LOCATION POST MTEstablished Series
TAXONOMIC CLASS: Very-fine, illitic, frigid Typic Natrixerolls
TYPICAL PEDON: Post silt loam, in cropland (colors are for dry soil unless otherwise noted).
Ap--0 to 6 inches, grayish brown (10YR 5/2) silt loam, very dark grayish brown (10YR 3/2) moist; weak medium platy structure; slightly hard, friable, slightly sticky and slightly plastic; many very fine and fine roots; many very fine tubular pores; neutral (pH 6.6); abrupt smooth boundary. (6 to 10 inches thick)
Btn/E--6 to 9 inches; B part (55 percent) very pale brown (10YR 7/3) and pink (7.5YR 7/4) silty clay, brown (7.5YR 5/4) moist; E part (45 percent) light gray (10YR 7/1) and light brownish gray (10YR 6/2) silt loam, brown (10YR 5/3) moist that interfingers into B part; strong medium columnar structure parting to moderate fine and medium angular blocky; very hard, firm, sticky and plastic; many very fine roots between prism faces and few very fine roots within peds; many very fine and few medium tubular pores; moderately alkaline (pH 7.9) clear smooth boundary. (9 to 12 inches thick)
Btn--9 to 21 inches; pink (7.5YR 7/4) clay, brown (7.5YR 5/4) moist; strong medium prismatic structure parting to moderate fine and medium angular blocky; very hard, very firm, very sticky and very plastic; many very fine roots between prism faces and common very fine roots within peds; common very fine and fine tubular pores; thick continuous brown (10YR 5/2) and brown (7.5YR 4/4) moist clay films on faces of peds and in pores; common moderately thick gray (10YR 5/1) silt film in pores and on ped faces; strongly alkaline (pH 8.6); gradual smooth boundary. (9 to 12 inches)
Bkn--21 to 27 inches; very pale brown (10YR 7/3) clay, brown (10YR 5/3) moist; consisting of 1/4- to 1/2-inch thick varves; massive; hard, firm, very sticky and very plastic; common very fine roots; common very fine tubular pores; 5 percent pebbles; few fine masses of soft lime between varves; strongly effervescent; strongly alkaline (pH 8.6); clear smooth boundary. (4 to 8 inches thick)
C--27 to 60 inches; very pale brown (10YR 7/3) clay, brown (10YR 5/3) moist; consisting of 1/4- to 1/2-inch thick varves; massive; hard, firm, very sticky and very plastic; 5 percent pebbles; few fine masses of soft lime between varves and calcareous fragments in the varves; strongly effervescent; strongly alkaline (pH 8.6).
TYPE LOCATION: Lake County, Montana; 420 feet east and 132 feet north of the SW corner of sec. 30, T. 20 N., R. 20 W.
RANGE IN CHARACTERISTICS:
Soil temperature - 44 to 47 degrees F.
Moisture control section - approximately between the depths of 4 and 12 inches.
Mollic epipedon thickness - 7 to 9 inches thick.
Control section - 60 to 80 percent clay and less than 10 percent fine sand or coarser.
Depth to Bk horizon - 17 to 26 inches.
Depth to varves - 17 to 26 inches.
Ap horizon - Value: 4 or 5 dry; 2 or 3 moist
Chroma: 2 or 3
Texture, less than 2 mm: silt loam or silty clay loam
Clay content: 20 to 35 percent
Rock fragments: 0 to 15 percent--0 to 5 percent cobbles; 0 to 10 percent pebbles
SAR: 4 to 10
Reaction: pH 6.1 to 7.3
Uncultivated areas have A1 horizons.
Bt/E horizons - Hue: 10YR or 7.5YR
Value: B part 6 or 7 moist
Chroma: B part 3 or 4, E part 2 or 1
Texture, less than 2 mm: B part silty clay loam or silty clay; E part silty loam or loam; mixed silty clay loam
Clay content: B part 35 to 45 percent; E part 18 to 27 percent; mixed 27 to 40 percent
Rock fragments: 0 to 15 percent--0 to 5 percent cobbles; 0 to 10 percent pebbles
SAR: 13 to 20
Reaction: pH 7.9 to 9.0
Some pedons have a white E horizon in uncultivated areas.
