Background: Soil chemical content can be usually influenced by tree speciespositively or negatively. It is necessary to study the effect of Acacia Melanoxylon and AlnusAccuminata tree species on some acidic soil content. The research was conducted in Rwanda forestry Authority (RFA)/ Ruhande Arboretum to assess the effect ofAcacia melanoxylon and Alnusaccuminataon nitrogen, soil organic matter, and soil acidity content in acidic soil. A Randomized Complete Block Design (RCBD) with three replications and three treatments were used. Soil sampling was done following the zigzag method per each treatment and each replication at two depths (0-15 and 15-30 cm). In each plot and each depth, ten soil samples were put together to make one composite sample. Results: The results indicated that the alter of soil chemical content can be influenced by tree species together with soil depths. The laboratory analyses indicated significant influence oftree specieson Soil pH, Organic Carbon, Organic Matter, Total Nitrogen, and Carbon to Nitrogen Ratio. The high values were observed under these two tree speciesand the low under fallow as a control for all parameters undertaken. The main factors should be the higher biomass and soil nutrients production under Alnusaccuminata comparatively to Acacia melanoxylon and fallow as control. Conclusion: The study indicated that tree species influence soil chemical content. Therefore, the establishment of Acacia melanoxylon and Alnusaccuminata tree species in farming systems like forestry and agroforestry under acidic soil is recommended andmuch should be done by Rwanda Ministry of Agriculture and Animal Resources (MINAGRI), Rwanda Forestry Authority (RFA) and Rwanda Environment Management Authority (REMA) to use these miracles tree species especially Alnus accuminata in farming systems where are suitable across the whole Country.