Geoscience Reference
In-Depth Information
also as Terra Roxa Estruturada, Sols Fersialitiques, Ferrisols, or Red Earths. In the
US Taxonomy they belong to Ultisols and to Alfi sols, e.g., Kandiudalfs.
Ferralsols are deep red or yellow soils of humid tropics that differ from Nitisols
by the absence of a nitic horizon. We can judge from their name that they are rich in
sesquioxides (Fe
2
O
3
, Al
2
O
3
) and perfectly crystallized kaolinites are predominant in
their clay fraction. Many local names refer to their color. The name has roots in the
Latin
ferrum
and
aluminium
. The red color is due to thin fi lms of hematite covering
the sand and silt particles and sooner or later fi lls parts of ultra-micropores. If a yel-
low color occurs, it is due to the dominance of goethite, a mineral named in honor
of a great writer and scientist. Although he did not discover the mineral, the famous
German poet was not only a great personality in biology and mineralogy, but he also
entered the discipline of soil science and other complicated geosciences through the
name, goethite. Goethite is found in every soil type where increased concentrations
of iron oxides appear in a horizon. It is transformed into limonite after hydration or
into hematite when dehydrated. The terrain of Ferralsols is level or moderately
undulating of Pleistocene age or more, i.e., 2 million years or more. The weathered
parent material of the same age has a stable microstructure. Due to the stable min-
eralogy and fi xing properties of sesquioxides, the microstructure of Ferralsols is
also stable. Even though their physical properties are good, owing to a very limited
stock of plant nutrients together with an unfavorable fi xing of phosphates similar to
Nitisols, their natural fertility is low. Moreover, owing to their acid reaction, they
are plagued by Al toxicity. In national classifi cation systems they are named, e.g., as
Latosols, Lateritic Soils, Sols Ferralitiques, or Ferralitic Soils. They are denoted as
suborders and great groups within the order of Oxisols in the US Soil Taxonomy.
6. Set of soils with perched water include two RSGs:
Planosols
and
Stagnosols
. In
both groups, waterlogging plays a principal role in their development.
Planosols occur in plains. They have a coarse-textured surface horizon with a
sharp transition into alluvial deposits typical with all signs of permanent or regu-
larly seasonal waterlogging. The name was derived from the Latin
planus
meaning
fl at. Their profi le refl ects geological stratifi cation more than soil evolution. Owing
solely to stratifi cation, they typically have a very low permeability. They are used
for planting rice as a single crop in Southeast Asia, but they require special care. In
other regions, even if they are drained, they offer usually poor yields. Their negative
features for agronomic use are less extreme if the waterlogging is only seasonal.
They appear as Pseudogley Soils in several national classifi cations. They are
equivalent to Albaqualfs, Albaquults, or Argiabolls in the US Taxonomy.
Stagnosols are soils periodically waterlogged due to surface water in regions
with humid to perhumid climate in fl at or gently sloping land on various sedimen-
tary materials with very low hydraulic conductivity. The name was derived from the
Latin
stagnare
meaning to fl ood. They usually have mottles from the topsoil to the
subsoil. The color patches are from greenish to dirty blue, irregularly interrupted by
bleaching which appears also in the top A (humus) horizon. All of their identifying
characteristics are due to stagnating water during wet rainy periods causing chemi-
cal reduction. Stagnosols differ from Planosols in that the long-lasting water satura-
