CARDI Soil and Water Science Technical Note No. 3 Soil and Landscapes of Basaltic Terrain in Ou Reang Ov District, Kampong Cham Province, Kingdom of Cambodia

Cambodia has an old national soil map and a more recent map of rice soils, but little is known about soil distribution and properties in the uplands. Such soil information is needed to support moves towards crop diversification in Cambodia in particular for the production of crops other than rice. This investigation was to identify the range and distribution of soil types in the Ou Reang Ov district, Kampong Cham province and to determine the limiting factors of these soils. The study area was on uplands of mid Pleistocene basaltic flows and associated lowland paddy soils of old and recent alluvium/colluvium. Soil classification was completed by detailed profile descriptions using FAO descriptors. These were combined with digital air photos, a digital elevation model, Landsat TM satellite images and expert knowledge to create 10 soil- landscape map units. Chemical and mineralogical properties of the detailed soil profiles were analysed. Soils on the basaltic terrain follow a toposequence from the Labansiek Soil group on the undulating surfaces of the plateau to the Kompong Siem Soil group on the lower slopes with an intermediate gravel-rich, brown clayey soil on the mid-slopes. The Ou Reang Ov Soil group is a newly proposed group for the mid- slope soils of the basaltic terrain. The abundance of gravel in the profile limits soil water storage which can be a constraint for crops under the erratic rainfall regimes of Cambodia. Although the soil occurs on hilly and sloping land, due to high permeability it is not prone to waterlogging or water erosion. The surface is prone to be hard when dry, but the sub-soil is generally friable and root penetration to 60 cm or deeper was observed. The Labansiek non-petroferric phase is a well-structured, acid red soil, with potentially deep root penetration and high soil water storage except where Al toxicity inhibits sub-soil root growth. A significant proportion of these soils are already used for rubber plantations in Ou Reang Ov. Kompong Siem is a deep black soil with no significant impeding layers. Slopes are low, and fields are often bunded so erosion risk is negligible. The soil has self-mulching properties so soil structure re-forms after tillage through wetting and drying cycles. Nutrient availability is not limiting as levels of most nutrients are satisfactory and pH is only moderately to slightly acid. Despite the high clay content, the Kompong Siem soil in the upland areas appears to drain well in the early wet season. However, its location on relatively flat, low-lying landforms results in shallow watertables (10-60 cm) during the main wet season. In conclusion, the clayey soils of the hills and slopes of Ou Reang Ov district have high potential for non-rice cropping. Their relative proximity to markets and good-quality all-weather roads adds to the potential for crop diversification.