During the 1990s, the Vietnamese government implemented salt-free projects to increase rice production in the areas affected by salinity intrusion in the Mekong Delta. In those areas, hydrological conditions and farming systems have dramatically changed. To clarify the present status of the farming systems and socio-economic conditions of the farmers in the areas, we conducted an interview survey covering a total of 193 farmers’ households in the villages of Dai An II, Long Phu and Tai Van in the Soc Trang Province. Forty one percent of the households practiced rice double cropping combined with livestock raising, 37% practiced a monoculture consisting of rice double cropping, and the others practiced rice double cropping combined with the cultivation of upland crops or fruits. However, almost all the farmers based their household income primarily on rice crops, with other products being only a minor source of income. Land productivity of rice was significantly lower than that of the other products, suggesting that expansion of the farm scale of the other products would be much more suitable for increasing the farmer’s income. Nevertheless, they strongly prefer rice to other products, whose cultivation may be constrained by availability of labor and land. The production scale would be restricted by the number of laborers in the family, because they do not use employed laborers for other products as they do for rice cultivation. Land availability is also a constraint because the upland crops are cultivated in the natural levees, and livestock are raised in the farmers’ back yard. Selling price of rice varied significantly less among the farmers than that of the other products, which may also account for the farmers’ preference for rice. The increase of land productivity through the expansion of the rice fields should have also facilitated their preference for rice over the other crops.
Clay-humus complexes are stabilized by clay, humic substances and cations in soils. Since there are few studies on clay-humus complexes in the tropical and subtropical regions, it is necessary, therefore, to examine the characteristics of clay, humic substances and cations in the soils of subtropical regions. Consequently, an investigation was conducted to examine the characteristics of the humic substances, which are considered be closely related to the properties of soils cultivated with sugarcane. Soil samples were collected from 9 fields in Kita-Daito island and 14 fields in Minami-Daito island, respectively. Clay content ranged for 514.0 to 920.0 g kg-1 in the soils of both islands, indicating that the soil texture consisted mainly of heavy clay. Kaolinite and illite were the predominant clays in the soils of both islands, while other clays included vermiculite-chlorite integrade, goethite and gibbsite. The degree of the humification of humic acids which was low was categorized as Rp type in both islands. Significant correlation coefficients were observed between the contents of exchangeable Ca and humin in the soils of Kita Daito (n=9, r=0.655*), Mimani Daito island (n=14, r=0.750**) and both islands (n=23, r=0.657*), respectively. It was suggested that the content of humin have been enhanced by the presence of exchangeable calcium in soils cultivated with sugarcane in the Kita and Minami Daito islands.