Geoscience Reference
In-Depth Information
organic debris on top of a mineral A horizon without mixing with its mineral parti-
cles. But let us return back to the origin of the description of horizons. With time
and with detailed fi eld studies, new horizons and sub-horizons denoted with small
letters taken from the process dominating their formation were added to the basic A,
B, C symbols (e.g., Ap-Bv-C profi le of the soil type cambisol). Here p means
plowed, and v is for the German Verwitterung (weathering). In this way a sort of
pedological stenography was developed. In some instances we can fi nd another
horizon between the A and B horizons, and then the sequence is A-E-B-C in our
pedological shorthand. More about this eluviated E horizon will be discussed later
on after we describe the processes in B horizon formation.
When clay and silt particles have been transported from the A horizon to deeper
positions during long time periods of a thousand or more years, a horizon rich in
those fi ne particles is formed below the A horizon. It is called an illuvial horizon
derived from the Latin lavere or lavare meaning to wash and inlavere later trans-
formed to illuvere meaning to wash in, to depose material washed out from the top,
or generally from somewhere else. The letter “n” was transformed into “l” during
the ancient Roman times just for ease of pronunciation. Using the pedological short-
hand of the end of the nineteenth century, this illuvial horizon is now recognized as
the B horizon. The main factor causing change within this B horizon has been the
climate during those years when the annual precipitation is higher than the annual
evaporation measured from the free water level of a meteorological station. Under
this climatic condition, it was typical that water fl owing down in the soil prevailed
over that fl owing up with the consequence of clay particles being transported and
deposited in the B horizon. Local soil particles were covered by a thin fi lm formed
by clay minerals with a typical orientation, and polyhedric structures were formed.
During their transport, since the clay and fi ne silt particles are frequently covered by
iron oxides originating due to chemical weathering, the B horizon has an intensive
brown color. If the soil is acidic with pH well below 6.5, the intensive weathering of
primary minerals releases free simple forms of Fe and Mn oxides that contribute to
the clay fi lm cover on the surfaces of soil particles. The color becomes more reddish
and rusty. Soil scientists in their detailed studies have recognized differences
between horizons described as, e.g., Bv, Bt, and Bhs where the small letters accen-
tuate the specifi c features of the horizon-forming processes. Having already
explained that “v” is for intensive weathering (German V erwitterung ), “t” is for
German T on meaning clay indicating that the horizon or sub-horizon is enriched by
clay particles compared with their content in neighboring horizons. The symbol
“hs” designates increased accumulation of humus and sesquioxides according to
chemical analysis.
In order to avoid any misunderstanding regarding the evolution of a B horizon,
the mutual arrangement of soil particles must also be considered. Coagulation and
microaggregation act against illuviation because they mechanically hinder the
transport of fi ne individual particles through soil pores.
Generally, the accumulation of matter in B horizons leads to the possibility of
waterlogging during periods of wet weather that leads to a lack of oxygen in chemi-
cal processes and the development of reduction processes. Because reduction
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