Genesis of Andosols at Kitakami, Iwate Prefecture, Northeast Japan
II. Relationships between Parent Materials and Soil Formation
1978 Volume 17 Issue 1 Pages 15-23
Details
1978 Volume 17 Issue 1 Pages 15-23
The distribution of Andosols and their relation to topography were clarified in the previous paper. In the present report the relationship between parent materials and soil formation is discussed.
Cumulative tepha layers from the top to the Murasakino pumice are divided into three groups; younger dacitic ash (1, 000 to 5, 000 years B.P.), older andesitic ash (10, 000 to 20, 000 years B.P.) and Murasakino pumice (20, 000 or more years B.P.). Each ash group also consists of several ash layers. Since most tephra layers were brought to the surface intermittently, they were subjected to surface weathering. Therefore, the degree of weathering of hypersthene and/or augite in the Andosols shows a tendency to become greater with the depth of soil profiles.
All the Andosols on the Kanngasaki terrace show the rejuvenation type of humus horizons formed of younger dacitic ash which covered the alluvial deposits.
The formation of Andosols on the Murasakino terrace was found to be closely related to the depositional features of the parent materials, to the mixing of the soils atrificially and to the truncation and redeposition of the soils. Tephra layers from the top to the Murasakino pumice were found in the profile of Normal Andosol. However, the Al horizon of this Andosol contains more or less the parent material belonging to the subsoil in addition to the parent materials which are common to the same horizon of Normal Andosol on the Kanagasaki terrace. It was noted that these materials were well mixed. Light-colored Andosol (II) was formed after the truncation of the Al horizon of Normal Andosol on the same terrace. Aquic Andosol was formed by the redeposition of eroded materials in the valley bottom.
Light-colored (II) and Aquic Andosols on the Nishine terrace were formed by truncation and redeposition, respectively, as observed in the Murasakino terrace. However, the formation of these soils was found to be more advanced on the Nishine terrace which is more strongly dissected.
