Agriculture Reference
In-Depth Information
Table 1.2. Major-Minor Horizon Designation in Soil Profiles
a
Designations
Characteristic
Major
O
Organic horizon may be partially or totally decomposed
organic matter
A
Topmost inorganic horizon characterized by relatively high
organic matter content
E
Interior horizon characterized by eluviation
B
Subsoil horizon showing evidence of weathering or illuviation
C
Lowest horizon in geologic material underlying the soil
showing little evidence of soil formation
Minor
(
Subdesignations
)
p
Plowed—applied only to topmost or A horizon
t
Accumulation of illuvial clay
a
Highly decomposed organic matter
h
Illuvial accumulation of organic matter
s
Illuvial accumulation of oxides of iron and aluminum
i
Slightly decomposed organic matter
e
Intermediately decomposed organic matter
g
Gleying
k
Accumulation of carbonates (lime)
n
Accumulation of sodium ions
z
Accumulation of soluble salts
a
Partial listing.
The small letter p, which stands for plow, is not used with any other horizon
designation.
With the passage of time a full set of horizons will develop in a regolith or
parent material. Major horizon designations and some of their distinguishing
characteristics are given in Table 1.2. Each horizon may be subdivided on the
basis of other distinguishing characteristics. The O, or organic, horizons can be
subdivided into those where the organic matter is just beginning, Oi, is well
along, Oe, or highly decomposed, Oa. Examples of these horizons can be seen
in Figure 1.1. Likewise, the B horizons can be characterized by increased clay
content, Bt, or reducing conditions, Bg [the meaning of the upper- and
lowercase letters are described in Table 1.2]. When two horizons are similar
but still distinguishably different, a number following the major horizon
designation indicates this. For example, a soil may have several Bt horizons
designated by Bt1, Bt2, Bt3. A number in front of a horizon designation
indicates that it is formed or is forming from a different parent material.
Examples of these different horizons can be seen in Figures 1.1 and 1.2.
Figure 1.1 also has two other distinctive horizons. The E horizon, which
stands for
eluviated
, is characterized by depletion of clay and is lighter in color
and coarser in texture than the over- or underlying horizons. The second dis-
