used (1) to optimize conventional soil mapping by minimizing the required number
of samples, (2) for soil suitability assessments and (3) is as such a key to sustainable
land use planning on the local scale, as it facilitates the participation of local people
in the decision making process.
While limestone areas are relatively rare in the tropics, they are extensive in
SEA and their characterization play an important role for land use planning.
Therefore, research into Paleozoic and Mesozoic limestone soils in the northern
parts of Thailand, Laos and Vietnam was carried out (see Sect. 2.5), with the
dominant soils found being Alisols and Acrisols in Thailand, Acrisols associated
with some Luvisols in Laos and Luvisols associated with Alisols in Vietnam.
Clay minerals showed a sequence running from illite through vermiculite to
kaolinite. Gibbsite was abundant in Thailand, where limestone showed intrusions
of iron ore, while goethite and hematite were also found. Kaolinite- and gibbsitedominated soils showed a limited chemical fertility, and most mineral nutrients
were restricted to the relatively fertile topsoil layer. These differences were due to
the different stages of soil development and young soil/sediment translocations.
In principle, limestone soils develop from dissolution residue, which shows –
with respect to mineralogy – less variation than normally expected. In our study,
aeolian processes seemed to be of minor importance, and besides an accumulation
of organic matter, clay mineral transformation, desilification and clay illuviation
were the dominant soil forming processes. At certain sites, ferralitisation was also
observed. Soil formation from limestone takes an extremely long time; it is
therefore of utmost importance to use the soils in a sustainable way. In particular,
erosion under sensitive crops such as upland rice may decrease soil fertility and
destroy the soils themselves. As a result, soils which have taken tens of thousands
of years to form can be lost to mankind within a few years (Sect. 2.6). Kaolinite
and gibbsite dominated soils in general tend to have a limited chemical fertility,
as most mineral nutrients are restricted to the relative fertile topsoil, which is
prone to erosion under current land use practices.
High rainfall intensities within tropical climates lead to severe soil erosion on
steep arable land, in fact, not only erosion but also nutrient depletion, and reduced
soil fertility and soil productivity are the consequences of ongoing land use
intensification in mountainous Southeast Asian areas. Section 2.6 outlines this
situation for northern Vietnam with respect to the disposition and impacts of land
use intensification on erosion and the effects of erosion on soil chemical status and
physical soil fertility levels. In the study, land use intensification on hillsides in the
research area was found to be equivalent to a reduced fallow area and to an
increased share of maize cropping on arable land – maize being the most remunerative cash crop, the consequences being an increase in the erosive slope length and
the area of uncovered soil surfaces at the beginning of the wet season, identified as
the most critical time for maize cultivation with respect to soil erosion. This has led
to middle and lower slope positions often being affected by severe erosion, with soil
on the basal slopes deteriorating through selective sedimentation of the poor
soil materials (see Chap. 3). Nitrogen and available cation stocks were found to
be reduced and the bulk density increased on arable land, causing a self-intensifying
34
K. Stahr et al.
of samples, (2) for soil suitability assessments and (3) is as such a key to sustainable
land use planning on the local scale, as it facilitates the participation of local people
in the decision making process.
While limestone areas are relatively rare in the tropics, they are extensive in
SEA and their characterization play an important role for land use planning.
Therefore, research into Paleozoic and Mesozoic limestone soils in the northern
parts of Thailand, Laos and Vietnam was carried out (see Sect. 2.5), with the
dominant soils found being Alisols and Acrisols in Thailand, Acrisols associated
with some Luvisols in Laos and Luvisols associated with Alisols in Vietnam.
Clay minerals showed a sequence running from illite through vermiculite to
kaolinite. Gibbsite was abundant in Thailand, where limestone showed intrusions
of iron ore, while goethite and hematite were also found. Kaolinite- and gibbsitedominated soils showed a limited chemical fertility, and most mineral nutrients
were restricted to the relatively fertile topsoil layer. These differences were due to
the different stages of soil development and young soil/sediment translocations.
In principle, limestone soils develop from dissolution residue, which shows –
with respect to mineralogy – less variation than normally expected. In our study,
aeolian processes seemed to be of minor importance, and besides an accumulation
of organic matter, clay mineral transformation, desilification and clay illuviation
were the dominant soil forming processes. At certain sites, ferralitisation was also
observed. Soil formation from limestone takes an extremely long time; it is
therefore of utmost importance to use the soils in a sustainable way. In particular,
erosion under sensitive crops such as upland rice may decrease soil fertility and
destroy the soils themselves. As a result, soils which have taken tens of thousands
of years to form can be lost to mankind within a few years (Sect. 2.6). Kaolinite
and gibbsite dominated soils in general tend to have a limited chemical fertility,
as most mineral nutrients are restricted to the relative fertile topsoil, which is
prone to erosion under current land use practices.
High rainfall intensities within tropical climates lead to severe soil erosion on
steep arable land, in fact, not only erosion but also nutrient depletion, and reduced
soil fertility and soil productivity are the consequences of ongoing land use
intensification in mountainous Southeast Asian areas. Section 2.6 outlines this
situation for northern Vietnam with respect to the disposition and impacts of land
use intensification on erosion and the effects of erosion on soil chemical status and
physical soil fertility levels. In the study, land use intensification on hillsides in the
research area was found to be equivalent to a reduced fallow area and to an
increased share of maize cropping on arable land – maize being the most remunerative cash crop, the consequences being an increase in the erosive slope length and
the area of uncovered soil surfaces at the beginning of the wet season, identified as
the most critical time for maize cultivation with respect to soil erosion. This has led
to middle and lower slope positions often being affected by severe erosion, with soil
on the basal slopes deteriorating through selective sedimentation of the poor
soil materials (see Chap. 3). Nitrogen and available cation stocks were found to
be reduced and the bulk density increased on arable land, causing a self-intensifying
34
K. Stahr et al.