Geoscience Reference
In-Depth Information
Plinthosols are mainly tropical soils exemplifi ed by an iron-rich, in some cases
also manganese-rich, humus-poor mixture of kaolinitic clay and gibbsites. The
consequence of this composition is a very small cation exchange capacity. When
this material exists in various forms from soft lumps to hardpan, it is described as
plinthite, and when it extends to continuous rocklike forms, it is denoted as petro-
plinthite. Laymen could confuse them with ironstone. The name was derived from
the Greek
plinthos
meaning brick. They are also known as Lateritic soils or Latosols
where the name was derived from the Latin
later
meaning brick. They developed
during extremely severe weathering conditions in wet hot tropical climates. As
weathering proceeded, bases were released and removed, while sesquioxides accu-
mulated within the profi le at the place of weathering. The process intensifi ed when
Fe in its bivalent soluble form was transported by groundwater from neighboring
slopes to a lower fl at terrain or by a great downward fl uctuation of groundwater
table, causing a precipitation of Fe
2
O
3
and giving an intensive red color to the
Plinthosols. They are more frequently and more vividly developed from basic rocks
than from acidic rocks. We recognize them in the fi eld by red mottles that are fi rm
when they are wet and very hard when dry. In either case, they are diffi cult to cut
with a knife and do not stain our fi ngers when we rub them. In some instances the
Plinthosols originate on old, many meters thick petroplinthites. The profi le of
Plinthosols is characterized by the presence of illuvial B horizon and even with the
eluvial E horizon above B horizon. Their sequences of horizons are designated as
A-B-C or A-E-B-C, respectively. Inasmuch as Plinthosols are shallow soils, their
agronomic use is primarily constrained by factors affecting plant nutrition. Their
disposition to retain adequate amounts of macronutrients is constrained by their
extremely small cation exchange capacity (CEC). The existence of plinthite restricts
the rooting depth of plants. Stoniness is frequent. With very low contents of both the
macro- and micronutrients, their natural fertility is usually poor. These soils are
known in some national systems as Groundwater Laterite Soils, Lateritas
Hydromorfi cas, or Sols Gris Latéritiques. The US Taxonomy recognizes them as
Oxisols and specifi es them at lower taxonomic levels.
Nitisols are deep well-drained reddish and red-brown soils of tropical rain forest
and high savanna with a typical nitic horizon below the relatively fertile and humus-
rich top horizon. The name of the RSGs is related to nitic horizon with its name
derived from the Latin
nitidus
meaning shiny, since the stable polyhedric aggregates
with strong angular structural elements have many very shiny walls. Since mottling
of the nitic horizon is always lacking, there is no infl uence of periodic waterlogging
on the development of these soils. This absence of mottling is one of the signs of
how Nitisols differ from Plinthosols. However, there is still a dominance of kaolin-
ite and halloysite in their clay fraction and their cation exchange capacity (CEC) is
still very low. Although the dominant color of Nitisols is red, it is less intensive than
that of Plinthosols. Nitisols, composed of A-B-C horizons, are profi tably used for
agricultural crop production. Even though their level of plant-available P is low
owing to a high content of free Fe, they are rich in soil microfauna and well homog-
enized without sharp boundaries between horizons, and all of their biologic diver-
sity contributes to good physical properties. They are recognized in national systems
