To examine the effect of successive rice cultivation by nonirrigated direct seeding on the fertility of soil, a comparison was made between a direct seeding lot and a transplanting lot in a loamy field representing a type of dry paddy fields.
　The successive cultivation of rice plants by nonirrigated direct seeding was observed to have many effects on the physical property of soil and influence the growth of the plants and the yield of rice. For example, in the direct seeding lot the percolation of logged water continued to increase for the first three years, reaching as much as 30 cm a day.
　The percolation, however, had no tendency to increase thereafter probably owing to changes of soil structure as seen in a gradual decrease of the solid phase ratio or an increase in number of soil aggregates not smaller than 0.5 mm and a decrease of those not smaller than 0.1 mm.
　As to the organic matter content of soil, almost no difference was observed between the two lots.
　In the case of the successive cultivation of rice plants by nonirrigated direct seeding, the application of rice straw was highly effective for maintaining the soil fertility because it improved the physical and chemical properties of the soil and suppressed the percolation of logged water.
　From these results, it seems that best attention must be given to the management of field as to fertilization and irrigation when rice plants are successively cultivated by nonirrigated direct seeding in such kind of nonirrigated paddy field as used in this experiment.