Rainfed lowland rice-growing soils of Cambodia, Laos, and North-east Thailand

There has been little critical assessment of the similarities and differences between the lowland rainfed rice soils of Cambodia, Laos, and North-east Thailand and their implications for soil management. The purpose of this paper is to review the published literature on soil properties and their spatial distribution in Cambodia, Laos and North-east Thailand, the main soil and water related constraints identified for lowland rainfed rice, and the similarities and differences in soil management technologies that relate to water use. While rainfed rice is the dominant crop, rainfall and its seasonal distribution varies significantly across the region so that cropping patterns vary, especially those for pre-rice and post-rice cropping with field crops. However, for lowland rainfed rice, surface hydrology can vary with natural and artificial drainage patterns and subtle topographic variations to the extent that locally it may override the influence of rainfall. This can make it difficult to regionally assess the prevalent hydrological regimes. Cambodia has a higher prevalence of seasonally flooded, alluvial soils than North-east Thailand or Laos. Substantial areas of sandy, high permeability soils are used for lowland rice in the region, but especially in North-east Thailand. Standing rainwater drains quickly from the soils of these fields exposing the rice to drought and high rates of nutrient leaching. However, loss of soil-water saturation may limit rice yield by inhibiting nutrient uptake more often than drought, per se. The interaction between water supply and nutrient acquisition requires further investigation. Shallow ground water is a potential resource for supplementary irrigation but the scope for using it has not been adequately examined. In North-east Thailand and parts of Laos, salinity in the ground water may be the major limitation on its use. Prospects for growing field crops in the lowlands depend on the amounts and reliability of early wet season rainfall or on amounts of stored water after harvesting rice. Apart from drought, waterlogging and inundation are significant water-related hazards for growing field crops in the early wet season. In addition, soil-fertility constraints in the early wet season and dry season will likely differ from those encountered by rice due in part to the different soil-water regime they encounter