Why is Rhizobium categorized as a ‘symbiotic bacterium’? How does it act as a biofertiliser?

Rhizobium forms a symbiotic relationship with certain plants such as legumes. The Rhizobium fixes nitrogen from the air into ammonia, which acts as a natural fertilizer for the plants. Current research is being conducted by Agricultural Research Service microbiologists to discover a way to utilize Rhizobium’s biological nitrogen fixation. This research involves the genetic mapping of various rhizobial species with its respective symbiotic plant species, like alfalfa or soybean. The goal of this research is to increase the plants’ productivity without using fertilizers.