Microbial richness of agricultural soils is an indicator of its health and fertility with a significant impact on crop yields.Present study analyzed the relationship between nitrogen-fixing bacteria and physicochemical characteristics of agricultural soils in the southern department of Atlántico, Colombia.Soil samples were collected from 10 sites of Repelón irrigation district, for physicochemical analysis (pH, organic matter, texture, moisture and available phosphorus) and isolation of nitrogen-fixing bacterial' strains using nitrogen-free culture media.Results demostrated the higher previlance of nitrogen-fixing bacteria in northern zone and central zone of the Repelón irrigation district (1.63 × 10 8 CFU g -1 for strain-1, 5.2 × 10 7 CFU g -1 for strain-2 and 4.5 × 10 7 CFU g -1 for strain-3).On the other hand, the physicochemical characteristics of soil show the adequacy to sustain nitrogen-fixing bacteria.The findings of the present research may serve as a baseline to identify soil micorganisms and defining strategies for sustainable management of agricultural soils in the region because these are integral component in ecosystem for nutrient recycling.