Geoscience Reference
In-Depth Information
According to ST, a small amount of plinthite in the soil does not form a continuous
phase; individual redoximorphic concentrations are not interconnected; a large
amount of plinthite may form a continuous phase. Individual aggregates of plinthite
in a continuous phase are interconnected, and the spacing of cracks or zones that roots
can enter is 10 cm or more. If a continuous layer becomes indurated, it becomes a
massive ironstone layer with irregular, somewhat tubular inclusions of
reductimorphic, clayey material. If the layer is exposed, these inclusions may be
flushed from the soil, leaving ironstone with coarse, tubular pores. Much that has
been called laterite is included in the meaning of plinthite. However, hardened
laterite, either vesicular or pisolitic, is not included in the definition of plinthite.
In ST plinthite is designated with the small-case letter “v.” Plinthite is nearly
always contained within the argillic or kandic horizon. Based on data from 45 soil
series, the mean depth to plinthite is 89 cm and the plinthite extends to an additional
89 cm or more. In some pedons, the plinthite layer is preceded by a concretionary
layer of iron oxides.
Most of the previous studies of plinthite have been carried out in either Brazil
(Anjos et al.
1995
,
2007
; Coelho and Vidal-Torrado
2003
; Fritsch et al.
2007
;
Miguel et al.
2013
) or the coastal plain of southeastern USA (Woods and Perkins
1976a
,
b
; Daniels et al.
1978
; Shaw et al.
1997
; Jacobs et al.
2002
; Aide et al.
2004
).
These studies focused on the field identification, role of parent material and
topography in the development of plinthite, and classification of plinthitic soils.
25.2 Properties of Soils with Plinthite
Figure
25.1
is a photograph of the Dothan soil series, a fine-loamy, kaolinitic,
thermic Plinthic Kandiudults. The plinthite appears as plates, polygons, or reticu-
late patterns, but concretionary forms (“petroplinthite”) are also common. Plinthite
is very acidic, low in SOC, has a very low cation-exchange capacity (i.e., low
activity clays), and has abundant Fe
d
but low levels of Al
d
(Table
25.1
). Unlike the
spodic horizon, plinthite has a low ODOE. Minerals in the clay-size fraction and
nodules are commonly kaolinite, hematite, and goethite but mica, gibbsite, and
anatase may also be found (Coelho and Vidal-Torrado
2003
).
25.3 Classification of Soils with Plinthite
There are 45 soil series in the USA, exclusive of US “territories,” with plinthite
(Table
25.2
). Plinthite occurs in two orders, four suborders, and five great groups,
including the Ultisols and Alfisols. However, the NRCS database includes pedons in
Micronesia that are classified as Oxisols. Although ST recognizes plinthite at the
great group level in all three of these orders, all of the soil series containing plinthite
occur in subgroups designated as Plinthic, Plinthaquic, Arenic Plinthic, and Arenic
Plinthaquic.
Search WWH ::
Custom Search