LAN DO THI, GERHARD GEROLD
Georg-August-Universität Göttingen, Department of Landscape Ecology, Germany
The mountainous areas of Vietnam cover ¾ of the national territory, with much difficulty for agricultural production. Nevertheless, the cultivated land /capita decreases annually (it was [0.109]hacapita in 1989, and only 0.089ha in 1999. According to UNDP, with a projected in the year 2010, the cultivated land/ capita will further decrease to 0.036ha). The conflict of increasing population on one hand and decreasing land/capita on the other hand leads to overuse and misuse of land - reason for soil erosion and degradation, which result to decline crop yields and hence decrease income as well as threaten food security situation for the farmer in this area. In addition, the off-site effects of soil erosion often have broader economic and environmental implications including sedimentation, flooding, and reduced water quality resulting in poorer living conditions for the people.
The examination of soil samples shows that pH value and base saturation in the area clearly decreased after cultivation for few years. The degradation of soil from the first to the fourth year of maize cultivation is indicated clearly by a decrease of organic matter, total nitrogen content and the cations exchange capacity between forestland and first year after slash and burn or between the first year and the fourth years after slash and burn. This change is due to the different level of soil erosion in the plot of first year to fourth year cultivated land. The availability of P content after Bray in both soil depths (0 to 30cm and 31 to 60cm) were very low especially in the plots that was used for long time and tended to decrease fast from the first year to the fourth years after cultivation. This indicates that land becomes degraded fast after few years cultivation. The reason lies in the fact that partly available P is lost through erosion process and also due to iron and aluminium enrichment at low pH values leading to reduce phosphorus availability through the irreversible phosphate fixation. Therefore, long time use of land without any soil conservation measures (soil erosion control) soil degradation leads to unusable grassland.
Keywords: Land resource, shifting cultivation, soil degradation, sustainable land use, Vietnam