Anisotropy of Drainable Macropores in Andosols and Alluvial Soils

Abstract

Since macropores significantly influence the movement of water and air in the soil, it seems that the structure of macropores should determine the processes of drainage. A study was conducted to investigate the structure of macropores and the associated drainage process. The drainage characteristics of volcanic ash and paddy field soils were investigated, and the following results were obtained. (i) The structure of entry pores depended on the direction of sampling. We constructed a model of entry pore structure, and found that this phenomenon is thought to reflect the shape of drainage macropores, especially the joints of tubular pores that have different diameters. (ii) The results of drainage experiments using a 100 cm3 cylindrical sampler were thought to have been greatly affected by the inner wall of the sampler. Nevertheless, it could be said that wall-affected results show soil drainage characteristics better than non-wall-affected results. We constructed a model of drainable macropores structure to explain this phenomenon. The shapes of pF-distribution curves were different depending on the direction of sampling when soil was sampled using a 100 cm3 cylindrical sampler. This could be explained by assuming that large tubular pores in the network of macropores were in a vertical direction and smaller tubular pores were in a horizontal direction.