We measured saturated hydraulic conductivities, unsaturated hydraulic conductivities and water retentivities of Bibai peat soils, consisted of highmoor peat (natural moss), transitional peat and lowmoor peat. The transitional moor soils exhibited remarkable anisotropy of permeability. The vertical saturated hydraulic conductivity was utmost about 400 times more than its horizontal saturated hydraulic conductivity probably due to the horizontal sedimentation of plant fiber. This anisotropy was most remarkable in the subsoil of a paddy field under the dressed soil and was less in the undisturbed soils under windbreak. Soil dressings and continuous cultivations may have affected the appearance of the intensive anisotropy. The unsaturated hydraulic conductivities of transitional peat and highmoor peat were quite small and were less than those of sand. This feature caused the necessity of relatively long time for the equilibration of water retention.
The effect of grass strips on the reduction of sediment and eutrophic salt loads is been widely recognized. However, there is very little knowledge of the effect of grass strips on the reduction of sediment and eutrophic salt losses. Attention was hence focused on the investigation of what grass strips should be managed. Soil suspension was supplied to the experimental plots under simulated rainfall. During the experiment, surface discharge and sediment concentration were measured. In order to observe the effect of grass strips on the reduction of sediment loss, the difference between the amounts of sediment supplied and sediment loss was investigated in each plot. While the sediment concentration and load from the bare upland field exceeded the concentration and load of the soil suspension supplied, the sediment concentration and load from all the grass strips were lower than the concentration and load of the supplied soil suspension. The difference between the sediment supplied and the sediment loss from the cutting weeds plot for pest control was approximately equivalent to that of the plot with the natural weeds. Therefore, it was concluded that cutting weeds for pest control can be applied to the grass strip for reduction of the sediment loss. There was little difference between the reduction of the sediment loss from the plot with 800 stems/m2 and that from the plot with 1,480 stems/m2 of Tall Fescue [F. arundinacea). However the plot with 2,330 stems/m2 of Tall Fescue was effective for the reduction of sediment loss comparing with 800 stems/m2 and 1,480 stems/m2. It suggests that stem density for grass strips should be maintain 2,330 stems/m2 to control soil loss from upland field.
For the proper management of water and fertilizer resources, distributions of water and solute in soil should be precisely predicted. The movement of water and solute in soil may be expressed with partial differential equations (PDEs). The PDEs should be numerically solved for real world applications because of complex initial and boundary conditions and high non-linearity in some cases. The recent development of a general PDE solver using the finite element method enabled us to more easily solve these PDEs governing water and solute transport in soil. FlexPDE Lite can be freely obtained through the Internet, and its applicability to solve water and solute transport in soil was investigated. FlexPDE Lite successfully simulated water infiltration into Haverkamp’s sandy soil with <2% of mass balance errors. However, it failed to calculate water infiltration into dry loamy sand when initial pressure head was < — 60 cm. Results of simulating solute transport with FlexPDE Lite also agreed well with analytical solutions especially when the convective transport was not dominant. Therefore, FlexPDE Lite, although its performances were purposely limited, could be an ideal tool for a training purpose or a small-scale simulation.
Areas affected by salinization in northeastern Thailand were studied using data obtained from ground geophysics methods, multi frequency electromagnetic induction terrain conductivity meter, and hydrological data from the routine piezometer method. The electromagnetic technique is not only a useful tool for identifying the spatial distribution of salt content in landscapes, but it may also be applicable for delineating local recharge and discharge areas by comparing results with hydrological data. It could also aid in reforestation planning in large-scale areas for rehabilitation of salt-affected soils.
The effects of subsoil improvements were examined on hardsetting soils in Kamikawa, Hokkaido. The soils were fine-textured gray terrace soils. They are clayey, hard, and compacted and have a shallow soil layer. For subsoil improvement, a new technique has been developed that makes use of soil amendments. The technique is a modified method of subsoiling. It requires the construction of trenches that are 10cm wide and 25-55 cm deep at 60-120 cm intervals. Soil amendments are put into the trench. The effects of the subsoil improvements and the construction standards for the trenches are as follows :
1. Three years after the subsoil was amended, soil hardness and bulk density decreased along the trench, and air filled porosity was maintained at around 0.10 m3 m'3. Penetration resistance was greater than 2.45 MPa at a depth of 25 cm before treatment; however, it decreased to 0.98 to 1.47 MPa after the treatment.
2. The amended parts of the field worked as a supplementary underground drain in the upland field and by the improvements agricultural machinery can be operated more effectively even after rainfall.
3. The subsoil improvements caused the root zone of crops to expand. Nutrient absorption improved remarkably, and yields increased. The improvements were especially effective to deep-rooted crops which are susceptible to wet injury.
4. The trench intervals were set at 60 cm when soil hardness was greater than 1.16 MPa ; however, the intervals were expanded to 120 cm when the soil hardness was 0.53 to 1.16 MPa.
This paper discusses the effects of subsoil breaking on the change of pressure head, soil temperature and runoff discharge in space and time in a sloping field. A controlled comparison experiment was conducted in a subsoil breaking field and a no-subsoil breaking field. The results showed that the runoff ratio of the subsoil-breaking field was about 1/10 of that in the no-subsoil breaking field. This indicates that subsoil breaking promotes the infiltration of rainfall into the soil and can reduce the surface runoff discharge. The spatial distribution of the equi-pressure heads during rainfall was complicated, and the coefficients of variation were large. However, that of the equi-pressure heads before and after rainfall showed the similar patterns of variation in both fields. As for soil temperatures, we didn’t observe a large difference between the subsoil breaking field and the no-subsoil breaking field, and the spatial variation was smaller than that of the pressure head.
Internal fissures in clayey soil are important for tillage efficiency. The roots reinforce the soil and absorb soil moisture, resulting in shrinkage and fractures in the massive soil structure. The objective of this study is to clarify the influence of roots on the prevention of wide cracks and the formation of small fissures during desiccation. Soil samples were taken from the surface layer of inter-row spaces of a clayey paddy three weeks after the release of ponding water in the summer 1998. In half of the sampling sites, the stems and leaves of the rice plants were cut away just before the release of the ponding water, in order to compare the effects of the two resulting water absorption conditions. After measurement of the volume fractions of the solid, liquid, and air phases, the oven-dried samples were soaked in water and then scoured with tap water in a sieve. The remaining roots were dried and weighed. The relationships among volumetric air ratio, moisture ratio, and root density were statistically analyzed, with the volumetric air ratio being regarded as the ratio of newly formed fissures due to desiccation. Positive direct correlations were observed between root density and air ratio for all plots. In he plots with transpiration, since the moisture ratio correlated with root density and air ratio, a large proportion of the variances in air ratio explained by moisture ratio and root density were shared. These results lead to the following conclusion : Rice plant roots inhibit the formation of wide cracks and induce small fissures in clayey well-puddled soil during desiccation. With transpiration, these effects are attributed to the compound functions of roots : absorption of water and soil reinforcement. However, even without transpiration, soil reinforcement by roots uniquely contributes to the development of small fissures.