Fine-textured brown lowland soils in the alluvial plain of Tokachi river used to have good chemical and physical properties and supported the high crop yields before heavy machinery was introduced to the fields. Recently, however, crop yields of the fine-textured brown lowland soils are decreasing. Therefore, we investigated the factors which decreased the crop yields in fine-textured brown lowland soils. We conducted soil profile surveys on an alluvial plain (900ha) at Makubetsu Town in Hokkaido, and prepared a basic soil map according to the "Classification of Cultivated Soils in Japan・Third Approximation". Brown lowland soils were dominantly distributed in our research area. Soil types with medium-texture were distributed a little apart from the small rivers. Soils with fine texture had more favorable chemical conditions. Clay mineralogy was almost the same among different soil types. With regard to the physical properities of soils, we found that the horizon just beneath the plow layer, especially in fine-textured brown lowland soils whose texture class belonged to SiC or SiCL, was characterized by a high bulk density and a low permeability. We assumed from these results that one of the factors responsible for the decrease of crop yields in the fine-textured brown lowland soils is the formation of tillage pan caused by the heavy agricultural traffics. We also presumed the reason for the formation of tillage pan in the fine-textured brown lowland soils with soil texture of SiC or SiCL. (1) Soil types with texture of SiC or SiCL had lower percentage of the pore space filled with easily available water than any other soil types, whether the effect of soil comperssion was large or small. (2) Soil types with texture of SiC or SiCL had a higher plasticity index than any other soil types. Therefore, the effect of soil comperssion by the agricultural traffics was more severe in this types of soils than any in other soil types. This caused the decrease in the percentage of pore space filled with gravitational water. Regradless of land use forms, micromorphology of the SiC or SiCL textured soil types showed a more compact state than the medium-textured soil types. We conclude from this that the soil types with the texture of SiC or SiCL have lower amount of easily available water than any other soil types, whether the effect of soil compperssion is large or small. According to the detailed soil map prepared by us, the soil types with the texture of SiC or SiCL occupy more than 50% of our research area.

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