火山性地土性調査法に就て(第二報)

抄録

In Japan there are many famous vocanos. Their explositive ejectamenta spread over the surface of the earth deeply in some cases it measures over few meters in thickness. The deposits of their materials deep enough to influence the nature of soils are called volcanogenous soils. This volcanogenous soils are distributed so widely that their managements are very important in Japan. The volcanogenous soils are not only different from the ordinary mineral soils in their characteristics especially in profile formation but also they are different in its own profile formations. On what foundations does the author wish to think of these differences? (1) Up to date, the volcanos erupted many times and fell various species of ejectamenta such as volcanic ash, volcanic sand, lapilli or lava ash, etc. The morphological characteristics of these materials are different from each other. Moreover, these are not always spread in one direction from the crater as the volcanos erupt but are spread according to the wind at erupted times, or the form of a crater, etc. Therefore if there are many volcanos in the area and erupt frequently that we have often encountered, we can easily imagine that there must be formed various phases of profile in volcanogenous soils. (2) The ejectamenta can spread widely and deeply over the surface of the earth anywhere bottoms, terraces and uplands- and so it often occurs that the geological origin of soils is of no practical significance. (3) During the time between the eruptions, the latest ejecta weathers in atomosphere, weeds and trees grow on it and humus are accumulated on the surface. When the new ejectamenta cover on the top layer, this humus zone is naturally ranged in the substratum. This means that the humus zone lays not only on the top but also in the substratum and generally the boundaries between the layers of the ejectamenta are distinctly drawn but not gradually as we observe in the mineral soils. It is needless to explain that it is the chief purpose in soil survey to classify the soil type. Generally the classification of soil types in soil survey is largely based on the geological origin of soils including topography, morphology of soil profile and othor features affecting its agricultural character. Nevertheless, as already mentioned the volcanogenous soils are entirely different in its forming process from the ordinary mineral soils and the geological origin of soil may often be of no practical significance and therefore it is evident that classifying method of soil type must be altered. Hereupon, the author ventures to propose his opinion in classifying the soil type in volcanogenous soils and the brief descriptions are as follows : (1) At first, classify the species of ejectamenta spread in the area and observe the characteristics of every ejecta. (2) Basing on the observation of characteristics of each ejecta, search for its distribution and variation in thickness. (3) Determine the relation of stratum between each ejecta. (4) On the basis of the above investigations and observations, divide the area into soil series according to the differences in profile of upper stratum and finally divide the soil series into soil type according to the differences in profile of substratum deep enough to influence the growth of the crops. As for instance of classifying the soil type in the volcanogenous soils the author describes the work of the investigation of volcanogenous soils in Nemuro and the eastern part of Kusiro district in Hokkaido Island of Japan.