Geoscience Reference
In-Depth Information
processes are typifi ed by small patches of rust and pale yellow colors, pedologists
speak about marmoreal sub-horizons. With those sub-horizons being less perme-
able than their neighboring sub-horizons, they contribute to more intensive water-
logging and increased acidic conditions. These lower pH reactions repeatedly
intensify reduction processes. This repetition stops in dry seasons with low precipi-
tation. Or more precisely speaking, when the oxidation starts, the Fe
2+
and Mn
2+
compounds originally dissolved in water are transformed by oxidation into chemi-
cal compounds not soluble in water. These compounds precipitate fi rst and are more
distinctly visible in coarse pores because they drain fi rst and the oxidation is more
intensive. Therefore, the visibility of marble colors is more distinct.
Soil horizons rich in sesquioxides are formed by different solubility in water
illustrated here with iron. Iron in the ferric form (with valence 3, i.e., Fe
3+
) is rela-
tively immobile. If soil drainage is poor and most or all soil pores are fi lled with
water, the minuscule amounts of soil air are rapidly used by microorganisms which
could not exist without oxygen. They are then forced to obtain oxygen from chemi-
cal transformation of ferric (trivalent) forms into ferrous (divalent) forms. Ferrous
iron (FeO) dissolved in soil water moves and migrates in the soil while kept in the
reduced form, i.e., during and after rainy periods whenever the soil remains water
saturated. During the whole period of waterlogging, the soil iron is free to migrate
locally within the horizon. Whenever the rainless period lasts suffi ciently long to
allow air penetration and oxidation reactions, the soil iron is transformed into water-
insoluble ferric forms. Precipitates of such ferric compounds are visibly identifi ed
as mottles. Amplifi ed lengths of such periods lead to intensifi cation of the process
and to more frequent mottling. In tropical climates the process is so strong that it
dominates the formation of thick horizons rich in iron and accounts for the birth of
latosols (lateritic soils). The name, derived from the Latin
later
meaning brick,
relates to the intensive red color of these soils that is similar to the color of bricks
produced of loess and baked at high temperature.
In cool regions with high values of annual precipitation, horizons poor in Fe
compounds are formed by leaching sesquioxides during wet periods when intensive
downward fl uxes of soil water prevail. Such a leached out horizon is evident by its
gray whitish color similar to ash, Russian
zola.
Together with Russian
pod
meaning
under or possibly with Ukrainian farmers' slang
poda
meaning soil, the name pod-
zol was commonly used in the countryside and eventually entered into soil science
literature. Podzolic horizons are formed overlaying a sesquioxide-rich horizon. The
leached or eluviated horizon was earlier denoted by A2, nowadays by E. The term
eluviate stems from the Latin
exlavere
which is a combination of
ex
meaning out of
and
lavere
meaning wash. Analogous to
illuvere
, the term
eluvere
was created.
Today, every indication of increased or decreased content of sesquioxides is easily
recognized in the fi eld, and the more distinct is the change of color in the direction
to rusty red, the more intensive is the process of horizon formation.
We have briefl y described the most frequently occurring main horizons and pro-
cesses leading to their formation. However, the spectrum of variation of horizons is
much broader and their characteristics go into many details. For example, the World
Reference Base (WRB) supported by the International Soil Science Society and
