Assisted Natural Regeneration (RNA) is one of practices that small farmers adopted to restore vegetation cover and improve land productivity in cultivated areas in Niger. The present study, conducted in the main cultivation areas in Niger, aims to assess the effects of RNA trees on soil physicochemical characteristics of cultivated tropical ferruginous soil in order to assess their fertility. Composite soil samples were collected at 0-20 cm deep under and outside trees crowns of Piliostigma reticulatum, Combretum glutinosum and Sclerocarya birrea respectvily to determine physicochemical characteristics i.e soil particle size composition, pH, CEC, organic matter contents, assimilable phosphorus. The results showed that trees have no influence on particle size composition because there are no significant differences between area under crown and area outside crown on granulometric composition under all species. However, the presence of these trees (Piliostigma reticulatum, Combretum glutinosum and Sclerocarya birrea) significantly improves chemical soil fertility. Indeed, organic matter content was 26, 3.8 and 4.2 times higher respectively under crown of P. reticulatum, C. glutinosum and S. birrea than that outside crown of these species. Moreover, available phosphorus content, often very low in cultivated tropical ferruginous soils in Niger, was 2.5, 1.2 and 2.1 times higher in soil under crown than soil outside without crown of P. reticulatum, C. glutinosum and S. birrea respectively. Thus, RNA practice improves soil chemical fertility of cultivated tropical ferriginous soils through input organic matter and nutrients by biomass plant. However, further studies can be performed to determine effects of RNA ligneous plants on soil hydrostructural properties.