Pedologist

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The following three kinds of soils have been regarded as the specific soils of Hokkaido, i.e. Volcanic ash soils, Peat soils and Heavy clay soils. But the term of the "Heavy soils" seems to be rather empirical or arbitrary than taxonomical from the pedogenetical point of view. Therefore, the author tried to set a suitable taxonomic situation with these soils, mainly distributed in northern Hokkaido especially along the coast of the Okhotsk Sea. In general, the extraordinally heavy texture, very sticky (when wet) and extreemly hard (when dry) consistency, which characterize these soils practically, could be explained by either the result of pedogenesis or the inherent abundant clay contents of parent materials of these soils. The former reason, however, may not be probable in these soils, because several genetic soil types are included in the so-called "Heavy clay soils", and no specific soil forming process responsible for the creation and the development of aforesaid diagnostic characteristics of these soils, could be found. Then, these "heavy" characteristics may be due to the latter reason, i.e. to the abundant clay contents of parent materials, whether they have been derived from originally clayey terrace deposits or from the ancient weathering crusts which are sporadically preserved on the highest terraces or more older gentle slopes along the coasts of the Okhotsk and Japan Sea. Thus, in order to set a suitable taxonomic situation to the so-called "Heavy clay soils" of Hokkaido, the dualistic, i. e. genetic in higher orders but non-genetic or geographic in lower orders, system of the soil classification as adopted by U.S.D.A., was found to be quite unsatisfactory. While the descending monistic sytem of the genetic soil classification adopted in USSR and the other European countries, could make it possible to give the proper taxonomical situation to the peculier property of the parent materials of these soils, at the category of the soil genus. Accordingly, a unifying scheme of the genetical soil classification was obtained on the so-called. "Heavy clay soils", including their diagnostic charactestic which is very significant both in practical and in taxonomical meanings.

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In this paper, the author deals with the successive changes of some soil properties caused by the cultivation of citrus trees. The data are obtained, from the soil survey in the important area of the citrus production in Nagasaki Prefecture during 1960-61. The orchard soils in the area are derived from the similar parent materials (weathering product of the andesite) on the steep slope, and cover different stages of development, from younger to older, in citrus cultivation. The results are summarized as follows: 1) According to the morphological characteristics of the soil profiles, these orchard soils could be divided into the following three stages of development which are represented by their horizon sequences: i) A/B (young soils), ii) A_1/A_2/B (most of orchard soils), and iii) A_1/A_2/B_1/B_2 (well drained orchard soils). It seems that A horizon becomes to differentiate into A_1 and A_2 subhorizons by tillage.

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