In hilly and mountainous areas, many terrace paddy fields have been abandoned due to lack of the successor to the farmer, difficulty of irrigation and so on. To clarify the changes of the soils after abandoned, the terrace paddy fields that were 3,12 and 20 years after farmers give up cultivation were investigated. The neighboring cultivated paddy fields, as the controls, were also investigated in Ohshima village, Niigata prefecture. The soil three years after abandoned had little differences in soil morphology and property compared with the control. In the cultivated paddy fields, the soils were classified into Gley or Gray Upland soils. On the well-drained sites (dominated by Miscanthus sinensis), the soils 12 and 20 years after abandonment had been desiccated and classified into Brown Forest soils. The surface (plow) and subsurface (plowsole) layers of these soils had changes in soil color from reduced gley or gray to oxidized brown and had weakly developed granular structure. In contrast, the soils 12 and 20 years after abandoned on the poorly drained sites (dominated by Phragmites communis) were classified into the same taxa as the controls. Basic intake rate, hydraulic conductivity and macro pore were increased, and exchangeable bases were decreased with the years after abandoned in the surface and subsurface layers of the soils on the welldrained sites. There, however, were not significant differences in these properties of the soils on
the poorly drained sites. After abandoned, soil morphological features and physical properties were remarkably changed on the well-drained sites. The changes of the soils have been influenced by microtopography
and drainage conditions.
This study was conducted to make clear the groundwater environment and the influences to the vegetation of forest land under the subway construction. The former was investigated by using the distributions of water table and. the cross sections. The latter was investigated by using the daily fluctuations of soil moisture tension in the planting site of higher trees and the observations of growth along the subway construction. Consequently, the groundwater environment was not influenced so much by the shield tunnel-ing of the subway construction, but much influenced by the excavation works. Because the excavation works are considered to cause the sudden change of groundwater environment. On the other hand, the sudden change of ground water influences to the soil moisture environment of the vegetation, so it is found that much higher dead trees and higher weakened trees were observed along the route of the excavation works.
This report describes the change of soil water properties of SHENYNAG Agricultural Universi-ty Experimental Farm for 6 months, from May to October in 1993. In order to study the properties, soil moisture suction was measured in the depth of 10 cm, 20 cm, 30 cm, 50 cm and 70 cm from ground surface at 6 : 00 PM every day. After the soil suction in the farm was measured, soil samples were obtaind in the each depth from ground surface. Soil moisture characteristic curves were determined by soil moisture external method and the soil gravity and the soil grain size distribution were measured from the samples. By scaling the data of each soil moisture characteristic curves , scaling factors were calculated. The following results were obtained :
(1) The water retentivity was influenced by not only the depth but also soil properties, for examples, the grain size distribution and the dry density.
(2) Rainfall during summer in 1993 was ordinary level. Rainfall of some months were very different from the normal year. Conditions in the soil water were very good for the growth of some vegetables in the field.
(3) The soil layer in the depth of 20cm and 30cm was very important for the vegetables growth, while it was found that the soil water supply layer under the 30 cm was also important.
(4) Soil moisture extraction pattern in the field was classified in all layer soil moisture extraction pattern.
The objective of this study is to investigate the effect of soil structural changes on raindrop erosion processes. Disturbed and undisturbed soils are sampled in embanked slope of Memanbetu, Hokkaido. According to the results of raindrop erosion test, splash ratio decreased after the repetition of freezing-melting process. Soil saturated hydraulic conductivity increased after the repetition. It was concluded that percolation mechanism is important to understand the raindrop erosion processes. Percolation volume was simulated on the bases of rheological properties of percolated suspen-sion and pF-moisture distribution curves. Results of the simulation represented observed changes in permeability due to the repetition of freezing-melting process in this study.
The relationship between change in soil structure induced by application of cow’s compost and yield of paddy rice was investigated in 1/2000 a pot trials. Three fertilization treatment were set up by compost application at the rate of 100%,50% or 0%,and adjusted to the total concentration of three major nutrients (NPK) with application of chemical fertilizer. Compost application gave considerable increase in rice yield. The increase in rice yield can be explained by the following effects. One was slower mineralization of compost than chemical fertilizer, hence, the nutrient-absorption pattern of rice in this situation increased yield. The other effect was the decrease in the solid phase of the soil induced by compost application. This study showed the difficulty of dividing the effects of compost application into chemical and physical effects.
A simulated rainfall study was conducted to evaluate the effects of slope gradient on interrill erosion of Shirasu, a volcanic ash soil. A simulated 30-min., 62 mm/h rainfall was applied to the slopes of 6°, 9°,12°,15° and 20° to measure interrill erosion and runoff. Whether runoff or soil loss rates all showed rapid increase in first ten minutes from the start of experiment except for slope 6° and 9°. Runoff became almost constant for slopes of 12°,15° and 20°, but soil loss rates showed a different variance according to slope gradient at the latter half of 30 min run. A third -degree polynomial for the relation of gradient with runoff and soil loss rates can be established based on the result of regression analysis of data. But variance in gradient for runoff differed from that for soil loss. For runoff, under the conditions of the experiment, the maximum was 15°, and there was a decrease beyond this slope gradient. Soil loss rates increased with slope gradient within the range of steepness in the experimental conditions. But increment rates differed according to the slope gradient, rapid increase was noted for slopes from 9° to 15°, and soil loss rates flattened beyond 15°.