土壌被の発達と透水性 : 細粒質ポドゾール性土の形態および理化学性の変化と透水性

抄録

The residual weathering crust derived from a fine textured sedimentary rock of Koetoi formation, which is one of the main parent material of podzolic soils and acid brown forest soil in the hilly area of northern Hokkaido, shows very high hydraulic conductivity (K=5×10^<-3>〜2×10^<-2>cm/sec). Vertical distribution patterns of hydraulic conductivity (=K-curve) of acid brown forest soil and related subtypes derived from this parent material have shown somewhat straight feature and hydraulic conductivity of the B-C horizon is in the range from 10^<-4> to 10^<-3>cm/sec. Genesis, morphological, physical and chemical properties of podzolic soils are closely related with their hydrological regime which is mainly controlled with the micro relief and hydraulic conpuctivity of the illuvial horizons, changes in the properties of podzolic soils with increasing humidity are illustrated by the following series of horizon sequences. Humus podzolic (Normal): A_1-A_2-B_<ti>-C Humus podzolic surface gleyed: A_1-A_<2g>-B_<ti>-C Podzolic pseudogley: A_1-A_<2(g)>-B_<tg> C_g or CG The K-curves of humus podzolic soils formed under the leaching-out water regime have the minimum value of about 10^<-5>cm/sec at the well developed illuvial horizon. During the humid season, upper soddy or illuvial horizons of humus podzolic surface gleyed subtypes are almost saturated with the stagnant water, which stagnate on the lower impermeable illuvial horizons (K=10^<-6>〜10^<-5>cm/sec). However, B-C or C horizons of this subtype of podzolic soils have moderately high hydraulic conductivity in the range from 5×10^<-4> to 5×10^<-3>cm/sec. The k-curves of podzolic pseudogley show the rapid decrease of hyaraulic conductivity down to the profile and conductivity of the Cg horizon of them is so slow (K≦10^<-5>cm/sec), that the B_<tg> horizon is left in the periodical stagnant water regime. According to the component analysis, variations of properties of these soils are closely linked to (1) the humus accumulation and related changes of physical and chemical properties, (2) formation of impermeable illuvial horizons and (3) changes of the soil water regime induced by the formation of impermeable layer in the profiles. From the results described above, it may be concluded that changes of physical properties, such as the reduction of coarse pore fraction and increase of bulk density, in the B horizons followed with the reduction of permeability lead the succession of heavy podzolic soils by controlling the water regime of them.