Morphological features and physic-chemical properties of soil profiles developed on several river terraces different of formation age, Tokachi district, Hokkaido

Fluvial terraces of different formation ages are in Tokachi region, Hokkaido. Fluvial terraces are classified as high terrace, middle terrace and low terrace as topographic classifications. In high terrace, Andosols was generated from parent material, which was Holocene volcanic ash and Late Pleistocene volcanic ash on diluvial formations. Geomorphic development by erosion and deficiency of tephra were conceded in high terrace. Wet type Andosols (HT-1 and HT-2 respectively) were distributed in this high terrace. The depth of groundwater influence in Andsols of HT-1 and HT-2 was different by effect of micro-topography. In middle terrace, Andosols was generated from parent material which was Holocene volcanic ash and Late Pleistocene volcanic ash. Dry and wet types Andosols (MT-1 and MT-2 respectively) were distributed by effect of habitat as well as in high terrace. Andosols in low terrace (LT) was generated from parent material, which was Holocene volcanic ash on fluvial deposit. In low land that was formed most newly, Alluvial soil (LL) was generated from fluvial deposits. Wet type Andosols (HT-1, HT-2 and MT-1) distributing at high and low terraces were observed iron mottle at a depth of > 50 cm. Therefore soil order could not be reflected that HT-2 and MT-1 was wet condition, by the Agricultural Soil Classification and Unified Soil Classification System of Japan. On the other hand, in the Japanese soil classification (2017), only HT-1 was classified as Wet type Andosols. Wet condition by snowmelt in early spring affects crop production in cool temperate area like Hokkaido. As a result, distinction of wet type soil and dry type soil is important. Our study suggested that is necessary to rethink criteria of wet condition to Andosols subgroup of Japanese soil classification (2017).