Seasonal change in thick regolith hardness and water content in a dry evergreen forest in Kampong Thom Province, Cambodia

In dry evergreen forests in Kampong Thom Province, Cambodia, regoliths over 10 m thick are widely distributed. At one study site in an evergreen forest, a 9.4-m-deep soil profile was excavated and certain physical properties, including soil hardness, soil water content, and extent of root systems, were measured in rainy and dry seasons. A new finding was that the evergreen forests used soil water from regolith in the dry season. The root system penetrated more than 9 m into the soil and withdrew soil water. Soil hardness from the surface to a depth of 4 m changed each dry and rainy season from extremely hard to soft. The reason for the change was related to soil texture; the silty soils readily changed their binding power in response to water content. A seasonal change in soil hardness was also observed at other points by penetration tests indicating very hard surface soils to a depth of 4.3 m at the end of the dry season, very soft surface soils to a depth of 1.0 m at the beginning of the rainy season (apparent soil thickness was 4.2 m), and. 3.5 m of soft surface soil at the end of the rainy season. Thick regoliths and silty surface soil layers were found to store water and maintain dry evergreen forests in central Cambodia. (c) 2008 Elsevier B.V. All rights reserved.