土壤有O、A、E、B、C、R等主要发生层，紧接O层之下是A层吗？ A、E、B、C或R均有可能。以美国土系数据库中的典型单个土体举例如下：

1.EPWORTH SERIES

TAXONOMIC CLASS: Sandy, siliceous, mesic Entic Haplorthods

TYPICAL PEDON: Epworth fine sand. (Colors are for moist soil unless otherwise stated) .

Oa--0 to 2 cm (1 inch); very dark grayish brown (10YR 3/2) well decomposed leaf litter; strongly acid; abrupt smooth boundary. [2.5 to 5 cm (1 or 2 inches) thick]

A--2 to 10 cm (1 to 4 inches); black (10YR 2/1) fine sand, dark grayish brown (10YR 4/2) dry; weak fine granular structure; very friable; common fine and many medium roots; strongly acid; clear wavy boundary. [2 to 13 cm (1 to 5 inches) thick]

E--10 to 18 cm (4 to 7 inches); grayish brown (10YR 5/2) fine sand; weak medium subangular blocky structure; very friable; many fine and medium roots; strongly acid; clear wavy boundary. [2 to 13 cm (1 to 5 inches) thick]

Bs1--18 to 33 cm (7 to 13 inches): strong brown (7.5YR 4/6) fine sand; weak coarse subangular blocky structure; very friable; many medium roots; moderately acid; clear wavy boundary.

Bs2--33 to 79 cm (13 to 31 inches): strong brown (7.5YR 5/6) fine sand; single grain; loose; few coarse roots; moderately acid; clear wavy boundary. [Combined thickness of the Bs horizon is 38 to 71 cm (15 to 28 inches).]

C1--79 to 130 cm (31 to 51 inches); brownish yellow (10YR 6/6) fine sand; single grain; loose; moderately acid; clear wavy boundary.

C2--130 to 152 cm (51 to 60 inches); yellow (10YR 7/6) fine sand; single grain; loose; moderately acid.

2.ADAMS SERIES

TAXONOMIC CLASS: Sandy, isotic, frigid Typic Haplorthods

TYPICAL PEDON: Adams loamy fine sand. (Colors are for moist soil unless otherwise noted.)

Oi -- 0 to 1 cm; very dark brown (7.5YR 2.5/2); slightly decomposed plant material; massive; very friable; many very fine, common fine and few medium roots throughout; extremely acid (pH 4.2), 1:2 calcium chloride; abrupt smooth boundary.

Oa -- 1 to 3 cm; black (10YR 2/1); highly decomposed plant material; weak fine and medium granular structure; very friable; many very fine, common fine and few medium roots throughout; extremely acid (pH 4.2); abrupt wavy boundary. (Combined thickness of the O horizons is 0 to 15 cm thick.)

E -- 3 to 10 cm; 60 percent reddish gray (5YR 5/2) and 40 percent gray (7.5YR 6/1); loamy fine sand; weak fine subangular blocky structure; very friable; common very fine, many fine and few medium and coarse roots throughout; extremely acid (pH 4.0); abrupt wavy boundary. (0 to 20 cm thick.)

Bh -- 10 to 17 cm; very dusky red (2.5YR 2.5/2); loamy fine sand; weak medium subangular blocky structure; friable; common very fine and few fine, medium and coarse roots throughout; extremely acid (pH 4.2); abrupt wavy boundary.

Bhs -- 17 to 34 cm; dark reddish brown (5YR 3/3); loamy fine sand; weak medium subangular blocky structure; very friable; common very fine and few fine, medium and coarse roots throughout; very strongly acid (pH 5.0); clear wavy boundary.

Bs -- 34 to 43 cm; 60 percent strong brown (7.5YR 4/6) and 40 percent brown (7.5YR 4/4); loamy fine sand; weak medium subangular blocky structure; very friable; few very fine and common fine and medium roots throughout; 1 percent gravel; strongly acid (pH 5.2); clear wavy boundary. (Combined thickness of the Bh, Bhs and Bs horizons is 10 to 60 cm)

BC -- 43 to 61 cm; yellowish brown (10YR 5/4) sand; weak medium subangular blocky structure; very friable; common very fine, fine and medium roots throughout; 2 percent gravel; strongly acid (pH 5.2); gradual wavy boundary. (10 to 60 cm thick.)

C1 -- 61 to 88 cm; brown (10YR 5/3) sand; single grain; loose; few very fine roots throughout; 2 percent gravel; strongly acid (pH 5.2); clear wavy boundary.

C2 -- 88 to 145 cm; light yellowish brown (10YR 6/4) sand; single grain; loose; few very fine roots throughout; 4 percent gravel; strongly acid (pH 5.2); clear smooth boundary.

