The primary organisms responsible for nitrogen fixation in soil are certain types of bacteria, particularly those belonging to the genus Rhizobium that form symbiotic relationships with leguminous plants. These bacteria convert atmospheric nitrogen into ammonia, a form usable by plants. Additionally, some free-living bacteria, such as Azoarcus and Klebsiella, also fix nitrogen independently. This biological process is vital for replenishing soil fertility and supports the entire food web, making nitrogen fixation essential for agriculture and ecosystems.