICS: Solum thickness typically is 40 to 55 inches, but it ranges to 70 inches in thickness. Reaction ranges from medium acid to very strongly acid in the upper part and from very strongly acid to strongly acid in the lower part. The depth to the top of the fragipan is greater than 20 inches. Sand content is less than 10 percent and rock fragment content is less than 1 percent in the series control section.
The A or Ap horizon has hue of 10YR, value of 4 or 5, and chroma of 2 or 3. Some undisturbed pedons have value of 3, and chroma of 1 or 2, but the horizon is less than 6 inches thick. The A horizons that have not been limed are generally strongly acid.
The Bt horizon has hue of 10YR, value of 5 or 6, and chroma of 2 to 6. It is typically silty clay loam. It generally has prismatic structure in the upper part and angular blocky structure in the lower part. Subangular blocky is not excluded.
The Btx horizon (fragipan) has hue of 10YR, value of 5 or 6, and chroma of 2 to 4. It is firm or very firm when moist and hard and brittle or fragile when dry. It is silty clay loam or silt loam.
COMPETING SERIES: These are the Haubstadt, Nabb, Pierpont, Plumfield, Sciotoville, and Shakamak soils in the same family and the Chalfont soils in a closely related family (cation exchange activity class not assigned). Haubstadt soils have loam fragipans formed in alluvium. Nabb and Shakamak soils have a series control section that extends to a depth of more than 70 inches. Pierpont soils are less acid than strongly acid in the lower part of the solum, typically have carbonates within a depth of 60 inches, and typically contain more rock fragments in the lower part of the series control section. Plumfield soils have a fragipan within a depth of 20 inches. Sciotoville soils have mica flakes throughout the profile and are stratified in the lower part of the series control section.
GEOGRAPHIC SETTING: Robbs soils are on nearly level to gently sloping ridgetops in unglaciated areas. The Robbs soils formed in 4 feet or more of loess underlain, commonly, by sandstone bedrock. Slope gradient ranges from 0 to 3 percent. Climate consists of hot summers and cold winters with mean annual temperature between 54 and 57 degrees F, and precipitation between 42 and 48 inches per year.
GEOGRAPHICALLY ASSOCIATED SOILS: The Grantsburg soils are the moderately well drained member of the drainage sequence and are on shoulders and ridges above the Robbs soils. Zanesville soils are also mapped in the association with Robbs soils and are on higher positions.
DRAINAGE AND PERMEABILITY: Somewhat poorly drained. The potential for surface runoff is low or medium. Permeability is slow or very slow.
USE AND VEGETATION: The major acreage cropped to corn, soybeans, small grain and meadow. Native vegetation is hardwood forest.
DISTRIBUTION AND EXTENT: Southern Illinois and Kentucky. Extent is small.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Indianapolis, Indiana
SERIES ESTABLISHED: Williamson County, Illinois, 1955.
REMARKS: Diagnostic horizons and features recognized in this pedon are:
Ochric epipedon -- the zone from the surface to a depth of 13 inches (Ap and E horizons);
Argillic horizon -- the zone from 13 inches to 49 inches (Bt1, Bt2, Btx1, Btx2 horizons);
Frangipan -- the zone from 26 to 49 inches (Btx1 and Btx2 horizons)