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Plinthosols

Definition

Plinthosols are soils with plinthite, petroplinthite, or pisoliths within 50 cm from the

surface soil, or within a depth of 100 cm when underlying a horizon which shows stagnic
properties or reducing conditions at sometimes of the year.

Distribution

The extent of Plinthosols is estimated worldwide at some 375 million rais. Soft plinthite
is most common in the wet tropics, especially in the eastern Amazon basin, the central Congo

basin, and parts of Southeast Asia.
In Thailand, Plinthosols cover about 15.5 million rais, distributing in all regions, notably

in the terrace, foothill slope, coalescing fans, residual areas from soil erosion, hills, and
mountains. They are mostly found in the north of the northeast region, followed by the upper

part of the east, the middle part of the south, the lower part of the north, and less common
in the central region, respectively.

Formation

Plinthosols are derived from an accumulation and segregation of iron and aluminum
oxides (sesquioxides). Also, the removal of bases and silica from the soil profiles causes the

reddish soils with high contents of the iron-rich, humus-poor mixture of quartz and kaolinite.

Aggregation of this mixture is dense enough to distinguish from the surrounding soils clearly.
Segregation of iron occurs through alternate wetting and drying to form mottles and concretions,
which gradually grow as iron and aluminum oxides are added until the whole soil mass becomes

cemented. Plinthosols are commonly developed from basic rock than acidic rock.

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