Agriculture Reference
In-Depth Information
Under most conditions little organic matter is eluviated down through the
soil. However, many soils have dark or black thick upper horizons, which are
high in organic matter. These horizons are found in soils developing under
high water conditions and grass vegetation where the grass and associated
roots die each year and contribute organic matter to the profile to a depth of
0.5 m depending on the type of grass and other environmental conditions.
When soil is saturated with water, the soil environment becomes reducing,
and under these conditions iron is reduced to the ferrous [Fe(II)] state. The soil
color becomes lighter and more yellow. Under reducing conditions soil also
develops variations in color called
mottling
or
gleying
. Thus any soil horizon
description, which includes a “g” designation, indicates that the soil is under
reducing conditions for a significant period of time during the year. It might be
expected that mottling or gleying would occur only in the lower horizons;
however, it can occur anywhere in a soil profile, even quite near the surface.
Iron in the ferrous state is more soluble than iron in the ferric state; indeed,
in the ferric state it is insoluble in most soil conditions . Under reducing con-
ditions ferrous iron may be leached out of soil, leaving it gray in color. This is
the origin of the term “gleying”.
Some types of vegetation and environmental conditions result in acid pro-
ducing litter on the soil surface. Under these conditions organic matter decom-
position products (humic, fluvic acids, etc.) can eluviate and be deposited
deeper in the soil. In Figure 1.4 the litter in the Oi horizon produces acid,
which allows the illuviation of aluminum, iron, and organic matter decompo-
sition products into the B horizons to form the Bhs horizon. The leaching of
aluminum, iron, and organic matter out of an area of the soil profile results in
the horizon becoming light gray or white, giving rise to the potential devel-
opment of an albic horizon.
To some extent the description of a soil profile gives an indication of some
of the chemistry and chemical conditions occurring in that profile. This in turn
provides the researcher and analyst with information about the types of com-
pounds and species likely to be found and the conditions necessary to isolate
them [3].
1.4.
SOIL NAMING
Various different soil naming systems are used throughout the world. In all of
these systems the horizons and their subdesignations vary somewhat accord-
ing to the different classification systems. In the United States the United
States Department of Agriculture (USDA) has developed the USDA Soil
Taxonomy system (simply referred to as
Soil Taxonomy
), which recognizes 12
soil orders. The United Nations, through its Food and Agriculture Organiza-
tion and United Nations Educational, Scientific and Cultural Organization
(FAO-UNESCO) and the International Society of Soil Science, has a system
that includes 26 soil groupings. There are many other systems, including those
