A 17-year chronosequence of Acacia auriculiformis fallows on Arenosols of the Batéké Plateau (D.R. Congo) was surveyed and compared with virgin savannah soils to assess chemical soil fertility changes induced by these N-fixing trees. Significant increases in organic carbon content, total nitrogen content, cation exchange capacity and sum of base cations were found after relatively short fallow periods of only 4 years and did not only affect the forest floor, but extended to at least 50 cm depth. The Acacia act as a major source of organic matter (OM), hence increasing organic carbon and nitrogen content and decreasing the C/N ratio. The increased OM content suggests that humification processes are the main cause of the significant decrease in pH. Total exchangeable cations initially increased slowly but doubled (topsoil 0–25 cm) and tripled (subsoil 25–50 cm) after 10 years. The point of zero net proton charge was systematically lower than soil pH and decreased with increasing OM content, thereby increasing the cation exchange capacity, although concurrent acidification retarded a significant beneficial impact at field pH on Acacia fallows of 10 years and older. Although the chemical soil fertility improves steadily with time, after 8 years of Acacia fallow the absolute amounts of available nutrients are still small and slash and burn practices are required to liberate the nutrients stored in the remaining biomass and litter before each new cropping period.