Soils in Greenhouse Plots in an Urban Area

Abstract

Although Osaka Prefecture has a large population (8.8 million people), agricultural lands occupied about 8% of the total area (13,920 ha) in 2010. Man-made soils are widely distributed in the agricultural fields, especially those with greenhouses. According to the Japanese Soil Classification system, soils over 35 cm thick consisting of materials transported from other sites that were dressed on the surface layer are classified as man-made soils. Here, one survey field, formerly a rice paddy, was located on a flood plain of the Yamato River. Soil dressing was conducted twice. Most soil properties were remarkably different among the horizons. Deeper buried horizons, separated from the upper horizons by an abrupt boundary, can be identified as dredged materials from nearby rivers, since the content of phosphorus (P) and exchangeable calcium (Ca) were relatively high and pH (H₂O₂) was remarkably lower than pH (H₂O). Previous land use for the paddy field was also confirmed at the deeper horizons by an upper wavy boundary, and a lower boundary separating it from a plow pan layer with iron mottles. In another research site located at Sakai City, known as Senboku new-town in the mountainous areas, the field site had been reclaimed 40 years before by clearing sections of old growth forest. Sulphur-containing marine clay makes up the deep layer of soil profiles in this area, necessitating dressing of soils from other sites. There were drastic differences in soil properties among soil horizons, especially the deeper layers characterized by reductive soil reactions, which consisted of marine clay. Relatively high contents of sulfuric compounds probably influence soil acidification by further cultivation of soils in the deeper layer.