Azotobacter vs. Rhizobium

Azotobacter is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts ( and also has hard crust )and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes that play an important role in the nitrogen cycle in nature, binding atmospheric nitrogen, which is inaccessible to plants, and releasing it in the form of ammonium ions into the soil (nitrogen fixation).

(biology) Any of various bacteria, of the genus Rhizobium, that form nodules on the roots of legumes and fix nitrogen.

the type genus of Rhizobiaceae; usually occur in the root nodules of legumes; can fix atmospheric oxygen

Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium species form an endosymbiotic nitrogen-fixing association with roots of (primarily) legumes and other flowering plants.