Author : Dja'far Shiddieq

Summary : A study was conducted to evaluate the effect of conservation of tropical rainforest to agriculture on the chemical, physical, and morphological properties of Ultisols, and to establish changes in the classification of Ultisol brought about by changes in soil properties. Twelve representative pedons used for detailed study were selected based on their parent material, elevation, slope, vegetation, and climate. Sampling of forest communities was done using point centered quarter method. The soils were classified following the U.S. Comprehensive Soil Classification System or Soil Taxonomy. Organic carbon (C), total nitrogen (N), cation exchange capacity (CEC), and base saturation were drastically decreased with the conversion of old and young secondary forest (natural forest) to agriculture. However, exchangeable aluminum (AI) and A1 saturation increased. Physical properties were affected by the decrease in organic carbon. Bulk density increased while available water holding capacity decreased due to the deterioration of soil structure. The soil chemical and physical properties of cogonlands were surprisingly improved by the presence of cogongrass relative to those under crops. The mineralogy of the soils in the study area, was dominated by kaolinite and followed by quartz. Vermicullite was very common in the soils under natural forest. Traces of gibbsite were observed except in cogongrass fields where it was fairly common. races of cristobolite and pyrophylite were found in all the soils. The soils under old and young secondary forests were classified under the family of Typic Haplohumult fine-loamy, kaolanitic, isohyperthermic. The soils under cultivated area were considered in the family of Typic Hapludult fine-loamy, kaolinitic, and those under cogongrass were under the family Humic hapludult fine loamy, kaolinitic, isohyperthermic. Conversion of natural forest to other land use had impacts on elanization, translocation, accumulation, and clay transformation processes.