The Hillgate series consists of very deep, well drained soils on old terraces. These soils formed in alluvium derived from mixed sources. Slope ranges from 0 to 9 percent. The mean annual precipitation is 14 to 22 inches, and the mean annual temperature is 58 to 62 degrees F.

Taxonomic class: Fine, smectitic, thermic Typic Palexeralfs

Hillgate loam, 0 to 2 percent slopes, rangeland. When described on November 21, 1988, the soil was dry throughout. (Colors are for dry soil unless otherwise noted).

A1—0 to 3 inches; yellowish brown (10YR 6/4) loam, dark yellowish brown (10YR 4/4) moist; moderate medium subangular blocky structure; hard, friable, slightly sticky and slightly plastic; common very fine roots; common very fine tubular pores; few fine distinct light brownish gray (10YR 6/2) iron depletions; slightly acid (pH 6.2); abrupt smooth boundary.

A2—3 to 11 inches; yellowish brown (10YR 5/4) loam, dark yellowish brown (10YR 4/4) moist; strong medium angular blocky structure; hard, friable, slightly sticky and slightly plastic; common very fine roots; common very fine and few fine tubular pores; common fine distinct light brownish gray (10YR 6/2) iron depletions; moderately acid (pH 5.6); clear smooth boundary.

A3—11 to 19 inches; yellowish brown (10YR 5/4) loam, dark yellowish brown (10YR 4/4) moist; strong medium angular blocky structure; hard, friable, slightly sticky, and slightly plastic; few very fine roots; common very fine and few fine tubular pores; common fine faint very pale brown (10YR 7/3) iron depletions; moderately acid (pH 5.6); abrupt smooth boundary.

2Bt1—19 to 38 inches; yellowish brown (10YR 5/4) clay, dark yellowish brown (10YR 3/4) moist; strong coarse prismatic structure; extremely hard, firm, moderately sticky and moderately plastic; few very fine roots; common very fine and few fine tubular pores; many moderately thick clay films; slightly acid (pH 6.2); clear smooth boundary.

2Bt2—38 to 53 inches; yellowish brown (10YR 5/4) clay loam, dark yellowish brown (10YR 3/4) moist; strong coarse prismatic structure; extremely hard, firm, sticky and plastic; few very fine roots; common very fine tubular pores; many moderately thick clay films; neutral (pH 7.2); clear smooth boundary.

2Bt3—53 to 63 inches; yellowish brown (10YR 5/4) clay loam, dark yellowish brown (10YR 3/4) moist; moderate medium subangular blocky structure; very hard, firm, sticky and plastic; few very fine roots; common very fine tubular pores; many moderately thick clay films; slightly alkaline (pH 7.5); clear smooth boundary.

2Bt4—63 to 73 inches; yellowish brown (10YR 5/4) clay loam; dark yellowish brown (10YR 3/4); moderate medium subangular blocky structure; very hard, firm, sticky and plastic; few very fine roots; common very fine tubular pores; many moderately thick clay films; slightly alkaline (pH 7.4).

Type location: Colusa County, California; located about 4 1/2 miles west of Maxwell, California 2400 feet south and 2500 feet east of the northwest corner of section 2, T. 16 N., R. 4 W., MDB&M; 39 degrees, 16 minutes, 10 seconds north latitude and 121 degrees, 16 minutes, 20 seconds west longitude; USGS Sites, California topographic quadrangle.

The mean annual soil temperature ranges from 63 to 65 degrees F. The soil temperature is above 47 degrees F the entire year. The soil moisture control section is dry in all parts from May 15 to Oct. 31, and is moist in some or all parts from Nov. 1 to May 15.

The A horizon is 10YR 6/3, 6/4, 5/3, 5/4, 7/2; 7.5 YR 5/4. Moist color is 10YR 4/3, 4/4, 5/3; 7.5YR 4/4. Bleaching occurs in the lower part of the A with one chip color difference. Texture is loam or clay loam and is gravelly in some pedons. Manganese accumulations are scattered in the upper 10 inches. Reaction is moderately to slightly acid.

The Bt horizon is 7.5YR 4/6, 5/4, 5/5, 5/6, 5/8; 10YR 5/3, 5/4, 5/6, 5/8, 6/3. Moist color is 7.5YR 3/4, 4/3, 4/4, 4/6, 5/4; 10YR 3/3, 3/4, 4/3, 4/4, 4/6, 5/4, 5/6. Texture is clay loam or clay. The horizon has an abrupt upper boundary with an absolute clay increase of at least 15 percent within 1 inch. Reaction is slightly acid to moderately alkaline. There are few very small white calcium carbonate masses that effervesce strongly. Base saturation is more than 75 percent in some part of the horizon.