Tropical forests are characterized by high biodiversity and aboveground biomass growing on strongly weathered soils. However, the distribution of plant species and soils are highly variable even within a tropical region. This paper reviews existing and novel knowledge on soil genesis, plant and microbial physiology, and biogeochemistry. Typically, forests in Southeast Asia are dominated by dipterocarps growing on acidic Ultisols from relatively young parent material. In the Neotropics and Africa, forests contain abundant legume trees growing on Oxisols developed in the older parent materials on stable continental shields. In Southeast Asia, the removal of base cations from the surface soil due to leaching and uptake by dipterocarp trees result in intensive acidification and accumulation of exchangeable Al3+, which is toxic to most plants. Nutrient mining by ectomycorrhizal fungi and efficient allocation within tree organs can supply phosphorus (P) for reproduction (e.g., mast fruiting) even on P-limited soils. In the Neotropics and Africa, nitrogen (N) fixation by legume trees can ameliorate N or P limitation but excess N can promote acidification through nitrification. Biological weathering [e.g., plant silicon (Si) cycling] and leaching can lead to loss of Si from soil. The resulting accumulation of Al and Fe oxides in Oxisols that can reduce P solubility through sorption and lead to limitation of P relative to N. Thus, geographical variation in geology and plant species drives patterns of soil weathering and niche differentiation at the global scale in tropical forests.