Study subject about soil physical properties, which are necessary for technical development of direct seeding and rice cultivation, are arranged and showed. Land leveling facilitates stand establishment of rice seeding, weed control, and water management for strengthening of lodging tolerance on wet-seeded and water-seeded rice culture. On dry-seeded rice culture, permanent water is applied earlier than before in roder to shorten dry rice field preiod and to decrease frequency of herbicide supply. Land leveling is necessary for dryseeded rice culture, too. Recentry, land leveling technology was developed. It is composed of crawler type tractor, laser controlled plow and leveler. The study on operation method of land leveling to avoid surplus compaction of soil is necessary in future. Development of dike management technique to prevent leakage of water is necessary for rice cultivation without puddling, such as dry-seeded rice culture and no-tilled transplanting rice culture. Farm land consolidation affecting water requirement is important to stabilize the dryseeded rice culture. The study on rice productivity decrease by continuation of no-tilled cultivation is necessary. The water seeding and wet seeding includes three types of seeding-method, i.e., broadcast seeding, stripe seeding and hill seeding. It is necessary to sow rice seed in constant depth for stable establishment of seeding, And, it is important to control the physical properties of puddled soil and to unify them in the paddy field in order to maintain constant sowing depth. The effect of soil properties and puddling method on the puddled soil physical properties should be studied, It is important to avoid the soil compaction and necessary to elucidate the role of organic matter in prevention of compaction when the sustainability of crop production by the large-sized mechanization agriculture is considered in future.
Field studies were carried out to know the effect of the continued non-tilled direct seeding rice culture on the soil compactness and water requirement in depth. The outline will be described below.
1) The compactness of topsoil was become harder due to the continuing of the non-tilled direct seeding rice culture. The compactness of the topsoil was about 500-1000 KPa.
2) The distribution of root system was not limited by the high compactness at the non-tilled direct seeding rice culture. The amount of root was converged into the surface of topsoil,0-3 cm, by continuing the nontilled direct seeding rice culture.
3) The thickness of organic layer on topsoil was about 14-18 mm in the way of surface application with rice straw, but about 6 mm in the way of carrying out of rice straw.
4) The water requirement in depth was increased due to the continuing of the non-tilled direct seeding rice culture. That was remarkable in the paddy field with low groundwater level, but there was no problem in the paddy field with high groundwater level.
5) The increase in the water requirement in depth was mainly due to the increase in the percolation through border.
Rice seeds sown directly in unwatered paddy fields are unreliable in their germination and young shoot emergence, though labour for cropping is much saved by such planting method. Effects of tillage operation on rice seed germination and shoot emergence out of soil were studied in unwatered paddy fields of Low-Humus Andosol soil (LHA) and Haplic Gray Lowland soil (HGL). Soil macropores holding moisture of more than -10 KPa of water potential were reduced by tillage operation in both soils, though pores corresponding to less than -10 KPa were not affected. After rain fall, tilled field more rapidly dried than untilled ones in both soils especially in LHA. Tillage operation lowered percentage of young shoot emergence in both LHA and HGL fields, and the percentage was higher in LHA field than HGL one regardless of the operation. Also stability of the emergence, which was evaluated based on coefficient of the variance, was reduced by tillage operation. Dominant part of no emergence seeds stopped their sprouting after beginning of root elongation, and percentage of such seeds in total seeds was over that of seeds normally emerging in the tilled HGL field. Only small part of total seeds was unable to emerge by stopping their growth after beginning of both root and shoot elongation, and such seeds were most often observed in tilled HGL field. In a laboratory experiment, beginning of shoot elongation of rice seeds was more delayed than that of root elongation by low soil moisture. Almost all rice seeds started their root elongation within 72 hr of seeding under soil moisture condition of more than -1 MPa, while initiation of shoot germination was repressed negatively corresponding to soil water potential at 72 hr.
An understanding of the transfer of heat and gases into and out of water is critical to understand the function of surface water layer in a low land paddy field. This study is intended to clarify the phenomena of heat and dissolved oxygen (DO) transfer in the paddy field water layer. Field experiment was conducted for temperature profile and DO concentration. The result was then verified with a lab-scale model under natural solar and artificial infrared radiation which were convective and non-convective system, respectively. The continuous higher temperature at the soil surface indicated that the soil surface acts as the plane from which the heat is conducted upward and downward to water and soil, respectively. The heating at the bottom of water layer imposes the buoyancy forces that brings the convection current. Rayleigh and Prantdl numbers also confirms the turbulent convection in the water layer. The heat that is accumulated in the soil layer during the heating phase of the day is transferred to the soil surface during the cooling phase. Then the heat transfer from the water to the atmosphere is regulated by convection. The diurnal variation in DO concentration indicated that there is an excess supply of oxygen in the day time and an over consumption in the night time. In the day time the DO generated by micro algae is transferred from the water to the atmosphere. In contrast, during the night time the oxygen transferred from the atmosphere into the water layer by reaeration. But, there was no distinctive concentration gradient in the ponded water either from the water surface or from the soil surface throughout the day. The corresponded temperature data confirmed the existence of a convection current in the water layer. This convection resulted in a uniform DO concentration throughout the profile regardless of reaeration or deaeration. The resulting temperature and DO profiles generated under the convective system was compared with the profiles generated in a non-convective system. This comparison lead to the conclusion that convection plays a significant role in the heat and DO transfer in the ponded paddy field.
Discharge from paddy fields after puddled is regarded as one of causes of water pollution of rivers and lakes. The problem is considered to be most serious on Uso-river basin in Shiga prefecture. The author investigated a practical method to clarify the puddled water by gypsum addition to it. A critical coagulation condition of the puddled water was estimated on the basis of the Hetero coagulation theory. The calculated results showed the Ca ion concentration greater than 2m mol// will flocculate the puddled water. The estimated value agreed well with experimental results. Then the most favorable flocculation aid was searched among eight soil amendments and fertilyzers. Gypsum was found to be the best aid because it showed most effective flocculation and moderate pH change. The optimum gypsum application rate was examined in test plots made on two paddy fields of the area. The rate of 30 kg/lOa gypsum deceased the suspended soild concentration below 0.1% in 12 hours. The critical Ca ion concentration at rapid flocculation was found to be ranging from 1 to 2m mol/Z, which agreed with the estimated critical concentration. The rate of 30kg/lOa gypsum was estimated to reduce 70 to 80% of the whole effluents of the suspended solids of the puddled water.