Bt horizon - Hue: 7.5YR or 10YR
Value: 7 or 6 dry
Chroma: 2, 3, or 4
Structure: moderate or strong columnar in the upper part
Clay content: 60 to 80 percent, 2 to 8 percent fine sand or coarser
Rock fragments: 0 to 15 percent--0 to 5 percent cobbles; 0 to 10 percent pebbles
EC: 0 to 4 mmhos/cm
SAR: 15 to 40
Reaction: 8.5 to 9.0
Bk horizon - Hue: 10YR or 7.5YR
Value: 6 or 7 dry; 5 or 6 moist
Chroma: 6 or 7 dry; 5 or 6 moist
Clay content: 60 to 80 percent, 6 to 15 percent fine sand or coarser
Rock fragments: 0 to 15 percent--0 to 5 percent cobbles; 0 to 10 percent pebbles
Calcium Carbonate equivalent: 5 to 15 percent
EC: 2 to 8 mmhos/cm
SAR: 25 to 45
Reaction: pH 7.9 to 9.0
C horizon - Hue: 10YR or 7.5YR
Value: 6 or 7 dry; 5 or 6 moist
Chroma: 2, 3, or 4
Clay content: 60 to 70 percent, 6 to 15 percent fine sand or coarser
Rock fragments: 0 to 15 percent--0 to 5 percent cobbles; 0 to 10 percent pebbles
EC: 4 to 8 mmhos/cm
SAR: 20 to 45
Reaction: pH 8.5 to 9.0
Some pedons have an 8- to 10-inch thick silt, silt loam layer between the varves.
COMPETING SERIES: There are no competing series.
GEOGRAPHIC SETTING: The post soils are on glacial till moraines and fan terraces. Elevations are 2,700 to 3,200 feet. Slopes are 0 to 15 percent. These soils formed in strongly alkaline clayey glacial till or glaciolacustrine deposits. The climate is characterized by long, cold winters; moist falls; moist springs; and hot, dry summers. Mean annual precipitation is 14 to 18 inches. Mean annual temperature is 42 to 45 degrees F. The frost-free period is 105 to 135 days.
DRAINAGE AND PERMEABILITY: Well drained, slow to rapid runoff; very slow permeability.
USE AND VEGETATION: Post soils are used mainly for irrigated crops, nonirrigated crops, pastureland, and rangeland. Principal crops are wheat, barley, alfalfa, and corn. Potential native vegetation is mainly rough fescue, bluebunch wheatgrass, Idaho fescue, and basin wildrye.
DISTRIBUTION AND EXTENT: Post soils are of moderate extent in the mountain valleys of northwestern Montana.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Bozeman, Montana
SERIES ESTABLISHED: Lake County, Montana, 1929 soils report "Soil Survey of the Lower Flathead Valley Area Montana." Post is a place name.
REMARKS: Soil interpretations record: MT0424. Diagnostic horizons and features recognized in this pedon are: a mollic epipedon from the soil surface to 7 inches (Ap, BtE horizons mixed); an upper natric horizon having strong columnar structure with interfingering of albic materials from 6 to 9 inches (Bt/E horizon); and a natric horizon from 9 to 21 inches (Bt horizon). Post soils have a frigid temperature regime and a xeric moisture regime.
LABORATORY DATA: S65MT-47-7; S65MT-47-6; clay mineralogy--S65MT-24-6; B21t, 8-13 inches; "a moderate amount of mica (or illite)" and "Lake County samples are all similar containing a moderate to abundant proportion of mice (or illite)"--memo December 6, 1967, Lincoln Lab. S84MT-047-000 Post series, glacial till, Bt, 16-27 inches is silty clay loams, illitic and 6 to 14 percent calcium carbonate and 16 to 27 sodium absorption ratios--memo August 19, 1985, Lincoln.