Very low amounts of available phosphorus (P-Bray1) were found in all soils
(ranging between 0.12 and 18 g m
À2
). In general, the highest P-Bray1
concentrations were found in A-horizons (up to 5.5 mg kg
À1 ) and in the home
gardens (1 TS HG; up to 43.1 mg kg
À1 ). Lower P-Bray1 stocks were detected in
catena 2 (Fig. 2.19f) than in catena 1.
Based on our observations and on the soil data, it can be deduced that beside
available nutrients, soil organic matter plays a key role in soil fertility during the
vicious cycle of events caused by soil erosion and soil fertility decline. Diminishing
soil organic matter content due to erosion leads to a degradation of the soil’s
physical properties, with aggregate stability – crucial for soils susceptible to soil
sealing, like the soil derived from clastic sediments (CK-S) – being reduced (2).
Compaction due to land use increase can cause a reduction in the number of coarse
pores and in infiltration rates and a hardening of the soil structure, which makes
intensive tillage necessary. Tillage in turn leads to a disruption in pore continuation
and supplies loose, easily erodible soil material. The common praxis of burning
plant residuals inhibits any increase in organic matter, despite the production of
large amounts of organic material, plus reduces biological activity and therewith
porosity. In the Chieng Khoi soils, a close relationship between organic matter
(OM) content and the sum of total nitrogen, plant available phosphorus and
S-values was found (Clemens et al. 2010), proving the importance of soil organic
matter with respect to soil fertility.
2.6.5 Conclusion Regarding Soil Erosion
The study area is highly susceptible to erosion and all the cultivated soils are
affected by it. Particularly land use intensification is associated with reduced fallow
periods and the continuous cultivation of crops and fosters therefore erosion
susceptibility. In respect of the necessity to develop a more sustainable land use
system, the following factors need to be considered. The susceptibility of soils with
regard to erosion depends mostly on the parent material, and soil organic matter
content plays a key role in improving soil conditions. Annual crops are the most
critical cultivars, because they leave the soil surface partly or part of the time
unprotected, particularly at the beginning of the wet season, resulting in splash and
soil sealing. Increasing the erosive slope length increases the amount and velocity
of overland flows, making mid- and lower slopes the most erosion prone terrain
positions. Even if fertilizers are able to compensate for the nutrients lost through
erosion, the sustainable cultivation of annual crops on steep slopes under monsoonal climates is barely attainable, and is only possible if measures are taken to
avoid the negative impacts of the actual land system shown here. A combination of
reduced erosive slope length and an increase in the stocks of soil organic matter are
considered to be necessary measures. Several experiments within the research area
have shown that it is possible to reduce erosion and increase soil fertility without
using additional fertilizer inputs (see Chap. 3).
102
K. Stahr et al.
(ranging between 0.12 and 18 g m
À2
). In general, the highest P-Bray1
concentrations were found in A-horizons (up to 5.5 mg kg
À1 ) and in the home
gardens (1 TS HG; up to 43.1 mg kg
À1 ). Lower P-Bray1 stocks were detected in
catena 2 (Fig. 2.19f) than in catena 1.
Based on our observations and on the soil data, it can be deduced that beside
available nutrients, soil organic matter plays a key role in soil fertility during the
vicious cycle of events caused by soil erosion and soil fertility decline. Diminishing
soil organic matter content due to erosion leads to a degradation of the soil’s
physical properties, with aggregate stability – crucial for soils susceptible to soil
sealing, like the soil derived from clastic sediments (CK-S) – being reduced (2).
Compaction due to land use increase can cause a reduction in the number of coarse
pores and in infiltration rates and a hardening of the soil structure, which makes
intensive tillage necessary. Tillage in turn leads to a disruption in pore continuation
and supplies loose, easily erodible soil material. The common praxis of burning
plant residuals inhibits any increase in organic matter, despite the production of
large amounts of organic material, plus reduces biological activity and therewith
porosity. In the Chieng Khoi soils, a close relationship between organic matter
(OM) content and the sum of total nitrogen, plant available phosphorus and
S-values was found (Clemens et al. 2010), proving the importance of soil organic
matter with respect to soil fertility.
2.6.5 Conclusion Regarding Soil Erosion
The study area is highly susceptible to erosion and all the cultivated soils are
affected by it. Particularly land use intensification is associated with reduced fallow
periods and the continuous cultivation of crops and fosters therefore erosion
susceptibility. In respect of the necessity to develop a more sustainable land use
system, the following factors need to be considered. The susceptibility of soils with
regard to erosion depends mostly on the parent material, and soil organic matter
content plays a key role in improving soil conditions. Annual crops are the most
critical cultivars, because they leave the soil surface partly or part of the time
unprotected, particularly at the beginning of the wet season, resulting in splash and
soil sealing. Increasing the erosive slope length increases the amount and velocity
of overland flows, making mid- and lower slopes the most erosion prone terrain
positions. Even if fertilizers are able to compensate for the nutrients lost through
erosion, the sustainable cultivation of annual crops on steep slopes under monsoonal climates is barely attainable, and is only possible if measures are taken to
avoid the negative impacts of the actual land system shown here. A combination of
reduced erosive slope length and an increase in the stocks of soil organic matter are
considered to be necessary measures. Several experiments within the research area
have shown that it is possible to reduce erosion and increase soil fertility without
using additional fertilizer inputs (see Chap. 3).
102
K. Stahr et al.