2.5.3 Soil Associations or Soil Distribution in the Karstic Areas
of Southeast Asia
In the three research areas, Bor Krai in Thailand, Huay Sang in Laos and Muong
Lum in Vietnam, quite a variety of soils was found. In total, 14 of the 32 reference
groups from the World Reference Base of Soil Resources were observed, and as in
other areas when using a quantitative approach, only a few reference soil groups
were shown to dominate, these being Alisols and Acrisols, besides Cambisols,
Leptosols and Luvisols. However, the associated soil reference groups may be
used to characterize and especially differentiate the landscapes from each other.
Bor Krai in Thailand represents a typical karst area with all the associated
phenomena such as caves and dolines (Fig. 2.12). Leptosols were found there on
very steep slopes and on convex elevated landscape positions. Nudilithic Leptosols
appeared with no real A-horizon, but with a cover of lichens and a little bit of soil
material. Leptosols can develop from other limestone soils via erosion and then
have a clay-loam texture and a sub-angular blocky structure. By far the dominant
soils were found to be of a clay illuviation type, soils that showed a strong clay
increase with depth, from a clay loam or silt loam in the topsoil to a high clay
content and clayey texture with angular blocky structures at a depth of 1–1.5 m.
Alisols were mostly found at the lower altitudes (equals higher temperature and
lower rainfall), and in Thailand were found to have a red soil color, indicating that
rubefication had taken place during their formation, which is also true for the
Acrisols, which were widespread in altitudes above 800 m.a.s.l. In both RSGs,
the A-horizon directly overlaid the argic horizon, though a prominent E-horizon
was generally missing. Acrisols may grade into Ferralsols; however, especially in
Bor Krai, this may have been related to a lithological change, because Ferralsols
were only found where iron ore intruded the limestone (Herrmann et al. 2007). The
texture of the Ferralsol was silty-clay to clay and the structure was characteristically
a granular to sub-angular blocky structure which is very strong. Also, there was an
increase in the amount of clay found with depth.
Those soils with the highest organic matter content and abundant biotic
structures were found around karst springs, associated with the occurrence of
freshwater limestone. These soils could be classified as Calcic to Luvic
Chernozems, and had a very dark and more than 50 cm thick topsoil. The occurrence of secondary lime in the topsoil was characteristic, though karstic infillings in
the depressions often contained Umbrisols. Charcoal and brick stone remnants
pointed towards human influence and long-term slash and burn cultivation
activities. Floodplain soils (Fluvisols) and groundwater affected soils (Gleysols)
are extremely rare in this environment, since surface streams and surface groundwater are rare in karst areas; however, where they occur, they represent very fertile
soils and are frequently used for legumes and other garden crops which demand
irrigation. Overall, the Bor Krai area was composed of 66 % Alisols, 20 % Acrisols,
8 % Cambisols and 1 % Leptosols, while Umbrisols, Ferralsols, Chernozems,
Fluvisols and Gleysols together representing less than 1 %.
82
K. Stahr et al.
of Southeast Asia
In the three research areas, Bor Krai in Thailand, Huay Sang in Laos and Muong
Lum in Vietnam, quite a variety of soils was found. In total, 14 of the 32 reference
groups from the World Reference Base of Soil Resources were observed, and as in
other areas when using a quantitative approach, only a few reference soil groups
were shown to dominate, these being Alisols and Acrisols, besides Cambisols,
Leptosols and Luvisols. However, the associated soil reference groups may be
used to characterize and especially differentiate the landscapes from each other.
Bor Krai in Thailand represents a typical karst area with all the associated
phenomena such as caves and dolines (Fig. 2.12). Leptosols were found there on
very steep slopes and on convex elevated landscape positions. Nudilithic Leptosols
appeared with no real A-horizon, but with a cover of lichens and a little bit of soil
material. Leptosols can develop from other limestone soils via erosion and then
have a clay-loam texture and a sub-angular blocky structure. By far the dominant
soils were found to be of a clay illuviation type, soils that showed a strong clay
increase with depth, from a clay loam or silt loam in the topsoil to a high clay
content and clayey texture with angular blocky structures at a depth of 1–1.5 m.
Alisols were mostly found at the lower altitudes (equals higher temperature and
lower rainfall), and in Thailand were found to have a red soil color, indicating that
rubefication had taken place during their formation, which is also true for the
Acrisols, which were widespread in altitudes above 800 m.a.s.l. In both RSGs,
the A-horizon directly overlaid the argic horizon, though a prominent E-horizon
was generally missing. Acrisols may grade into Ferralsols; however, especially in
Bor Krai, this may have been related to a lithological change, because Ferralsols
were only found where iron ore intruded the limestone (Herrmann et al. 2007). The
texture of the Ferralsol was silty-clay to clay and the structure was characteristically
a granular to sub-angular blocky structure which is very strong. Also, there was an
increase in the amount of clay found with depth.
Those soils with the highest organic matter content and abundant biotic
structures were found around karst springs, associated with the occurrence of
freshwater limestone. These soils could be classified as Calcic to Luvic
Chernozems, and had a very dark and more than 50 cm thick topsoil. The occurrence of secondary lime in the topsoil was characteristic, though karstic infillings in
the depressions often contained Umbrisols. Charcoal and brick stone remnants
pointed towards human influence and long-term slash and burn cultivation
activities. Floodplain soils (Fluvisols) and groundwater affected soils (Gleysols)
are extremely rare in this environment, since surface streams and surface groundwater are rare in karst areas; however, where they occur, they represent very fertile
soils and are frequently used for legumes and other garden crops which demand
irrigation. Overall, the Bor Krai area was composed of 66 % Alisols, 20 % Acrisols,
8 % Cambisols and 1 % Leptosols, while Umbrisols, Ferralsols, Chernozems,
Fluvisols and Gleysols together representing less than 1 %.
82
K. Stahr et al.