Soil Characteristics of Lowland Rice Fields along Topographical Positions in an Intermountain Basin of Northern Laos

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Information related to environmental conditions such as soil characteristics is essential for the development of an effective lowland rice production strategy in the mountainous region of northern Laos. In this study, we investigate soils of lowland rice fields located at various topographical positions in an intermountain basin in order to evaluate the soil characteristics with special reference to mineralogical properties, charge characteristics, and P status of the soils. Five rice field sites were selected, including those located in the lower part of the basin (LP), upper part of the basin (UP), floodplain (FP), and fallow land at a hill slope (MF). The clay mineral composition was dominated by kaolin minerals and quartz in UP and MF and by kaolin minerals in LP. A higher proportion of illite and predominance of quartz was found in FP. All the soils showed point of zero salt effect (PZSE) values of less than 4.0, irrespective of horizon and site, which are 2.0-3.0 units lower than pH_w. The σ_p values ranged from 3.88 to 9.76 cmol_c kg^<-1> at the A horizon. The Feo/Fed and Alo/Ald ratios ranged mostly from 0.2 to 0.4 and 0.2 to 1.0, respectively. On the basis of these results, the soils in the study area were considered to be at a stage prior to a strongly weathered phase. The total P and Bray-P contents of the soils ranged from 139-503mg kg^<-1> and 1-63mg kg^<-1>, respectively. The Bray-P contents were in the order FP>UP>LP>MF. The total P contents and most of the inorganic P fractions extracted by sequential P analysis were highest at the A horizon in FR. The amount of inorganic P fractions in the surface soils of UP were higher than those of MF and LP. Therefore, it was suggested that eroded soil materials, including nutrients such as P from MF, were translocated to UP; however, they were prevented from reaching LP owing to its topographical position. The soils in FP seemed to be affected by the different soil materials transported by occasional flooding. These results indicated that it is necessary to establish lowland rice field management technologies by taking into account the difference in topography and its influence on soil fertility.
2011-06-30

Soil Characteristics of Lowland Rice Fields along Topographical Positions in an Intermountain Basin of Northern Laos

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