Research relating to the use of organic amendments on soils has focused largely on agricultural soils, and there is a lack of information worldwide on their efficacy as amendments for urban soil management, especially in tropical urban environments. A pot experiment was conducted to assess the influence of biochar and organic compost on urban soil properties and on tree growth performance in Singapore. Biochar and compost were mixed with topsoil in different proportions, and two urban tree species commonly grown in Singapore (Samanea saman and Suregada multiflora) were used. There were significant additional height increments for both the tree species following application of biochar. S. saman exhibited greater stem elongation compared with S. multiflora in response to organic amendments. A significantly higher foliar N content was found in both tree species in biochar-amended treatments along with significant increases in P and K. Increases in soil nutrient concentrations were also observed in combined biochar–compost treatments for both species. Combined compost and biochar had the strongest effects on soils and growth of the two urban tree species examined and applications containing biochar resulted in the most significant soil improvements.