LOCATION TILMA ID+WAEstablished Series
TAXONOMIC CLASS: Fine, mixed, superactive, mesic Xeric Argialbolls
TYPICAL PEDON: Tilma silt loam - on a 7 percent southeast-facing slope in a cultivated field. (Colors are for dry soil unless otherwise stated.)
Ap--0 to 8 inches; dark grayish brown (10YR 4/2) silt loam, very dark brown (10YR 2/2) moist; weak very fine granular structure; slightly hard, friable, slightly sticky and slightly plastic; many very fine and fine, few medium roots; many very fine interstitial pores; few uncoated (bleached) silt grains; medium acid (pH 6.0); abrupt smooth boundary. (6 to 10 inches thick)
A--8 to 14 inches; dark grayish brown (10YR 4/2) silt loam, very dark brown (10YR 2/2) moist; weak fine subangular blocky structure; slightly hard, friable, slightly sticky and slightly plastic; many fine and very fine roots; many very fine interstitial pores; slightly acid (pH 6.1); clear smooth boundary. (3 to 7 inches thick)
Bw--14 to 20 inches; grayish brown (10YR 5/2) heavy silt loam, very dark grayish brown (10YR 3/2) moist; weak medium prismatic structure parting to weak medium subangular blocky; hard, friable, sticky and plastic; many fine and very fine roots; many very fine, common fine tubular pores; some organic stains on faces of peds; many uncoated (bleached) silt grains; slightly acid (pH 6.2); abrupt smooth boundary. (6 to 10 inches thick)
E--20 to 23 inches; light gray (10YR 7/2) silt loam, dark grayish brown (10YR 4/2) moist; massive; slightly hard, very friable, slightly sticky and slightly plastic; many fine and very fine roots; few very fine tubular pores; few fine black concretions; slightly acid (pH 6.4); abrupt smooth boundary. (1 to 3 inches thick)
Btb1--23 to 30 inches; brown (7.5YR 5/3) silty clay, light yellowish brown (10YR 6/4) rubbed, dark brown (7.5YR 4/3) moist; strong medium columnar structure parting to weak medium and coarse subangular blocky structure; extremely hard, extremely firm, very sticky and very plastic; few fine and very fine roots that penetrate peds; many very fine tubular pores; continuous moderately thick clay films on peds; many small and medium black and brown concretions; medium acid (pH 6.0); abrupt smooth boundary. (5 to 12 inches thick)
Btb2--30 to 34 inches; brown (10YR 5/3) silty clay, light yellowish brown (10YR 6/4) rubbed; brown (7.5YR 5/3) moist; moderate medium prismatic structure; very hard, very firm, very sticky and very plastic; few fine roots; common very fine tubular pores; continuous moderately thick clay films on peds; many black and brown concretions larger than 1 mm.; slightly acid (pH 6.1); abrupt smooth boundary. (4 to 7 inches thick)
Btb3--34 to 42 inches; brown (10YR 5/3) silty clay, light yellowish brown (10YR 6/4) rubbed, dark brown (7.5YR 4/3) moist, brown (7.5YR 5/3) rubbed moist; strong medium subangular blocky structure; very hard, firm, sticky and plastic; few very fine roots; many very fine tubular pores; continuous moderately thick clay films on peds; slightly acid (pH 6.1); abrupt smooth boundary. (6 to 10 inches thick)
Btb4--42 to 60 inches; light yellowish brown (10YR 6/4) silt loam, very pale brown (10YR 7/3) rubbed, yellowish brown (10YR 5/4) moist, brown (7.5YR 5/3) rubbed moist; weak medium prismatic structure parting to strong medium and fine angular blocky; extremely hard, extremely firm, slightly sticky and slightly plastic; brittle and compact; common very fine tubular pores; continuous moderately thick clay films in pores; many small black concretions and patchy stains on peds; neutral (pH 6.8).
