SOIL PROPERTIES BELOW EXOTIC TREE PLANTATIONS AT THE SAITHONG SILVICULTURAL RESEARCH STATION IN PRACHUAP KHIRI KHAN PROVINCE, THAILAND

Often, exotic trees grow very fast in their new environments and sometimes even faster than the indigenous ones. These exotics seem to be tougher and live through harsher conditions. They may be drought resistant, and they can survive long periods with little to no rainfall. These trees are fast growing and are resistant to pests and diseases which tend to damage the locally growing trees. Moreover, these trees can help improve soil physical and chemical properties. Hence, the objective of this study was to compare the physical and chemical properties of soil under different tree plantations, namely Eucalyptus urophylla, Acacia crassicarpa and Acacia aulacocarpa at the Saithong Silvicultural Research Station. Three soil composite samples were collected from soil depths of 0-10, 10-30, and 30-50 cm, in the three stands aged 27-year old and planted at a spacing of 2 x 2 m.  Soil type in these stands was sandy loam. Soil moisture percentage (12.85%) was significantly the highest in the E. urophylla plantation. Phosphorus (18.2 mg/kg DW), magnesium (6.98 cmol_c/kgDW), and potassium (6.98 cmol_c/kgDW) contents were also the greatest in the E. urophylla plantation. Organic matter content was the highest in the A. crassicarpa plantation (7.59%), while nitrogen (0.04%) and calcium (36.17cmol_c/kg DW) contents were the highest in the A. aulacocarpa plantation. Significant differences were also observed in the soil bulk density and porosity attributable to structure, tillage, cropping practices, soil depth and compaction. Characterization of soil pore system is equally important in understanding soil physical and parent material composition, which has a direct impact on soil chemistry and fertility. Parent material rich in soluble ions (calcium, magnesium, potassium and sodium) is desirable, as these chemicals are easily dissolved in water and readily available for plants. For soils with poor nutrient content found in warm and humid tropical climates with low water supply, E. urophylla, A. crassicarpa and A. aulacocarpa have the potential to enhance the aboveground stand production. Undoubtedly, when planted, they can help restore the soil nutrients faster and therefore, can be used in forest plantations or in agroforestry farms.