疑似グライ土における構造の生成過程に関する一考察

抄録

Comparative physical and micromorphological studies were carried out on four main subtypes developed on the fine textured Pleistocene deposit, aiming at making clear the genesis of subsoil structure of pseudogleys and related soils. Pseudoogleys are characterized by very closely packed coarse prismatic structure of the streaked and marmolized Bg and Cg horizons. Physical properties of the clods composing these horizons are very different from those of the fossil C horizons (ancient red weathering crust) of acid brown earth and the completely gleyed CG horizons of stagnogleys (cf. Fig. 4-10). Micromorphological studies on these horizons show that the clods have very closely packed primary peds and narrow channels because of filling up with colloidal materials. These colloidal materials settled on the surface of peds usually have gelatin like appearance in plain light and are often overlain by simple channel argillan with strong continuous orientation in the Bg or Cg horizons, especially in the Cg. These micromorphological properties of the Cg horizons seem to show their fossil characters. In the structure of Cg horizons of profile-2, transitional feature from the plasma of fossil red soil to that of the Bg and Cg horizons of pseudogleys is observed as shown in Photo. 4 in Plate 1. Transformation of plasmic structure occurs strongly at outer part of primary peds facing coarse interpedal voids and diffuses into the matrix of primary peds. This transformed part of the primary peds has bleached appearance and usually accompanies black or blackish brown concretions. Based on the facts described above, we can conclude that the changes of soil structure in the process of pseudogleization occurs under weak unaerobic condition, caused by periodical stagnant water regime, with the aid of water soluble organic matter which is brought into the Bg or Cg horizons by the downward movement of gravitational water through coarse interpedal voids. During this process, loss of iron, transformation of the plasmic structure and dispersion of colloidal materials progress in the outer part of the Bg and CG, and fill up their interpedal voids. On the other hand, inside of primary peds in which the bulk of plasma is in the peptized condition, primary particles are strongly packed by the desiccation of the profile during early summer.