Geoscience Reference
In-Depth Information
1
Definitions, Concepts and Tools
In this chapter we shall review the definitions, concepts and methods
on which the study of soil formation is based.
1.1 BASIC CONCEPTS
1.1.1
Horizons, Pedons and Profiles
When we dig a pit for studying a section through the soil, we notice
the soil is not uniform. It is divided into layers of varying thickness
called horizons because of the more or less horizontal arrangement of
their boundaries. These horizons are differentiated by their colour and
by their physical, chemical and biological properties. The sequence
of characteristic horizons of a given soil is termed soil profile . Rather
incorrectly, specialists say they dig a 'profile' when they open a soil pit. The
rock that has weathered to form the soil is called the parent material .
By definition, a soil profile does not have defined width. It therefore
seems practical to define an elementary volume of soil, the pedon
(Simonson and Gardner 1960). It is a theoretical individual because the
soil is a continuum. In the vertical direction the pedon corresponds to the
profile including a small thickness of the upper part of the unweathered
rock. In the horizontal plane its width is several decimetres or a few
metres and is always small enough for the pedon to be considered free
of lateral variability.
Definitions
The idea of sequentially naming the horizons of a soil profile by the
first letters of the alphabet, written in capitals, comes from German
Nomenclature of horizons
 
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