C3 -- 145 to 190 cm; brown (10YR 5/3) sand; single grain; loose; 5 percent gravel; strongly acid (pH 5.4).

3.TOYATTE SERIES

TAXONOMIC CLASS: Dysic Lithic Cryofolists

TYPICAL PEDON: Toyatte moderately decomposed plant material in an area of woodland on a muskeg, on a convex mountain slope of 25 percent slope at an elevation of 94 m (When described on June 18, 2014, the soil was moist from the surface to a depth of 32 cm and wet from a depth of 32 to 42 cm.)

Oe--0 to 16 cm; moderately decomposed plant material; very strongly acid (pH 4.5); clear wavy boundary

Oa--16 to 32 cm; highly decomposed plant material; very strongly acid (pH 4.5); abrupt wavy boundary

Bw--32 to 42 cm; silt loam, 80 percent light gray (2.5Y 7/2) and 20 percent brown (7.5YR 4/3) dry, 80 percent grayish brown (2.5Y 5/2) and 20 percent very dark brown (7.5YR 2.5/3) moist; weak medium subangular blocky structure; soft, very friable, nonsticky and nonplastic; common fine and medium roots throughout; common very fine and fine irregular pores; 2 percent medium iron depletions, gray (2.5Y 6/1) moist, and 3 percent fine strong masses of oxidized iron, strong brown (7.5YR 4/6) moist; 4 percent subrounded gravel; noneffervescent (HCl, 1N); very strongly acid (pH 4.6); abrupt irregular boundary

2R--42 cm; bedrock

4.KNOB LOCK SERIES

TAXONOMIC CLASS: Dysic, frigid Lithic Udifolists

TYPICAL PEDON: Knob Lock moderately decomposed plant material, in a map unit of Hogback-Knob Lock Complex, 35 to 60 percent slopes, very rocky, very bouldery in a very rocky wooded area. (Colors are for moist soil unless otherwise noted.)

Oe -- 0 to 8 cm (0 to 3 in); dark reddish brown (5YR 2.5/2) broken, crushed and rubbed moderately decomposed (hemic) plant material; about 45 percent fiber, 27 percent rubbed; weak fine granular structure; friable; many fine, very fine and medium roots; extremely acid; clear smooth boundary.

Oa -- 8 to 18 cm (3 to 7 in); black (5YR 2.5/1) broken, crushed and rubbed highly decomposed (sapric) plant material; about 27 percent fiber, 15 percent rubbed; weak fine and medium granular structure; friable; many fine and very fine, and common medium roots; extremely acid; clear smooth boundary. (The combined thickness of the O horizons is 1 to 20 inches.)

Bs -- 18 to 23 cm (7 to 9 in); dark reddish gray (5YR 4/2) very fine sandy loam; weak medium and fine subangular blocky structure; friable; common very fine and fine, and few medium roots; very strongly acid; abrupt smooth boundary. (0 to 4 inches thick.)

R -- 23 cm (9 in); Marcy Anorthosite.

5.MAHOOSUC SERIES

TAXONOMIC CLASS: Dysic Typic Cryofolists

TYPICAL PEDON: Mahoosuc slightly decomposed plant material on a 75 percent southeast-facing slope in a forested area. (Colors are for moist soil.)

Oi -- 0 to 8 centimeters; dusky red (2.5YR 3/2) slightly decomposed plant material consisting of needles and twigs; massive; very friable; many very fine roots; extremely acid; abrupt smooth boundary. (5 to 20 centimeters thick.)

Oe -- 8 to 20 centimeters; black (N 2/0) mucky peat (hemic material); moderate medium granular structure; very friable; many roots; 10 percent wood fragments; extremely acid; abrupt irregular boundary. (3 to 20 centimeters thick.)

C1 -- 20 to 51 centimeters; fragmental materials consisting of cobbles, stones, gravel and boulders with about 15 percent organic soil material in the interstices; diffuse irregular boundary.

C2 -- 51 to 165 centimeters; fragmental materials consisting of stones, boulders and cobbles.

6.LOS PENONES SERIES

TAXONOMIC CLASS: Euic, isohyperthermic Lithic Udifolists

TYPICAL PEDON: Los Penones mucky peat--wildlife habitat (Colors are for moist soil).

Oe--0 to 8 inches; black (10YR 2/1) mucky peat, very dark brown (10YR 2/2) crushed and rubbed; about 70 percent fiber, 30 percent rubbed; massive; friable; many very fine, fine and medium roots, few coarse roots; moderately alkaline; abrupt wavy boundary. (3 to 15 inches thick)

R--8+ inches; hard consolidated limestone bedrock.