Geoscience Reference
In-Depth Information
material between our fi ngers, we can roughly judge if the soil is sandy, loamy, or
clayey, i.e., the soil texture. We focus our attention upon the potential differences in
texture between the horizons, especially between those horizons lying below the top
humus horizon. Further important features for identifying individual horizons are
the density of fi ne roots including the depth of their penetration, the origin of a spe-
cifi c neoplasm, and whether or not the transition from one horizon to the next is
sharp or gradual. The word neoplasm, derived from the classical Greek
neo
mean-
ing new and
plasma
meaning formation, denotes newly formed bodies originating
from the products of weathering and humifi cation that are by-products of soil-
forming processes. After the soil horizons forming the entire soil profi le visualized
in the excavated pit are described, soil samples taken from each horizon are quanti-
tatively analyzed in the laboratory. The chemical and mineralogical content as well
as the soil physical and soil hydraulic properties of each sample are quantitatively
ascertained. All these data coupled with microscopic observations of soil micromor-
phology offer detailed keys to appreciate and understand how mineral particles are
arranged and interconnected by amorphous and humifi ed materials whenever we
observe thin, undisturbed soil sections in a microscope. Combining the fi eld descrip-
tion of individual horizons composing the soil profi le with results of laboratory tests
enables us to identify the soil taxonomic type.
The birth of soil - or soil genesis in scientifi c jargon - changes the parent mate-
rial from which the soil originated, and this transformation is generally recognized
even by the naked eye of a layman and confi rmed by the soil scientist. For example,
a soil originated from a loessial sediment is evident to everybody, but a more detailed
evaluation of its soil genesis leading to its taxonomic classifi cation is more compli-
cated and should start with the identifi cation of its soil horizons.
12.2
Soil Horizons
Soil horizons are defi ned as diagnostic soil horizons when they refl ect features that
are recognized as occurring in soils and can be used to describe and defi ne soil
orders, classes, families, or types.
Inasmuch as the horizon at the soil surface is characterized by accumulations of
humus, it is named the humus horizon. Its dark color that can even be black stems
from the color of humic substances, especially when they form a sort of fi lm on
solid silicate particles in addition to humus fi lling a small part of its soil pores. The
international designation is A horizon that has its roots in the original system of
horizons used by the classics of soil science. At that time the fi rst letters of the
alphabet were used, and since the humus horizon was the fi rst when we start the
description from the top, it got the abbreviation A horizon. The next horizon if pres-
ent got the abbreviated name B horizon. Below it where parent rock exists is denoted
as C horizon. Sometimes the C horizon is just below A horizon. This alphabetic
terminology for soil horizons was adopted without any modifi cation for about a half
century until O horizon was introduced to designate more or less decomposed
