Agriculture Reference
In-Depth Information
The C horizon
This horizon comprises unconsolidated material in various stages of weathering and
from which the soil is considered to have formed. Roots are often active within this
horizon and play important roles in weathering. Illuvial accumulations of organic
matter, clays and larger particles, calcium carbonate and silica are often present in
fractures and other voids within this horizon (Graham
et al.,
1994).
The R layer
This is the hard layer of underlying rock which may include quartz, basalt, sandstone
and other parent rock types. It supports no significant root activity.
1.3
Transitional and sub-horizons
The master horizon designations may be modified by the addition of qualifying suffixes
to permit differentiation between vertical subdivisions within horizons and to identify
the horizons in a general sense across different soils. One example is the need to reflect
different textural properties within the B horizon and such sub-horizons may be denoted
as B1, B2, B3, etc. Similarly, the designation Ap denotes a sub-horizon disturbed by
tillage and the designations Bh, Bt, Bk and Bs denote B horizons with, respectively,
accumulations of organic matter, translocated clay, pedogenic carbonate and sesquioxides
combined with organic matter. Indurated horizons are indicated by m. More than one
suffix may be used and an horizon that is largely or completely cemented with deposits
of pedogenetic calcium carbonate is denoted by the term Bkm. It should be noted that
the qualifying suffixes do not always have the same connotation in different systems
of classification.
The boundaries between adjacent master horizons may be discrete or transitional
horizons may occur with some of the properties of both. Such horizons are named from
their adjacent master horizons, that most ressembling the transitional horizon taking
precedence. Thus, a transitional horizon lying between the A and B horizons but most
ressembling the B would be designated BA. Alternatively, horizon boundaries can be
convoluted and the horizons may interdigitate into each other; occasionally small parts
of one horizon may become completely surrounded by the materials of that adjacent.
While many soils acquire dust and other inputs over long periods, certain soils may
have a layer of some other material, such as volcanic ash, loess or alluvium deposited on
their surfaces and these then become the parent materials of new soils. Over time, such
parent materials differentiate into new horizons and, where such additions recur over
time, sequences of buried horizons may be apparent in such exposures as road cuttings,
notably in the volcanically active areas of the world.
