与第12版相比，美国土壤系统分类检索第13版中增加了一个主要发生层——孔泡层，大写字母为V，即在土壤表面，或在岩石碎屑层（如荒漠砾幂）、物理或生物结壳、新近沉积的风成物质下面形成的矿质层次，其特征是以气孔为主，并具有片状、棱柱状或柱状结构体。至此，美国土壤系统分类使用的主要发生层或土层有10个主要发生层或土层：有机土壤物质发生层或土层（O）、有机或矿质湖积物发生层或土层（L）、矿质孔泡发生层（V）、矿质表层（A）、矿质淋溶层（E）、有发生学证据的矿质表下层（B）、无发生学证据的矿质表下层（C）、强胶结的坚硬岩石层（R）、人造物质形成的根系限制层（M）、水层（W）（其他信息可阅读土层符号），其中最常用到的还是O、A、E、B、C、R。O、A、E、R相对容易确定，用B还是C有时却难于确定。从下列4例可见，石砾含量与使用B还是C无直接关联，主要是看是否有成土过程的影响。现列举Official Soil Series Descriptions (OSD)中的4例：

1.CRAGO SERIES

TAXONOMIC CLASS: Loamy-skeletal, carbonatic, frigid Aridic Calciustepts
TYPICAL PEDON: Crago gravelly loam, in grassland (Colors are for dry soil unless otherwise noted.)
A--0 to 4 inches; light brownish gray (10YR 6/2) gravelly loam, grayish brown (10YR 5/2) moist; weak fine granular structure; soft, very friable, slightly sticky, slightly plastic; many very fine and fine roots; 20 percent gravel; slightly effervescent; moderately alkaline (pH 8.0); clear smooth boundary. (2 to 6 inches thick)
Bk1--4 to 10 inches; pale brown (10YR 6/3) gravelly loam, brown (10YR 5/3) moist; weak fine subangular blocky structure; slightly hard, very friable, slightly sticky, slightly plastic; common very fine and fine roots; 35 percent gravel; continuous distinct calcium carbonate casts on undersides of gravel; disseminated calcium carbonate; strongly effervescent; moderately alkaline (pH 8.0); clear smooth boundary. (2 to 8 inches thick)
Bk2--10 to 21 inches; very pale brown (10YR 7/3) very gravelly loam, pale brown (10YR 6/3) moist; weak fine subangular blocky structure; slightly hard, very friable, slightly sticky, slightly plastic; few very fine and fine roots; 60 percent gravel; continuous prominent calcium carbonate casts on surface of gravel; some cementation between individual gravel; disseminated calcium carbonate; strongly effervescent; moderately alkaline (pH 8.2); clear smooth boundary.
Bk3--21 to 37 inches; very pale brown (10YR 7/3) extremely gravelly sandy loam, pale brown (10YR 6/3) moist; weak fine subangular blocky structure; soft, loose, nonsticky, nonplastic; 75 percent gravel; continuous distinct calcium carbonate casts on gravel; some calcium carbonate cementation between individual gravel; disseminated calcium carbonate; violently effervescent; moderately alkaline (pH 8.4); gradual wavy boundary. (Combined thickness of the Bk2 and Bk3 horizons - 10 to 46 inches.)
2C--37 to 60 inches; brownish yellow (10YR 6/6) extremely gravelly loamy sand, yellowish brown (10YR 5/6) moist; massive; loose, nonsticky, nonplastic; 75 percent gravel; disseminated calcium carbonate; strongly effervescent; moderately alkaline (pH 8.4).

2.DEVILLE SERIES

TAXONOMIC CLASS: Loamy-skeletal, mixed, superactive, frigid Lithic Haplustepts

TYPICAL PEDON: Deville very channery loam, stony, in rangeland (colors are for dry soil unless otherwise noted).

A--0 to 4 inches; dark grayish brown (10YR 4/2) very channery loam, dark brown (10YR 3/3) moist; moderate fine granular structure; slightly hard, very friable, nonsticky and nonplastic; many fine and very fine roots; many fine and very fine interstitial pores; 35 percent channers; neutral (pH 6.8); clear wavy boundary. (3 to 5 inches thick)

Bw1--4 to 7 inches; pale brown (10YR 6/3) very channery loam, brown (10YR 5/3) moist; moderate medium subangular blocky structure; slightly hard, very friable, slightly sticky and nonplastic; many fine and very fine roots; common fine and very fine tubular and interstitial pores; 5 percent flagstones and 35 percent channers; slightly alkaline (pH 7.4); clear wavy boundary.

Bw2--7 to 11 inches; brown (10YR 5/3) very channery loam, brown (10YR 4/3) moist; weak fine subangular blocky structure; slightly hard, very friable, nonsticky and nonplastic; common very fine roots; common fine and very fine pores; 10 percent flagstones and 50 percent channers; slightly alkaline (pH 7.4); gradual wavy boundary. (Combined thickness of Bw horizons is 4 to 10 inches).

