Saturday, 15 July 2006

Clay Mineralogical Composition Map of Paddy Soils in Miyagi Prefecture, Northeastern Japan.

Oki Sano1, Toyoaki Ito1, Tadashi Ando1, Masami Nanzyo2, Genya Saito1, Kimio Saito3, and Masahiko Saigusa1. (1) Field Science Center, Graduate School of Agricultural Science, Tohoku Univ, 232-3, Yomogida, Naruko, Miyagi, Japan, (2) Graduate School of Agricultural Science, Tohoku Univ, 1-1 Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai, Japan, (3) Miyagi Pref. Furukawa Agricultural Experiment Station, 88, Fukoku, Ohsaki, Furukawa, Miyagi, Japan

Clay mineralogical compositions in paddy soils affect the soil properties that are related to rice productivity and environmental pollution. Clay mineralogical compositions in alluvial paddy soils are closely related to surface geology in mountains located on upper reaches. In paddy soils, the transformation of clay minerals is marginal. Sedimentation of clay minerals is closely related to topography. Fine and easily dispersed clay minerals are more dominant in back marshes than in natural levees due to the clay sorting effect. In terrace and hilly lands, volcanic ash soils and residual soils are prevalent and their clay mineralogy is strongly affected by volcanic materials, wind-blown loess or surrounding rocks. A clay mineralogical composition map of cultivated soils in Nagasaki Prefecture, the Kyushu Islands, Japan has been made (Nakashima, 1998). However, information on topography was not fully integrated into the map. In this research, we developed a method to make a clay mineralogical composition map of paddy soils by integrating clay mineralogy point data, information on surface geology of mountains in upper reaches, topography types and soil types. Following this, we made a clay mineralogical composition map of paddy soils in Miyagi Prefecture, Northeastern Japan.We collected paddy soil samples from the Soil Survey for Improved Utilization and Conservation of Soil Resources in Miyagi Prefecture (74 soil samples) and 4 supplemental samples. The crystalline clay mineralogical composition of the clay fraction was determined by X-ray diffraction used in combination with various treatments. The amounts of amorphous clay minerals were evaluated with acid-oxalate (pH 3.0) extractable silica. The soils were then classified into 6 clay mineralogical composition types including 1) smectic type, 2) mixed type (including smectite and 2:1-2:1:1 intergraded minerals etc. evenly), 3) mixed type rich in chlorite, 4) mixed type rich in kaolin minerals, 5) mixed type rich in 2:1-2:1:1 intergraded minerals and 6) amorphous type. The clay mineralogical composition map was made by integrating the information from land use maps, topographical classification maps, surface geological maps, soil maps and watershed mesh maps with the point data. The operation was conducted using geological information system software “ Arc GIS 9 ”. In alluvial areas, where 94 % of paddy soils exist, point data on clay mineralogical composition type were expanded to incorporate plane information, based on the assumption that clay mineralogical composition is uniform in a “cell”, which is a topographical plane subdivided by watershed. Distribution of paddy fields was determined using the land use map. Paddy fields were then divided into 6 topographical types by superimposing the topographical classification maps. Of the six topographical types classified, four were alluvial including 1) valley basin and fan, 2) natural levee, 3) back marsh and 4) polder. Two types were non-alluvial, including 5) terrace and 6) hilly land and mountain. Furthermore, one topographical plane was subdivided into some watersheds by overlapping the watershed mesh map. We defined one topographical plane subdivided by watershed as a “cell”. It is imperative concept to this study that the clay mineralogical composition of paddy fields in a cell is the same as that of the point data in the cell. Moreover, we assumed that a cell with no recorded clay mineralogical data, positioned between several cells with same clay mineralogy had the same clay mineralogy. We expanded the point data on clay mineralogy of a paddy soil to the plane data in this way. Most alluvial paddy soils in Miyagi Prefecture were classified as being of mixed type and smectic type. It is considered that the paddy soils abundant in smectite are strongly influenced by the tertiary tuff, containing smectite present in the mountains where many rivers originate. Paddy soils with clay mineralogical compositions of mixed type rich in chlorite existed in valley basins of the northeastern area and soils abundant in kaolin minerals were distributed into watersheds affected by granite rock. Paddy soils abundant in 2:1-2:1:1 intergraded minerals were distributed on terrains and hills of the western area where non-allophanic Andosols and residual soils existed. Paddy soils with clay fractions dominated by amorphous clay were distributed on the Eastward side of the Zao volcano, which is located in the western part of Miyagi Prefecture.

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