SOIL FORMATION OVER IGNEOUS ROCKS IN SINGAPORE

Abstract

This Academic Exercise endeavour to study soil formation over the igneous rocks in Singapore. Soil formation is a complex function of geology, climate, topography and vegetation. In Singapore, climate is relatively uniform in space and therefore does not cause any noticeable difference in the resulting soil types. The local variation in soil characteristics are mainly attributed to the geologic and topographic factors which exert a strong control on the processes of soil formation. The effect of climate on soil formation is evident through its active influence over soil-forming processes. The residual system is dominated by minerals that occur under intense weathering conditions such as kaolinite and oxides of iron and aluminum. The soluble bases and cations are rapidly leached out of the soil system. The rapid decomposition of organic matter produces thin organic horizon and limits the supply of chelating substances to complex the metallic ions. Translocation of sesquioxides and clay colloids occur and the precipitation of iron oxides produce an oxic B horizon. To examine the influence of parent rock on soil characteristics, the Rengam (over granite) and Jerangau (over granodiorite) soil series were compared. Differences in mineral and chemical composition, and physical properties could mainly be attributed to the mineral composition of the parent rocks. The lack of significant variation in the chemical properties is due to the overwhelming influence of climate which has obliterated to some extent the effect of geology. To examine the influence of topography on soil formation, a toposequence over granite located at Bukit Timah Nature Reserve was studied. Soil variability was studied with respect to slope gradient and slope position. Besides field survey, laboratory analysis was also conducted to test soil pH and moisture content. The soils at different sections of the slope exhibited variability in soil depth, degree of horizonation and texture. On the hill-top and upper slope characterised by gentle gradient, thick soil profiles occur with well developed soil horizons. On the steeper mid-slope, shallow soil profiles occur with soil textures being more coarse compared to other sections of the slope. At the lower slope, soil solum thickens and the soil profiles generally have a more pronounced and clay rich B horizon.