Describe the role of blue green algae and bacteria in fertility of soil.

Blue green algae(Cyanobacteria)  act as biofertilizer. Biofertilizers are introduced in the soil. They helps in replenishing the nutrients in the soil which are then taken in by the plants .They have the ability to fix the atmospheric nitrogen.  

Other bacteria  such as Rhizobium also have the ability to fix atmospheric nitrogen. Bacteria also help in decomposition of organic wastes and in doing that  provide the nutrients to the soil which are then taken up by the plants.