BC--11 to 17 inches; brown (10YR 5/3) extremely channery sandy loam, brown (10YR 4/3) moist; weak fine granular structure; slightly hard, very friable, nonsticky and nonplastic; common very fine roots between channers; 15 percent flagstones and 65 percent channers; neutral (pH 7.0); clear wavy boundary. (3 to 8 inches thick)

R--17 inches; brown (10YR 5/3) hard fractured shale.

3.ANAKEESTA SERIES

TAXONOMIC CLASS: Loamy-skeletal, isotic, frigid Humic Dystrudepts
TYPICAL PEDON: Anakeesta channery loam, on a 65 percent north-northeast facing slope at 5000 feet in elevation, forested.
Oe--0 to 1 inches; moderately decomposed plant material and root mat
A1--1 to 4 inches; very dark grayish brown (10YR 3/2) channery loam; moderate fine to coarse granular structure; very friable; many medium to coarse roots, common fine and very fine roots; 25 percent by volume slate channers; extremely acid; clear wavy boundary.
A2--4 to 8 inches; dark brown (10YR 3/3) channery loam; moderate fine to coarse granular structure; very friable; many medium to coarse roots, common fine and very fine roots; 20 percent by volume slate channers, 7 percent by volume metasandstone channers; extremely acid; clear wavy boundary. (7 to 14 inches)
Bw1--8 to 14 inches; dark yellowish brown (10YR 4/4) extremely channery loam; weak medium and coarse subangular blocky structure; friable; few fine and medium roots; 60 percent by volume slate channers, and 10 percent by volume quartz gravel; very strongly acid; clear wavy boundary.
Bw2--14 to 30 inches; dark yellowish brown (10YR 4/4) extremely channery loam; weak medium and coarse subangular blocky structure; friable; 65 percent by volume slate channers, and 10 percent by volume quartz gravel; strongly acid; clear smooth boundary.
Bw3--30 to 45 inches; dark yellowish brown (10YR 4/4) extremely channery coarse sandy loam; weak medium and coarse subangular blocky structure; friable; 65 percent by volume slate channers, and 10 percent by volume quartz gravel; strongly acid; clear smooth boundary. (20 to 40 inches)
R--45 inches; fractured Anakeesta slate; some earthen material in fractures, which are greater than 4 inches apart.

4.BACKROAD SERIES

TAXONOMIC CLASS: Loamy-skeletal, mixed, superactive, frigid Andic Dystrudepts

TYPICAL PEDON: Backroad gravelly ashy silt loam, forested (colors are for dry soil unless otherwise noted).

Oi--0 to 2 inches; undecomposed and slightly decomposed forest litter.

A--2 to 5 inches; grayish brown (10YR 5/2) gravelly ashy silt loam, dark grayish brown (10YR 4/2) moist; weak, fine granular structure; soft, very friable, nonsticky and nonplastic; many very fine, fine, medium, and coarse roots; 20 percent pebbles; slightly acid (pH 6.2); clear wavy boundary. (2 to 4 inches thick)

Bw--5 to 14 inches; light yellowish brown (10YR 6/4) gravelly ashy silt loam, dark yellowish brown (10YR 4/4) moist; weak fine and medium subangular blocky structure; soft, very friable, nonsticky and nonplastic; many very fine, fine, medium, and coarse roots; 25 percent pebbles; slightly acid (pH 6.2); clear wavy boundary. (7 to 11 inches thick)

2E--14 to 23 inches; very pale brown (10YR 7/3) very gravelly silt loam, brown (10YR 5/3) moist; moderate fine and medium subangular blocky structure; slightly hard, friable, nonsticky and nonplastic; common very fine, fine, medium, and coarse roots; 10 percent cobbles, 40 percent pebbles; slightly acid (pH 6.4); clear wavy boundary. (6 to 15 inches thick)

2E/Bw1--23 to 34 inches; E part (80 percent) is very pale brown (10YR 8/3) extremely gravelly silt loam, pale brown (10YR 6/3) moist, Bw part (20 percent) is very pale brown (10YR 7/4) extremely gravelly silt loam, yellowish brown (10YR 5/4) moist; moderate medium subangular blocky structure; slightly hard, friable, nonsticky and nonplastic; common very fine, fine, medium, and coarse roots; 10 percent cobbles, 60 percent pebbles; slightly acid (pH 6.4); gradual wavy boundary. (8 to 20 inches thick)

2E/Bw2--34 to 60 inches; E part (60 percent) is very pale brown (10YR 8/3) extremely gravelly silt loam, pale brown (10YR 6/3) moist, Bw part (40 percent) is very pale brown (10YR 7/4) extremely gravelly silt loam, yellowish brown (10YR 5/4) moist; massive; slightly hard, friable, nonsticky and nonplastic; few fine, medium, and coarse roots; 15 percent cobbles, 70 percent pebbles; slightly acid (pH 6.4).