Soil physicochemical properties in a high-quality tea production area of Thai Nguyen province in northern region, Vietnam

This study investigated general physicochemical properties of tea garden soils at the alluvial plain of Cong River in Tan Cuong commune, Vietnam. Four gardens were selected as study sites on three transect lines established perpendicularly to the river. Soil samples were collected from the surface (0-10 cm) and subsurface (20-30 cm). Soil texture classes varied from sandy loam to light clay, which was affected by different terrains along the transect lines as well as severe disturbance such as terracing and earth excavation. The levels of total C and total N were correlated with increasing garden age, suggesting the replenishment of soil organic matter pool by the addition of plant residue and manure. Meanwhile, the soils showed strongly acidic nature with the average pH(H2O) of 3.7 at the surface and 3.9 at the subsurface. The effective cation exchange capacity (ECEC) was low at 4.7 and 4.9 cmolsub(c) /kg, respectively, and dominated by exchangeable Alsup(3+). Soil acidification was exacerbated with increasing garden age. However, a relatively large saturation of exchangeable calcium (Casup(2+)), potassium (Ksup(+)), and magnesium (Mgsup(2+)) on the ECEC was found in the surface soils. The levels of available P were high, occasionally exceeding 1000 and 500 mg /kg at the surface and subsurface, respectively. In spite of strongly acidic condition, ammonium (NH4-N) applied as fertilizer was converted to nitrate (NO3-N) to move down to deeper layers. The levels of the bases, P, and mineral N seem to be principally determined by management practices. Significant portion of these nutrients was likely to exist in water soluble forms without adsorption onto soils. It should be required to develop proper schemes and to educate the owners for adequate fertilizer managements.