Tan Cuong commune in Thai Nguyen province is praised for the best tea quality among the northern region of Vietnam. In the commune, the market price of tea leaves was high at the gardens close to the Cong River and tended to lower with increasing distance from the river. In this context, soil morphological, mineralogical and chemical properties were investigated to characterize the soils in four tea gardens which were located on a transect line established perpendicularly to the river: TB1 close to the river, TB2 to TB4 away from the river and located on the previous dikes. The investigation was also conducted at two forested patches (FB1 and FB3) adjacent to TB1 and TB3. In addition, one garden was selected in Song Cau commune (TS1), located on a hillslope. All of the tea garden soils were strongly acidic throughout the profile with high contents of exchangeable Al. At the riverside, the TB1 and FB1 soils were classified as Typic Ustipsaments. They were sandy and predominated by quartz and kaolin minerals in the clay fraction. Meanwhile, the TB2 to TB4 and FB3 soils far from the river were Typic Dystrusteps while the TS1 soil was Typic Paleustults. These soils were relatively fine textured with various kinds of 2:1 clay minerals. The (Fe_d-Fe_o)/Fe_t ratio was the lowest in TB1 and FB1 and the highest in TS1. The analysis of total element contents agreed with the difference in these mineralogical properties. In addition, the influence of soil disturbance was found in TB2 and TB4. While the former was weakly reflected only by discontinuity of the horizon stratification and mineralogical properties, the latter was so severe resulting in a very shallow soil depth with abundant rock fragments.

We evaluated the effects of soil mixing by deep-plowing under Chinese yam cultivation on soil profile characteristics and soil classification in Tokachi district of Hokkaido. A grower field with six-time Chinese yam cultivation history was chosen, and two soil profiles within a field were evaluated in higher elevation (upper) and lower elevation (lower) locations. A backhoe was used to make a 5-m trench perpendicular to the planting rows, and soil profiles were exposed with areas where soil had been disturbed or undisturbed by soil mixing. Soil samples were collected from each horizon or at certain depth in undisturbed and disturbed parts, respectively, and the samples were analyzed for soil particle size, physico-chemical characteristics, and Al, Fe, and Si by selective dissolution. The width of undisturbed part within the 5 m trench was only 20 to 40 cm, and most of the profile was disturbed. Separation of soil horizons were unclear in disturbed profile, and appropriate evaluation of soil profiles will be challenging unless undisturbed parts are found. In this study, we found undisturbed part, and the soils were classified as Brown-humic Allophanic Andosols (relocatic phase) In the disturbed part, soil color and physico-chemical characteristics were homogenized and would be difficult to classify the soil at the subgroup level. When subsoils have lowland soil or upland soil materials, the soil profile may no longer be classified as Andosols.