What is the principle underlying the use of cyanobacteria in agricultural fields for crop improvement ? 

Explanation

Some cyanobacteria live in mutually beneficial relationship with plants obtaining food and shelter and fixing nitrogen for the plant. They also reduce soil alkalinity and improve soil texture.
Nostoc, Anabaena and Oscillatoria are (BGA) Blue - green algae that can fix atmospheric nitrogen.
In paddy fields cyanobacteria serve as an important biofertiliser.
BGA also add organic matter to the soil and increase its fertility.