Soils in Reclaimed Land After Drainage in Isahaya Bay

Abstract

Development of polder along water front regions in Japan to create new agricultural areas has occurred since the Medieval Ages. In Nagasaki Prefecture, reclamation in the Isahaya Bay area began in 1330AD, and has grown to 3500 ha, making it the largest farmland in Nagasaki Prefecture until modern times. The recent reclamation project in the bay area started in 1992 and was finished in 1999, with the project as a whole being completed in 2008. After the provisional closing of the tidal gate for construction of the tidal embankment in April 1997, the site became a stretch of dry land about 0.5 m above the water level of the regulation reservoir. In addition, artificial modifications, such as cutting ditches, were allowed for part of the dry lands. The annual flooding frequency is about four times per year, with a flood duration of no more than 24 h. As a result, pioneering vegetation developed at the investigation site during at least the first five years following the provisional gate closure. In the reclaimed area, there are sporadically dry fields covered with goldenrod vegetation. A moderate amount of angular blocky soil structure was observed below the water table in the soil profile, indicating that the subsoil was relatively dry due to a lowering of the groundwater table. Frequent repetition between wet and dry conditions due to changes in the water level can promote the formation of a blocky structure. During the sedimentation process the changes in depth of the water at some specific locations presumably promoted the formation of angular blocky soil structures. The reclaimed land along the seacoast can be considered a specific environment that forms a specific soil structure at a relatively fast rate.