Effects of tea cultivation on soil quality in the northern mountainous zone, Vietnam
Dang, Minh Van
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The objectives of the study were to assess soil quality and its relationship to the sustainability of tea cultivation in the Northern Mountainous Zone, Vietnam. Overall soil quality declined with increasing age of the tea plantations as evidenced by decreases in soil OC, total N, K and S, available P and K, mean weight diameter of aggregates, total porosity, plant available water capacity (PAWC), and earthworm populations. As well, total P, bulk density and mechanical resistance increased with increasing cultivation intensity. Because these soil properties were sensitive to cultivation effects, they were considered to be good indicators of soil quality. Soil properties that were less sensitive to change, and limited as soil quality indicators included texture, clay mineralogy and sesquioxides, Cd concentration, and effective cation exchange capacity. Soil quality changes were greatest during the first 10 years of cultivation and were generally greatest in the surface 0-to 40-cm of soil. Soil and crop management factors (e.g., fertilization) were considered to be the most important factors affecting soil quality. Decreases in long-term crop yields were found to correspond with decreases in soil quality. In terms of crop productivity, the most important soil quality indicators (based on a multiple regression analysis) were OC, available P, total K and PAWC. Economic analysis of the yield and production cost data indicated that, under current conditions, tea cultivation in the Northern Mountainous Zone is sustainable for periods of about 40 years. Thus, measured values of soil quality indicators in the 40-yr tea soils were considered to represent the "critical levels" for economic sustainability of tea cultivation. In addition to quantitative assessments of soil quality, qualitative assessments involving farmer interviews were used to evaluate the overall efficiency of current management practices to sustain long-term tea production. The major socio-economic indicator of sustainability was farm prosperity, which reflected the willingness of farmers to adopt soil conservation technologies. Government policies related to land ownership and market access also were important factors influencing sustainability. Generally, farmer observations of the changes in soil quality were in good agreement with the quantitative assessments. Qualitative information obtained from on-farm surveys supplement the quantitative data obtained through soil analyses, and should be incorporated into future studies.