Correlation between Percolation Pattern and Soil Horizon Differentiation in a Stratified Soil Column Model of a Paddy Field with Volcanic Ash

Studies on the influence of a percolation pattern in seepage water on several phenomena in a layer (I)

Abstract

The purpose of this paper is to clarify the correlation between the percolation pattern and soil horizon differentation to oxidation and reduced layers. The soil of a paddy fields that had open system percolation was used in this investigation. The stratified soil model column is composed of Kuroboku soil sampled disturbedly from plow and plowsole layer and composed of yellowish-brown loam sampled undisturbedly from subsoil in the paddy field. In the experiment, DO, pressure head in seepage water, air entry value and Eh were measured at short intervals in the vertical plane, and then, the drainage level was controlled by the soil column. Velocity of the model was about 50 mm/d and that of the other model was about 100 mm/d.
As a result of this observation, the following information was obtained.
1) In the case of a low drainage level (about 70 cm), surface soil became the closed system percolation, low DO (1 ppm level), and reduced layers. Both plowsole and subsoil became the open system perco) ation, high DO (6 ppm level), and oxidized layer.
2) In case of a high drainage level (about 20 cm) in the same column, all layers became a closed system percolation, low DO soon, but became reduced layers after about 80 days.
Judging from the above, it is clear that the open system percolation caused penetration of air into the layer and produced oxidized layer. The closed system percolation caused separation of air from the layer and produced a condition of reduced surroundings. When the activation of microbes was poor, the advance of reduced condition became slow, but the closed system percolation finally brought about a reduced layer.