limestone areas in Southeast Asia are predominantly characterized by unique
landforms consisting of karst towers (Gunn 2004) – a reason, why soil development
is comparably young. In general, soil development started in the Paleogene period or
later; nevertheless, limestone dissolution in the semi-humid tropic areas seems to have
been rather quick and; therefore, in the karstic depressions, one can find layers several
meters thick derived from the residue of limestone. Due to their common steep and
rugged slopes, these areas are often found in the highlands, where there is also poor
infrastructure. As a result, they tend to be inhabited by socially disadvantaged groups,
and especially by ethnic minorities. In these areas, fallow-based farming systems that
use little mineral fertilizer and that work alongside subsistence-oriented animal
husbandry systems prevail. However, a recent change from long-term to shorter fallow
periods or permanent cropping-based farming systems has involved the increasing
application of agrochemicals, which is problematic because limestone areas are
generally karstified and characterized by high subsurface discharge. Tests with tracers
in Son La province revealed translocation velocities between 95 and 235 m/h in karstic
underground areas (Nguyet 2006). A behavior, which may affect downstream the
functionality of ecosystems and the quality of drinking water. In addition, the continuous clearing of forests and cropping on sloping land has led to tremendous soil losses
occurring, those which will not be reconstituted for thousands of years based on
natural soil formation processes. Limestone soils represent environmental barriers to
xenobiotica between the surface and the karstic underground areas; therefore, provide
important ecosystem services. Due to the fact that soils in the karstic areas of Southeast
Asia are not well-understood nor analyzed (Vijarnsorn and Eswaran 2002; Kubiniok
1999), this paper aims to present basic information on limestone soils in the region, to
highlight soil formation processes in such areas and the resulting characteristics and
constraints that arise. Examples from Thailand, Laos and Vietnam will be presented.
There are two main initial hypotheses used, when working with sub-humid
tropical soils on limestones. The first is that the soils will be derived in the main
from limestone residues, and with addition of aeolian material not expected or of
minor importance when compared with limestones in subtropical areas (Jahn 1997).
The second hypothesis is, that it will be expected for the soil development process
to show tropical soil formation characteristics, such as with ferralitic soils, but only
when the land surface is sufficiently old.
The following questions will be addressed here: (1) What factors influence soil
formation in the study area, (2) what processes prevail in sub-humid Southeast
Asia, and (3) how are these related to those in other tropical and subtropical zones?
Finally, it should be checked as to how vulnerable these soils are to chemical and
physical soil degradation.
2.5.1 Materials and Methods
Materials: Three karst catchments, each a few km
2 in size, were selected in northern
Thailand, Laos and northern Vietnam, with the ages of the limestone ranging from
Carboniferous in Laos to Permian in northern Thailand and finally Triassic in
78
K. Stahr et al.
landforms consisting of karst towers (Gunn 2004) – a reason, why soil development
is comparably young. In general, soil development started in the Paleogene period or
later; nevertheless, limestone dissolution in the semi-humid tropic areas seems to have
been rather quick and; therefore, in the karstic depressions, one can find layers several
meters thick derived from the residue of limestone. Due to their common steep and
rugged slopes, these areas are often found in the highlands, where there is also poor
infrastructure. As a result, they tend to be inhabited by socially disadvantaged groups,
and especially by ethnic minorities. In these areas, fallow-based farming systems that
use little mineral fertilizer and that work alongside subsistence-oriented animal
husbandry systems prevail. However, a recent change from long-term to shorter fallow
periods or permanent cropping-based farming systems has involved the increasing
application of agrochemicals, which is problematic because limestone areas are
generally karstified and characterized by high subsurface discharge. Tests with tracers
in Son La province revealed translocation velocities between 95 and 235 m/h in karstic
underground areas (Nguyet 2006). A behavior, which may affect downstream the
functionality of ecosystems and the quality of drinking water. In addition, the continuous clearing of forests and cropping on sloping land has led to tremendous soil losses
occurring, those which will not be reconstituted for thousands of years based on
natural soil formation processes. Limestone soils represent environmental barriers to
xenobiotica between the surface and the karstic underground areas; therefore, provide
important ecosystem services. Due to the fact that soils in the karstic areas of Southeast
Asia are not well-understood nor analyzed (Vijarnsorn and Eswaran 2002; Kubiniok
1999), this paper aims to present basic information on limestone soils in the region, to
highlight soil formation processes in such areas and the resulting characteristics and
constraints that arise. Examples from Thailand, Laos and Vietnam will be presented.
There are two main initial hypotheses used, when working with sub-humid
tropical soils on limestones. The first is that the soils will be derived in the main
from limestone residues, and with addition of aeolian material not expected or of
minor importance when compared with limestones in subtropical areas (Jahn 1997).
The second hypothesis is, that it will be expected for the soil development process
to show tropical soil formation characteristics, such as with ferralitic soils, but only
when the land surface is sufficiently old.
The following questions will be addressed here: (1) What factors influence soil
formation in the study area, (2) what processes prevail in sub-humid Southeast
Asia, and (3) how are these related to those in other tropical and subtropical zones?
Finally, it should be checked as to how vulnerable these soils are to chemical and
physical soil degradation.
2.5.1 Materials and Methods
Materials: Three karst catchments, each a few km
2 in size, were selected in northern
Thailand, Laos and northern Vietnam, with the ages of the limestone ranging from
Carboniferous in Laos to Permian in northern Thailand and finally Triassic in
78
K. Stahr et al.