TYPE LOCATION: Benewah County, Idaho; 560 feet west and 560 feet south of section corner in the NE1/4 NE1/4 sec. 25, T. 45 N., R. 6 W.
RANGE IN CHARACTERISTICS:
Mollic epipedon thickness - 16 to 23 inches Organic matter content in mollic epipedon (weighted average) - 1 to 3 percent Volcanic ash content - less than 20 percent in the silt and sand fractions Depth to E horizon - 15 to 23 inches Depth to argillic horizon - 18 to 26 inches Average annual soil temperature - 47 to 56 degrees F. Days continuously moist - 60 to 90 in late winter and spring Days dry - 45 to 80 consecutive days in late summer
A or Ap horizon Value - 3 through 5 dry, 2 or 3 moist Chroma - 2 or 3, dry or moist Reaction - moderately acid through neutral
Bw horizon (or Bt horizon in some pedons) Value - 3 through 5 dry, 2 or 3 moist Chroma - 2 or 3, dry or moist Clay content - less than 1.2 times the amount of clay in the A horizon. Reaction - moderately acid through neutral
E horizon Value - 4 or 5 moist Clay content - 5 to 18 percent Content of iron-manganese concretions - few to common, ranging from about 1 to 4 mm. in diameter
Btb1, Btb2, Btb3 horizons have an abrupt textural change with the overlying E horizon Hue - 7.5YR or 10YR Value - 5 or 6 dry, 4 or 5 moist Chroma - 3 or 4, dry or moist Texture - SICL, SIC Clay content - 35 to 50 percent Content of fine sand and coarser sand - less than 10 percent Reaction - moderately acid through neutral The clay is dominantly montmorillonite
Btb4 horizon moist consistence - extremely firm to very firm; resembles a weak fragipan in some pedons.
COMPETING SERIES: These are the Latah, Quicksell, and Setnum (T) series. Latah soils have mottles in the Bt horizons and are somewhat poorly drained. Quicksell soils have mollic epipedons 10 to 16 inches thick. Setnum soils are moderately deep.
GEOGRAPHIC SETTING: These soils are on plains and hills on plateaus, mainly near drainageways. Slopes range from about 7 to 25 percent. Elevations are 2,300 to 3,200 feet. The soils formed in older loess, probably mostly of the recent or Pinedale surface, Early Bull Lake or Pre Bull Lake. The mean annual precipitation is 18 to 23 inches. The frost-free period is 110 to 160 days.
GEOGRAPHICALLY ASSOCIATED SOILS: These are the Palouse, Naff, and Thatuna soils. Palouse and Naff soils have fine-silty control sections and lack an E horizon, and are on smooth back slopes. Thatuna soils have a fine-silty control section and are on back slopes and foot slopes that are commonly concave.
DRAINAGE AND PERMEABILITY: Moderately well drained; medium to rapid runoff; slow permeability.
USE AND VEGETATION: Most areas are under cultivation. Small grains, hay, pasture, lentils, and peas are the major crops. Principal vegetation of uncultivated areas is Idaho fescue, wild rose, snowberry and small forbs.
DISTRIBUTION AND EXTENT: Loessal region in the prairies in northern Idaho and eastern Washington. Moderately extensive.
MLRA OFFICE RESPONSIBLE: Portland, Oregon
SERIES ESTABLISHED: Whitman County, Washington, 1975.
REMARKS: Diagnostic horizons and features recognized in this pedon are:
Mollic epipedon - The zone from the surface to 20 inches (Ap, A, and Bw horizons)
Albic horizon - The zone from 20 to 23 inches (E horizon)
Abrupt textural change - at 23 inches (upper boundary of Btb1 horizon)
Argillic horizon - The zone from 23 to 60 inches (Btb1, Btb2, Btb3, and Btb4 horizons)
Particle size control section - The zone from 23 to 43 inches - (Btb1, Btb2, and part of the Btb3 horizon)