Effects of dressing pyroclastic deposits to the crust forming soils distributing in Biei hill area are summarized as follows.
1) Crust forming soils in Biei hill area shows difficulty in plowing, which becomes the limiting factor for the crop production. This soil is not clayey, but easily dispersed after rain fall, and readily forms crust on drying. This crust forming soil has remarkably large solid percentage, high bulk density, and low coarse porosity. The soil hardness also increases remarkably on drying.
2) Pyroclastic deposits improves these soil physical properties, and the yield of field crops increase steadily by dressing the pyroclastic deposits to the thickness of 5-10 cm. Larger dressing of pyroclastic deposit showed little effect on the yield of field crops due to the decrease of soil fertility.
3) Soil dressing decreased the specific resistance on traction when field is plowed. Soil dressing of 5-10 cm thickness was effective enough to the decrease the specific resistance to the lowest value.
4) Critical thresholds of the necessity of soil dressing for improving the physical properties of soils were evaluated. These values were (1)silt+clay contents > 0.35 kg kg-1,(2) Crust hardness > 0.8 MPa, (3) bulk density > 1.35 Mg m-3. When either one of the three criteria is applicable, it is judged that soil dressing is necessary. The goal value of the improving soil texture was set at 0.35 kg kg.
The project areas to consolidate paddy fields have been shifting from flat to sloping terrain since around 1980. Studies from various aspects on shifting to sloping areas were carried out to solve the foreseeable problems about such consolidation of paddy fields. However, further technical problems occurred while implementing these projects. For example uneven rice growing areas appeared in the central areas of paddy lots after consolidation. Areas where rice was growing unevenly were becoming overlaid with piled surface soil during consolidation works. The survey revealed the lack of plowsole, and sedimentation of the original surface soil occurred in the same location where uneven rice was growing right after consolidation. The survey also revealed that the uneven rice fields remained after 15 years of consolidation, and that bearing capacity of foundation soil around the central part of a paddy lot was lower than the periphery. The consolidations have long-term effects on the conditions of soil and rice cultivation. Therefore, it is important to consolidate farmland foundations in order to make farmland adequate for rice production.
Little is known about colloidal properties of beidelite, a member of smectite group. Also, colloidal behavior of mixed-clay suspension is not wholly clear. In this study, critical flocculation concentration (CFC) of Na-saturated montmorillonite, beidellite and illite suspension, and of mixtures between of them was measured. Na-saturated clay minerals having diameter smaller than 2//m were mixed with NaCl solution to prepare different electrolyte concentration. Flocculation of these suspensions was detected by transmission of 700 nm wave length light. CFCs of montmorillonite, beidellite and illite were 0.02, 0.005-0.01, and 0.0125-0.025 mol L_1, respectively, depending on clay concentration of 0.3 to 0.1 kgm-3. CFCs of the mixed clay suspension were smaller than the greater CFC of the either single clay suspension. Observation of flocs and x-ray diffraction pattern suggested that binding between different clay minerals occurred in the process of flocculation.
The clayey soils of Kameyama Mountain in Mie prefecture were studied experimentally in the laboratory with the idea of improving some of its engineering properties by using a very small amount of ordinary Portland cement. Laboratory tests on shear strength, compressive strength, consistency limits, compaction and specific gravity of soil were carried out with cement content of 0 %, 0.2 %, 0.4 % and 0.6 % by weight. It was observed that the cohesion cohesion which is a major component regarding the strength of soil was increased with the increase in the amount of cement and angle of internal friction at higher cement content was decreased. Compressive tests under two different room temperatures showed the compressive strength to be increased with the increase in the quantity of cement depending upon the temperature providing on it. Plasticity index, a measure of soil plasticity was found to decrease with the increase in the percentage of cement mixed with it. A close observation of results plotted in the plasticity chart also noticed that the addition of cement reduced the compressibility, dry strength and toughness of soil whilst increasing the volume change ratio and specific gravity of soil.
To assess the effect of vegetation change on water balance in terms of wetland ecosystem conservation, we evaluated the evapotranspiration, the largest output from the Bibai mire, with the lysimeter method in a bamboo grass-invading area and in a Sphagnum bog. Cumulative evapotranspiration in 2004 and 2005 except for snow season was greater in the Sphagnum bog than in the bamboo grass-invading area. Therefore, we should deny a hypothesis such that because the bamboo grasses had higher ability of evapotranspiration than the indigenous plants, groundwater table lowering would take place when the bamboo grasses invaded into the Sphagnum bog, resulting in the expansion of the bamboo grass-invading area even more into the indigenous vegetation area. Based on the measured evapotranspiration rates under several groundwater table levels in the lysimeter method, we concluded that the Sphagnum growth would not be disturbed even if groundwater table level dropped down to —20 cm while the evapotranspiration rate in the bamboo grass area would be affected significantly with groundwater table level changing. And from comparison of the data from the lysimeter method with those from the other two methods, it was suggested that the lysimeter method can evaluate the evapotranspiration rate in a field scale as precisely as the Bowen ration method or the Penman method.
The main aim of farm-land consolidation projects is to preserve the environment, while keeping high agricultural productivity. In this research, soil moisture of a clustered field of wild iris (Iris ensata var. spontanea) in Hanamaki, Iwate, was measured to grasp its temporal change, and to consider their relationships with meteorological and hydrological conditions. In this area, the number of flower stalks has decreased remarkably since the farm land consolidation project. The results revealed that the soil at the test site is almost saturated, and the temporal change in shallow soil moisture is larger at the distant point from pond water and a cut-off wall than the near point. Although change of the volumetric water content by precipitation was not significant, its reduction by fall of pond water level was clear. Thus, it was suggested that a moderate change in soil moisture between drying and wetting is essential for the growth of wild iris and environmental preservation.