LOCATION PLATMAK WYEstablished Series
TAXONOMIC CLASS: Fine, smectitic, mesic Aridic Paleustolls
TYPICAL PEDON: Platmak-rangeland. (Colors are for dry soil unless otherwise stated)
A--0 to 2 inches; brown (10YR 5/3) loam, very dark grayish brown (10YR 3/2) moist; weak medium and thin platy structure; soft, very friable, slightly sticky and plastic; many very fine roots; neutral (pH 7.0); abrupt smooth boundary. (Combined A and E horizons 1 to 7 inches thick)
Bt--2 to 13 inches; dark yellowish brown (10YR 4/4) clay loam, dark brown (10YR 3/3) moist; moderate medium prismatic structure parting to moderate medium and fine subangular blocky; common fine and few coarse roots; hard, firm, sticky and plastic; many prominent clay films on faces of peds; slightly alkaline (pH 7.6); clear smooth boundary. (8 to 14 inches thick)
Bk1--13 to 17 inches; pale brown (10YR 6/3) clay loam, yellowish brown (10YR 5/4) moist; moderate medium and coarse angular blocky structure parting to moderate fine and very fine subangular blocky; hard, friable, sticky and plastic; few faint threads and seams of secondary calcium carbonate; strongly effervescent; slightly alkaline (pH 7.8); clear smooth boundary.
Bk2--17 to 60 inches; very pale brown (10YR 8/4) loam, yellowish brown (10YR 5/4) moist; moderate medium and coarse subangular blocky structure; slightly hard, sticky and plastic; few faint masses of secondary calcium carbonate; strongly effervescent; strongly alkaline (pH 8.5).
TYPE LOCATION: Sheridan County (Sheridan Area), Wyoming; SW1/4, NW1/4, sec. 29, T. 57 N., R. 83 W.
RANGE IN CHARACTERISTICS: The mollic epipedon ranges from 7 to 15 inches thick. The mean annual soil temperature ranges from 47 to 53 degrees F. The soil temperature at a depth of 20 inches is 41 degrees F. or more for 145 to 192 days. The soils are moist in some or all parts of the moisture control section from April 15 to June 15 but are dry in all parts for 60 consecutive days following July 15 and 90 cumulative days from July 15 to October 25. Rock fragments range from 0 to 15 percent throughout. EC is less than 2 mmhos throughout.
The A horizon has hue of 10YR or 2.5Y, value of 4 or 5 dry, 2 or 3 moist, and chroma of 2 or 3. It is loam, very fine sandy loam or fine sandy loam. It is neutral or slightly alkaline. Those pedons that do not have an A horizon, have an E horizon.
The Bt horizon has hue of 10YR or 2.5Y, value of 4 to 6 dry, 3 or 4 moist, and chroma of 2 to 4. Texture is clay or clay loam with 35 to 50 percent clay with more than 15 percent fine or coarser sand. It is neutral to moderately alkaline. A Btk is present in some pedons.
The Bk and C horizons have hue of 10YR or 2.5Y, value of 4 to 8 dry, 3 to 5 moist, and chroma of 2 to 4. Texture is loam or clay loam. Calcium carbonate equivalent ranges from 5 to 15 percent when averaged. It is slightly alkaline to strongly alkaline.
COMPETING SERIES: These are the Dawes, Fondis, Iliff, Jonpol, Platner and Platsher series. Dawes soils have a coarse sand 2C horizon. Fondis series have less than 15 percent fine or coarser sand in the Bt horizon. Iliff soils have a lithic contact at 20 to 40 inches. Jonpol soils have a paralithic contact at 20 to 40 inches. Platner soils have soil temperatures at 20 inches that are 41 degrees F. or warmer for over 192 days. Platsher soils have gravelly substrata between 20 and 40 inches.
GEOGRAPHIC SETTING: Platmak soils are on alluvial fans, fan remnants, flats, plateaus, terraces, mesas and lower hills. They formed in residuum and alluvium derived from shales. Slopes are 0 to 25 percent. Elevations range from 3,500 to 5,000 feet. The mean annual precipitation is 10 to 17 inches of which about half falls as snow or rain in April, May, and early June. The frost-free period is about 105 to 120 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the competing Jonpol and Platsher soils and the Absted, Arvada, Bidman, Cambria, Cushman, Forkwood, Recluse, Renohill and Ulm soils. Absted and Arvada soils have natric horizons. Bidman, Cambria, Cushman, Forkwood, Renohill and Ulm soils lack a mollic epipedon. Recluse soils have fine-loamy particle-size control sections. Absted, Arvada, Bidman, Cambria, Forkwood, Platscher, Recluse and Ulm soils occur on similar positions. Cushman, Jonpol and Renohill soils occur on shoulders and summits on hills and ridges.
DRAINAGE AND PERMEABILITY: Well drained; slow or medium runoff; slow permeability.
USE AND VEGETATION: Rangeland, irrigated and dryland crops, and wildlife habitat. Native vegetation consists of prairie junegrass, western wheatgrass, needleandthread grass, Idaho fescue and big sagebrush.
DISTRIBUTION AND EXTENT: North central and northeastern Wyoming. The series is of moderate extent.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Denver, Colorado
SERIES ESTABLISHED: Sheridan County (Sheridan Area), Wyoming; 1987.
REMARKS: Diagnostic horizons and features recognized in this pedon are: Mollic epipedon - 0 to 13 inches (A,Bt); Argillic horizon - 2 to 13 inches (Bt); This pedon has an absolute increase in clay at the upper boundary of the Bt horizon of more than 20 percent within a vertical distance of 7.5 cm; Aridic subgroup - Usually dry in some part or all of the moisture control section when the soil temperature is 41 degrees F. or warmer; SIR - WY0759