LOCATION PLAZA CAEstablished Series
The Plaza soils occur at elevations below 175 feet in a dry subhumid, mesothermal climate having a mean annual rainfall of 15 to 25 inches, with hot, dry summers and cool, moist winters; an average January temperature of about 45 degrees F.; and average July temperature of about 80 degrees F.; a mean annual temperature of about 62 degrees F.; and a frost free season of about 260 days.
The Plaza soils occur in the same general area as the Capay, Sunnyvale, Tehama, and Willows soils, occupying intermediate positions between the well drained Tehama soils and the poorly drained Willows soils in basin positions. They resemble the well drained Codora, Esparto, Jacinto, Tehama, and Wyo series and the poorly drained Willows. In addition the Codora soils have grayish brown (10YR hue) sola; the Esparto soils developed in medium textured alluvium and have 10YR hues; the Jacinto soils developed in moderately coarse textured alluvium; the Tehama soils are noncalcareous and developed in medium textured alluvium; and the Wyo soils developed in medium textured alluvium. The Willows soils are fine textured, grayish brown or gray soils containing varying amounts of alkali salts.
TAXONOMIC CLASS: Fine-loamy, mixed, superactive, thermic Aeric Epiaqualfs
TYPICAL PEDON: Plaza silt loam (nearly level at 120 feet elevation in a fallowed field formerly sown to rice).
Ap--0 to 10 inches; light brownish gray (2.5Y 6/2) silt loam, dark grayish brown (2.5Y 4/2) when moist with common fine, distinct, strong brown (7.5YR 5/6) mottles; massive breaking to subangular blocks; hard when dry, friable when moist; abundant very fine roots; many very fine pores; low in organic matter; medium acid, pH 5.8; clear, smooth lower boundary. 6 to 12 inches thick.
Bt--10 to 25 inches; light olive brown (2.5Y 5/3) clay loam with grayish brown (10YR 5/2) coatings on ped faces, olive brown (2.5Y 4/3) with dark grayish brown coatings (10YR 4/2) and common fine, distinct strong brown mottles; massive breaking to angular blocks; very hard when dry, very firm when moist, sticky and plastic when wet; very few roots; many very fine mainly tubular pores; moderately thick, continuous clay films on ped faces and in tubular pores; neutral, pH 7.0; clear, smooth lower boundary. 10 to 18 inches thick.
Btk--25 to 34 inches; light yellowish brown (2.5Y 6/3) clay loam with grayish brown (2.5Y 5/2) coatings, olive brown (2.5Y 4/3) with dark grayish brown (2.5Y 4/2) coatings and common fine, distinct, strong brown mottles; massive breaking to angular blocks; hard when dry, friable when moist, sticky and plastic when wet; very few roots; common very fine pores; thin, continuous clay films on ped faces and in some pores; moderately alkaline, pH 8.1; slightly calcareous with disseminated lime and a few small hard concretions; gradual, smooth lower boundary. 6 to 10 inches thick.
C1--34 to 46 inches; light yellowish brown (2.5Y 6/3) clay loam, light olive brown (2.5Y 5/3) when moist with many fine, distinct, strong brown mottles; massive; hard when dry, friable when moist, sticky and plastic when wet; very few roots; common very fine pores; moderately alkaline, pH 8.2; strongly calcareous with a few small, soft lime concretions; gradual, smooth lower boundary. 10 to 18 inches thick.
C2--46 to 60 inches; mottled pale olive and light gray (5Y 6/3 and 7/2) silty clay loam, light olive brown (2.5Y 5/3) when moist with many fine, distinct, strong brown mottles; massive; hard when dry, friable when moist, sticky and plastic when wet; no roots; common very fine pores; moderately alkaline, pH 8.1; strongly calcareous with a few small, hard and soft lime concretions, lime mainly disseminated. Many inches thick.
RANGE IN CHARACTERISTICS: The A horizons range in color from light brownish gray to grayish brown (2.5Y and 10YR hues); in texture, from silt loam to clay loam or silty clay loam; and in reaction from slightly acid to neutral.
The Bt horizons are light olive brown to light yellowish brown (2.5Y hue) and pale brown to yellowish brown (10YR hue); clay loam or silty clay loam but distinctly finer textured than the A horizon; and neutral to mildly alkaline.
C horizon colors range from light gray to light olive brown (2.5Y or 5Y hues), are moderately fine textured, and moderately or strongly alkaline. The lower B and C horizons are calcareous, lime being segregated in soft and hard masses and disseminated. Nearly all of the Plaza soils have been used for rice or irrigated pasture which has caused these areas to have more acid and more mottled surface horizons than in unirrigated areas. Slightly and moderately alkali-affected areas interfinger into alkali basin areas. The lower B and upper C is weakly cemented with silica and lime in places. Fluctuating high water tables are common within the upper C horizon depending upon the season and nearness to flooded rice fields.
GEOGRAPHIC SETTING: Nearly level lower edges or nearly level to very gently undulating fingers of older alluvial fans extending into basin areas.
DRAINAGE AND PERMEABILITY: Moderately well and imperfectly drained; medium to slow runoff; moderate permeability, very slow where lower solum is weakly cemented. All of the Plaza soils currently mapped occur in rice producing areas and consequently water tables persist for longer periods than would be expected under natural conditions.
VEGETATION: Annual grasses and forbs.
USE: Primarily for rice, irrigated pasture, and ladino clover; a few irrigated shallow rooted field crops; dry farmed grain and range.
DISTRIBUTION AND EXTENT: West central portion of the Sacramento Valley, California.
MLRA SOIL SURVEY REGIONAL OFFICE (MO) RESPONSIBLE: Davis, California
SERIES ESTABLISHED: Soil Survey of Glenn County, California, 1951.
SOURCE OF NAME: Plaza School, Glenn County, California.
TYPE LOCATION: SW 1/4 of SW 1/4 of section 1, T. 19 N., R. 3 W., 2 miles ENE of Willows, Glenn County, California.
REMARKS: This soil is classified as follows: USDA Yearbook: Noncalcic Brown Revised Classification: 7.45-7.13 Univ. of Calif., Storie and Weir: Calcic Brown, Profile Group III.
OSED scanned by SSQA. Last revised by state on 2/60.
Series reclassified 5/95. Competing series not updated at that time.