The intensive land use on the Ridge has resulted in very deep erosion and weathering of its soil (12-25 m). As such, the humus (organic matter) content of the soil is low, and its nature strongly reflects the characteristics of the rocks that lie underneath. The rocks on Kent Ridge are from three formations: the Kallang, Tekong, and Jurong formations (Public Utilities Board, 1976). These are mostly sandstone and mudstone formations, and so the soils they form are mostly sediment that is deficient in many forms of plant nutrients. The soil is known to be deficient in potassium (K) and calcium (Ca) (Lee, 1995); hence Ridge plants have to be well adapted to grow under such conditions. The evidence for the deep erosion of the soil is the numerous boulders that can be seen in several places along Kent Ridge Road. While there is leaf litter present, the humus layer is much thinner compared to primary forest, and even more so in areas covered by grass. Organic matter on the ground acts as a 'sponge' to absorb water when the rain comes, (Holttum, 1954b) so without it, the water holding capacity of the soil is reduced. Kent Ridge soil exhibits high surface runoff and a compact nature. (Sim, 1991; Mudaliar, 1984) Therefore, water is not absorbed and retained well by the soil, instead flowing off; in addition, the soil is poorly aerated. Together with the higher soil temperature, due to the greater light penetration to the surface compared with primary forests, this causes the environment to be drier than primary forest, hence pioneer belukar plants have to be adapted to lower water availability. Ironically, while the open nature of the canopy causes higher rainfall penetration to the ground, this results in erosion of the soil that eventually causes the water deficiency. The acidity of the soil is also high, and this can decrease the solubility and hence availability of macronutrients, in particular nitrogen and phosphorus, while increasing the solubility of toxic substances like aluminum. Therefore, only plants that can tolerate this can survive. The relative absence of legumes (members of the family Leguminosae [Fabaceae]), except for the exotic Acacia, means that they cannot carry out their nitrogen fixing function for the soil.