Water saving irrigation and soil salinization prevention supported by capillary barrier of soil

土壌塩分化を効果的に防ぐことができれば、かんがい農業の持続性が高まる。土壌中の根群域下部の比較的浅い深さに礫層を敷設すると、土壌と礫の保水性の違いにより、地下水からの毛管上昇（上向きの不飽和流れ）を遮断できる。地下水に塩分が含まれる場合、この毛管上昇と蒸発による塩分集積を予防できることになる。礫層とその上部に相対的に粗粒の土を配した土層構造をキャピラリーバリアと呼ぶ。両者の土の保水性の違いは、地表面から下向きに流入する浸潤水（下向きの不飽和流れ）を遮断する働きを持ち、その結果、礫層の直上部に浸潤水が貯留されることになる。したがって、この礫層が比較的浅い位置に敷設されると、植物は効率よくこの貯留された土中水を生育に利用できるようになる。節水かんがいが可能となる。本研究では、このような土のCB による地下水からの毛管上昇の遮断ならびに地表面からの浸潤水の貯留効果を、圃場での植栽実験を通して明らかにした。Effective prevention of soil salinization is required for sustainable irrigation. Capillary barrier of gravel soil is introduced and investigated to solve this requirement in the study. The gravel being embedded in a layer within the shallow depth of field soil, an upward unsaturated water movement from groundwater in the deeper soil will be cut off by this gravel layer. In the case of saline groundwater, a soil salinization which is caused by the upward saline water movement from the groundwater and evaporation of soil water near the soil surface will be effectively prevented. This excellent cutoff of soil water movement is brought physically by a clear difference in a water retention capacity between gravel and field soil (relatively finer soil), and is call the capillary barrier (CB) of soil. The CB also works properly to stop infiltrating water from the soil surface supplied by irrigation or rainfall, with the result that the soil water accumulates above an interface between the upper field soil and the gravel layer. As vegetables can sufficiently uptake this soil water for their growth even under the scarcity of irrigation supply of water, the CB of soil will be attractive in developing a efficient water saving irrigation method. In the study, a field experiment was conducted in order to investigate a practical applicability of the CB of soil to the soil salinization prevention and the water saving irrigation. Sand was compacted into polypropylene container boxes buried in the field soil, each about 100,000cm^3 in volume, then 5cm-in-thickness gravel layer was embedded in the sand. After Mizuna (Potherb mustard) being planted, the saline groundwater 10,000ppm was supplied into the bottom of the container box with a constant head of water. Soil water content and electric conductivity were monitored along the depth of the container box, and the growth of Mizuna was observed during about seven weeks