Established Series


The Seattle series consists of very deep, very poorly drained organic soils formed in herbaceous and woody deposits in depressions in river valleys and glacial till plains. Slopes are 0 to 1 percent. Average annual precipitation is about 40 inches. Mean annual temperature is about 50 degrees F.

TAXONOMIC CLASS: Euic, mesic Hemic Haplosaprists

TYPICAL PEDON: Seattle muck, pasture. (Colors are for moist soil unless otherwise noted.)

Oap--0 to 11 inches; black (5YR 2/1) muck, black (10YR 2/1) dry; about 10 percent fibers, less than 1 percent rubbed; strong fine granular structure; hard, very friable, nonsticky, nonplastic; many roots; strongly acid (pH 5.4); clear smooth boundary. (10 to 13 inches thick)

Oe--11 to 17 inches; dark reddish brown (5YR 2/2) mucky peat, black (10YR 2/1) and dark brown (10YR 4/3) dry; about 60 percent fibers, 25 percent rubbed; laminar structure; very hard, very friable, nonsticky, nonplastic; common roots; strongly acid (pH 5.1); clear smooth boundary. (3 to 12 inches thick)

Oa1--17 to 21 inches; black (5YR 2/1) muck, black (5YR 2/1) dry; about 5 percent fibers, less than 1 percent rubbed; massive; very hard, very friable, slightly sticky, slightly plastic; few roots; very strongly acid (pH 4.8); clear wavy boundary. (0 to 6 inches thick)

Oa2--21 to 35 inches; very dark brown (7.5YR 2/2) muck, very dark grayish brown (10YR 3/2) and pale brown (10YR 6/3) dry; about 45 percent fibers, 15 percent rubbed; massive, very hard, very friable, nonsticky, nonplastic; few roots; moderately acid (pH 6.0); clear smooth boundary. (12 to 17 inches thick)

O'e--35 to 60 inches; dark brown (7.5YR 3/4) mucky peat, very dark brown (10YR 2/2) and brown (10YR 5/3) dry; about 70 percent fibers, 35 percent rubbed; massive; very hard, very friable, nonsticky, nonplastic; few roots; moderately acid (pH 5.8 water).

TYPE LOCATION: King County, Washington; 200 feet east, 500 feet north of center sec. 2, T.24N., R.5E.

RANGE IN CHARACTERISTICS: The organic deposits range in depth from 51 inches to more than 40 feet. Mean annual soil temperature is 47 to 52 degrees F. The difference between mean winter and mean summer soil temperature is 15 to 30 degrees F. Fibers are mostly from sedges but some pedons are up to 25 percent wood fragments. The subsurface and bottom tiers (12 to 51 inches) contain 10 to 25 inches of hemic material. The soils are very strongly acid or moderately acid.

The surface tier (0 to 12 inches) has hue of 10YR, 7.5YR, or 5YR, value of 1 to 4 moist, 2 to 6 dry, and chroma of 1 to 4 moist or dry. It is sapric material, with 5 to 40 percent fibers, unrubbed, and less than 10 percent rubbed. This tier has moderate or strong granular structure.

The subsurface tier (12 to 36 inches) has hue of 10YR, 7.5YR, or 10YR, value of 2 or 3 moist, 2 to 7 dry, and chroma of 1 to 5 moist or dry. It is more than one-half sapric material. It is sapric and hemic material with thin layers of sedge peat. The fiber content ranges from 30 to 70 percent, 10 to 35 percent rubbed.

The bottom tier (36 to 51 inches) is similar in color to the subsurface tier and fiber content is generally higher than the subsurface tier.

COMPETING SERIES: This is the Pinnebog series and the similar McMurray series. Pinnebog soils have a difference between mean winter and mean summer temperature of more than 30 degrees F and the soil reaction of Pinnebog soils are typically slightly alkaline. McMurray soils are dominantly hemic material in the subsurface tier.

GEOGRAPHIC SETTING: The Seattle soils are in depressions on glacial till plains and river terraces at elevations ranging from near sea level to 1,000 feet. Seattle soils formed in organic materials. These soils occur in a mild marine climate having cool dry summers and mild wet winters. The average annual precipitation is 25 to 50 inches. The average January temperature is 38 degrees F, the average July temperature is 64 degrees F, and the mean annual temperature is 50 degrees F. The average frost free season is 150 to 250 days.

GEOGRAPHICALLY ASSOCIATED SOILS: These are the Alderwood, Buckley, Everett, and Shalcar soils. Alderwood, Buckley, and Everett soils are all mineral soils. In addition, Alderwood soils are loamy-skeletal. Everett soils are sandy-skeletal. Buckley soils are fine-loamy. Shalcar soils have a loamy substratum within 51 inches.

DRAINAGE AND PERMEABILITY: Very poorly drained; very slow to ponded runoff; moderate permeability.

USE AND VEGETATION: Most Seattle soils have been cleared and drained for use as cropland or pasture. Hay, corn silage, blueberries, and truck crops are common crops. In the natural state the vegetation consists of red alder, western red cedar, black cottonwood and Sitka spruce with an understory of sedge, rush, cattail, hardhack, trailing blackberry, red elderberry, devilsclub, Siberian miners lettuce, trillium, salmonberry, and ladyfern.

DISTRIBUTION AND EXTENT: This series is of moderate extent in the Puget Sound Basin of Western Washington.


SERIES ESTABLISHED: King County, Washington, 1970.

REMARKS: Diagnostic horizons and features recognized in this pedon are a surface tier from the surface to 12 inches that is sapric material from 0 to 11 inches and hemic from 11 to 12 inches, a subsurface tier from 12 to 36 inches that is sapric for 18 inches and hemic for 6 inches, and a bottom tier from 36 to 51 inches that is hemic throughout.

National Cooperative Soil Survey