$Azolla$ possesses a symbiotic association with which of the following?

Modern farmers have been able to increase rice production by about $50\%$ by using which of the following?

The common nitrogen fixer in paddy fields is . . . . . . .

$A$ farmer has applied chemical fertilisers in his crop field for many successive seasons. In the next season,the crop growth was poor as the soil lost its fertility. Suggest the suitable micro-organisms that replenish the fertility of the soil in his field.

Which organism is used as a biofertilizer in the cultivation of soybean crops?

Farmers have reported over $50\%$ higher yields of rice by using which of the following biofertilizer?