Soils Under Different Forest Types in the Dry Evergreen Forest Zone of Cambodia: Morphology, Physicochemical Properties, and Classification

We studied the morphology and physicochemical properties of soils under three different types of forest, i.e., dry evergreen forest (DEF), dry deciduous forest (DDF), and mixed forest with evergreen and deciduous trees (MF), in the dry evergreen forest zone of Kampong Thom Province, Cambodia. The morphological features of soils varied among the three different forest types. The physical characteristics of the soils in the study area were strongly correlated with soil texture. Clay content was clearly higher in DEF soils than in the DDF or MF soils. Bulk density was generally high (1.27–1.92), except in the surface horizons. It was especially high at depths of 100–200 cm and 160–200 cm in the DDF and MF soils, respectively. Total soil porosity was 0.32–0.44 (m3 m−3), except in the surface horizons, and was slightly higher in DEF soils. The DEF soils were characterized by a higher percentage of fine pores (less than −49 kPa) than the other pore classes. DDF soils were characterized by decreasing percentages of coarse pores (0 to −0.2 kPa; the point of capillary saturation), medium pores (−0.2 to −4.9 kPa), and small pores (−4.9 to −49 kPa), and by a concomitant increase in fine pores with depth. In MF soils, the proportion of small pores slightly decreased with depth. The soils were generally poor in cation-exchange capacity (CEC) and exchangeable cations (ECEC). ECEC and CEC were closely related to clay content. The stock of exchangeable Ca2+, Mg2+, and K+ was larger in DEF soils than in DDF soils. The pH (H2O) of DDF soils was clearly higher than that of the other soil types at 0–50 cm in depth and showed different patterns in vertical changes. The stock of total carbon at 0–70 cm in depth was highest in MF soils.