Pedologist

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Terraces of three stages are distributed on the foot of Mt. Yoneyama, along the coast of the Sea of Japan, Central Honshu. Characteristic reddish layers are found within the higher terrace deposits. In some place, the reddish layer lies unconformably on the Tertiary basement, while in the other place, sandy beds are intercalated between the reddish layer and the basement. On the other hand, the reddish layers are related with the upper soil horizons in the following two ways. The one is that the recent A horizon, with or without the transitional brownish soil layer beneath it, is developed on the reddish layer, while the other is that the A-C solum derived from the reddish layer, is covered with the dune sand deposit which had also differentiated to the considerably matured soil profile. The ferric oxides responsible for the reddish hue (2.5 YR in the MUNSELL'S color notation) of these layers, seem to be very stable, judging from the fact that the hue persists over 2 months under the incubation treatment with starch. The reddish layers are dominated by the clay fraction (-2μ), the SiO_2/R_2O_3 mol ratio of which tends to be somewhat lower than those of the adjacent layers, and contents of free A1- and Fe-oxides of the formers relatively higher than the latters, though the type of weathering of the reddish layers does not seem to be so strongly allitic. One of the most important factors of the genesis of these soil layers may be the high content of clay in their parent materials, which may have prevented from free drainage and resulted in reduction and solvation of free iron compounds. After the upheaval of the terraces, water regime may have turned to be well-drained, and facilitated to oxidize ferrous iron. Then, the limonitic coating of clay particles may have gradually altered to the hematitic compounds. The parent material of these layers is supposed to have deposited during Riss/Wurm interglaciation, and the red weathering to have taken place mainly in Wurm I/Wurm II interstadial age, though it seems that the weathering has gone on progressively up to today.

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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.

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New factors controlling nature of organic matter in paddy soils appear successively with time during long term utilization of alluvial soils in paddy field. Characteristics of organic matter of immature paddy soils derived from alluvial deposits are more or less dependent on nature and amount of debris of aquatic organisms which are sedimented with mineral particles. In the case of lake deposits, phytoplankton as well as reed are shown to be main sources of organic matter of the soils, which is verified by examining organic matter content, content of chlorophyll like substances, C/N ratio, free iron content and soil texture. Oxidative destruction of the chlorophyll like substances and of mineralizable nitrogen compounds contained in the soils is concurrently caused by lowering ground water table, but the former is more intensive than the latter. Comparison of these two values suggests a possible method to evaluate degree of oxidative condition developed in the paddy soils. Long term farming of the alluvial soil in paddy field with low ground water table is supposed to allow furrow slice to reach a stationary state in respect to organic matter content. This organic matter content of the furrow slice is found to be positively correlated with the capacity of phosphate retention of the furrow slice.

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