Soil pore-structure in a surface layer compacted with a roller for the dry-seeded rice cultivation

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  • 水稲乾田直播栽培におけるローラによる地表面鎮圧が作土の間隙構造に及ぼす影響
  • スイトウ カンデン ジカマキ サイバイ ニ オケル ローラ ニ ヨル チヒョウメン チンアツ ガ サクド ノ カンゲキ コウゾウ ニ オヨボス エイキョウ

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To prevent water leakage from direct-sown dry paddy rice fields, a compaction roller 2 m in width with a total load of 9.4 kN was developed. Even when the soil was dry, and thus difficult to compact, the roller success-fully decreased permeability after five times compactions. However, the mechanism of the observed permeability de-crease has not been well investigated. We therefore stud-ied the progressive changes in the pore size distributions based on the water retention curves as a result of the roller compactions. Observations of the soil profile and measure-ments of penetration resistance identified a densely com-pacted layer less than 2 cm above the plough pan. Posi-tion markers inserted in the soil profile revealed that the soil at several centimeters below the surface was highly compacted. The pore size distributions had three stages as the number of roller compactions increased. Firstly, the volume of the larger pores and the total porosity de-creased. Secondly, the pore size distribution shifted from larger pores to smaller pores while keeping the total poros-ity almost constant. Lastly, the pore size distribution did not change anymore regardless of the further roller com-pactions. Although water infiltration decreased steeply in the first stage, the compaction was insufficient to prevent leakage. In the second stage, infiltration reduced suffi-ciently to stop leakage. At this stage, the ratio of pores larger than 0.2 mm decreased, whereas macropores larger than 0.1 mm still remained in the compacted soil layers.

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