LOCATION NILRAP WYEstablished Series
TAXONOMIC CLASS: Clayey over loamy-skeletal, smectitic over mixed, superactive, frigid Aridic Argiustolls
TYPICAL PEDON: Nilrap silt loam--on a northwest facing slope of 2 percent utilized as cropland (small grains). (Colors are for dry soil unless otherwise stated.)
Ap--0 to 8 inches, grayish brown (10YR 5/2) silt loam, very dark brown (10YR 2/2) moist; moderate medium and fine granular structure, slightly hard, very friable, slightly sticky and slightly plastic; common fine roots; neutral (pH 7.2); abrupt smooth boundary. (4 to 10 inches thick)
Bw--8 to 15 inches; grayish brown (10YR 5/2) silt loam, dark grayish brown (10YR 3/2) moist; moderate medium and fine granular structure; hard, friable, sticky and plastic; many fine roots; neutral (pH 7.2) clear wavy boundary. (0 to 8 inches thick)
Bt1--15 to 32 inches; brown (7.5Y 5/2) clay loam, dark brown (7.5Y 4/2) moist; moderate fine angular blocky structure; hard, firm, sticky and plastic; common fine roots; thin continuous clay films on faces of peds; mildly alkaline (pH 7.4), clear wavy boundary.
Bt2--32 to 37 inches; light brown (7.5Y 6/4) clay, brown (7.5Y 5/4) moist; weak coarse angular blocky structure; very hard, firm, sticky and plastic; few fine roots; few thin clay films on faces of peds; mildly alkaline (pH 7.4), clear wavy boundary. (The combined thickness of the Bt is 17 to 25 inches.)
2Bk--37 to 60 inches; pink (7.5Y 7/4) very gravelly loam, light brown (7.5Y 6/4) moist; massive, hard, firm, sticky and plastic; few fine roots; 35 percent gravel; strongly effervescent, lime is segregated in seams and soft masses; moderately alkaline (pH 8.2).
TYPE LOCATION: Weston County, Wyoming; 875 feet north and 50 feet east of the center of sec. 8, T. 47 N., R. 60 W.
RANGE IN CHARACTERISTICS: The mollic epipedon ranges from 7 to 15 inches thick. Depth to calcareous material ranges from 20 to 37 inches. Depth to the base of the argillic horizon is 20 inches or more. The Bk horizon contains 35 to 50 percent gravel and less than 15 percent calcium carbonate equivalent. The mean annual soil temperature is 44 degrees F., and the mean annual summer temperature is 60 degrees F. or more. Depth to bedrock is greater than 60 inches.
The A horizon has hue of 7.5YR or 10YR, value of 4 or 5 dry, 2 or 3 moist, and chroma of 2 or 3. Texture is typically a silt loam but may be loam. Reaction is neutral or mildly alkaline.
The Bt horizon has hue of 7.5YR or 5YR, value of 4 through 6 dry, 3 through 5 moist, and chroma of 2 through 4. It is typically a clay but may be a clay loam. Clay ranges from 35 to 50 percent. Reaction is neutral or mildly alkaline.
The Bk horizon has hue of 7.5YR or 5YR, value of 6 or 7 dry, 5 or 6 moist, and chroma of 3 or 4. Texture is very gravelly loam or very gravelly clay loam with 35 to 50 percent coarse fragments. Lime occurs as segregated seams and soft masses. Calcium carbonate equivalent is 5 to 14 percent. Reaction is mildly through moderately alkaline.
COMPETING SERIES: These are the Ethridge, Holderness, Morop, Nunnston, Scobey, Tanna, and Ulric. Ethridge soils have accumulations of gypsum below 24 inches. Holderness soils lack carbonates above 40 inches. Morop soils have 15 to 35 percent coarse fragments in the control section. Nunnston soils have Bt horizons with hue of 10YR or yellower. Scobey soils formed in till. Tanna soils have a paralithic contact at 20 to 40 inches. Ulric soils have bedrock at 40 to 60 inches.
GEOGRAPHIC SETTING: Nilrap soils are on nearly level to sloping alluvial fan aprons, fan piedmonts, and terraces formed during the late Pleistocene epoch. Slopes range from 0 to 10 percent. These may be dissected in some areas by ephemeral streams. Elevation is 5,000 to 6,000 feet. The average annual precipitation is 15 to 19 inches with over half of the annual precipitation falling in April, May, and June and less than one inch falling in each month of July, August, September, and October. The average annual temperature is 41 to 47 degrees F. The frost-free season is 90 to 100 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the Vasset soils. The Vassett soils are fine-silty and lack the coarse fragments in the substratum.
DRAINAGE AND PERMEABILITY: Well drained; medium and slow runoff; moderately slow permeability.
USE AND VEGETATION: These soils are used for hayland, cropland, and rangeland. The principal crops are winter wheat and alfalfa. Native vegetation consists of green needlegrass, rhizomatous wheatgrass, needleandthread, and blue grama.
DISTRIBUTION AND EXTENT: Black Hills area of northeast Wyoming.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Denver, Colorado
SERIES ESTABLISHED: Weston County, Wyoming; 1983.