Environmental Engineering Reference
In-Depth Information
horizons.
In Figure 2.4 an O horizon is also given. This is an organic horizon, which may be
called an Oi, Oe, or Oa, depending on the organic matter decomposition state (Oi
represents incompletely, Oe intermediately, and Oa completely decomposed organic
material). These horizons are not common, but are important because they are very
different from mineral horizons and require different handling, extraction, and analysis.
Horizons designated by two capital letters or a slash between capital letters are
transition horizons, depending on the mixing of the two layers. This then indicates that
the transition horizons contain characteristics of both the overlying and underlying
materials.
In soils developed under tree vegetation an E horizon will be the next major horizon
below A. It is lighter in color than the horizons above or below and is coarser. In soils
developed under other types of vegetation the A horizon will transition to a B horizon.
There will typically be several of these horizons. They are higher in clay and redder than
overlying horizons.
The B horizons transition to a C horizon, which is the material from which the soil is
developing. It shows no evidence of the effects of the soil-forming factors and may be
very thin or thick. Soils developing on underlying rock may have thin, almost nonexistent
C horizons, while those developing from glacial till or other deposited materials may
have C horizons that are several meters thick. Rock underlying this horizon will often be
referred to as the R horizon.
It is common to find situations in which there are more or fewer horizons. A soil may
have an A horizon that is underlain by a C horizon. The A horizon may not be present,
and the topmost layer may be a B horizon. In cases of severe erosion the topmost horizon
may be a C. In river valleys and stream and river terraces there can be buried soils. There
may be A and B horizons underlain by another set of A and B horizons. All possible
variations of this basic situation can and do occur (e.g., A and B might be underlain by a
different B horizon).
Horizons with a capital letter followed by a small letter have distinctive characteristics.
In Figure 2.4 the small letter t in Bt indicates that the B horizon has a high amount of clay
in it. There are many other ways in which differences in horizons may be designated.
These will not be described here; however, if needed they can be found in basic soils or
soil taxonomy texts [15]. The local soil conservationist or soil scientist can interpret more
complex horizon designations if need be.
Different soils and different horizons may need different sampling. For this reason
knowledge of the soils and their characteristics will be needed in developing a field
sampling plan.
2.3. SOIL TYPES
On the basis of the characteristics of a soil's profile it is described as belonging to one of
twelve soil orders. These range from the Entisols, which have a minimal A (or no)
horizon development to Oxisols, which have undergone intensive weathering. A
complete list of the twelve soil orders is given in Chapter 1, Table 1.1. Six of these
 
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