Pedologist

Detailed soil survey in general has been explicitly expected to be applicable for precision soil and environmental management. However, identifying and interpreting soil in a certin farm parcel requires more clear concept of object boundary of gradual change of soil properties in reality. We examined the validity of present definition of soil classification units up to series level through conducting a detailed soil survey in the dilluvial fan, Katashina, Gunma. We set a benchmark soil with inductive inference of the past activity of Mt. Haruna, which supplied tephra as parent material in the study area, and determined topo-sequential change of both morphological and physico-chemical properties of pedons from benchmark soil. We overlaid predicted soil units obtained from filed survey to 10m mesh digital elevation model with GIS to prepare 1:5000 soil map in the study area (ca. 60ha). The results showed that Haplic Volcanogeneous Regosol developed on convex slope, Humic Volcanogeneous Regosol develop on concave slope, while the area affected by land readjustment was characterized by Haplic Andosol which suggested the presence of buried horizon with andic soil properties in the area.

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Paddy soils (225 points) in southern part of northeastern Japan were divided according to clay mineralogical composition. In northern part of Miyagi Prefecture, semi-smectitic paddy soils were dominant in alluvial lowlands derived from Kitakami River and Eai River. Most paddy soils in alluvial lowlands derived from Hasama River and Yoshida River were dominated by smectite (Smectitic). Mixed mineralogy type soils, which had low content of smectite, existed in valley bottoms in Kitakami Mountains. In southern part of Miyagi Prefecture, smectitic soils were dominant in alluvial lowlands of Nanakita River and Natori River. Semi-smectitic soils were observed in alluvial lowlands of Abukuma River. In Shonai Plain, northwestern part of Yamagata Prefecture, smectitic paddy soils existed in alluvial lands near Mogami River. Paddy soils of semi-smectitic type and 2:1-2:1:1 intergraded mineral type were observed in the lowlands derived from Aka River and Nikko River. In Yamagata Basin, semi-smectitic and smectitic soils were distributed in alluvial lowlands near Mogami River and Su River. Paddy soils in Yonezawa Basin were diverse in clay mineralogical composition. In Aizu Basin, western part of Fukushima Prefecture, smectitic and semi-smectitic paddy soils were relatively dominant. Mixed and semi-smectitic paddy soils were observed in Kooriyama Basin and southern part of Fukushima Basin, respectively. In valley bottom of Abukuma Mountains, most of paddy soils were classified into mixed mineralogy type. In coastal region near Pacific Ocean, clay mineralogical composition types were different depending on the geology of upper reach of each river. Tertiary sedimentary rocks or tuffs, which contain substantial amount of smectite, were distributed in upper reaches of many alluvial paddy soils in the southern part of northeastern Japan.

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Field researchers and agricultural extension workers often encounter field-to-field variations in soil properties and rice yields that make it difficult for them to transfer appropriate agricultural technology and suitable management practices to local farmers. To address this constraint, we examined grain yield and shoot biomass of rice (Oryza sativa L.) and the general fertility status of topsoils (0-10cm) collected from 80 paddy fields with different topographical positions in an intermountain basin in northern Laos. The soils of studied rice fields showed fewer nutrients such as exchangeable bases, CEC, and T-C and T-N contents than the soils of lowland rice fields in Thailand. While higher levels of exchangeable K and available P and Si were found in rice fields at the upper part of the basin and the floodplains located in river catchments, the rice fields located in the lower part of the basin had lower soil nutrients probably due to less flooding and inflow of soil nutrients from the surrounding hill slopes. The yield of high yielding varieties (HYVs) was strongly correlated to CEC, exchangeable Ca, Mg, and K, and available P and Si. On the other hand, indigenous varieties (IVs) did not show a proportional relationship with those elements. Higher yields of HYVs were found in rice fields where soil fertility was relatively high. However, the yields of HYVs were not significantly higher than the yields of IVs under low fertility conditions, indicating that it would be invaluable to supply chemical fertilizer to grow HYVs in such infertile rice fields. In contrast, planting IVs in such conditions could be a better choice for local residents to obtain stable rice production without costly agricultural materials.

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