北海道におけるいわゆる"重粘性土壌"の成因について

抄録

There have been a number of investigations on the pedogenetic factor of so-called "Heavy clay soils" which are widely distributed in Hokkaido, but the problem of their genesis is not sufficiently solved yet. Based upon the results of soil and geological survey of northern Hakkaido in which so-called "Heavy clay soils" have especially wide distribution, the present author concluded that these soils have certain regularity in the mode of sedimentation of parent materials. That is, these soils have, as a rule, clayey upper horizon which is underlain by compact silty subsoil, and the horizons of alternate silt and clay layers are commonly found. Besides, from the analytical data of three phase distribution and mechanical component of soil samples, it is recognized that solid phase ratio is correlative with silt/(silt+clay) ratio in so-called "Heavy clay soils". Moreover, silt content has a close correlation to silt/(silt+clay) ratio and this correlation is expressed by the following regression line, Y=1.05x+1〜9 where, x: silt content (%) y: silt/(silt+clay) ratio (%) Analytical data showed that at least 22.5% of silt (clay content ranging 47 to 69% in this case) is indispensable for the genesis of compact soils such as so-called "Heavy clay soils". Maximum silt content is 50% and in that case 31 to 44% of clay is required for the genesis of so-called "Heavy clay soils". These facts show that not only clay content but also silt content is most important factor for the formation of compact soils.