Which substance of biological origin is used to maintain and increase soil fertility?

The use of biofertilizers can reduce the consumption of chemical fertilizers containing which mineral elements?

The aquatic fern which is an excellent biofertilizer is

The organisms which are used to enrich the nutrient quality of the soil are called

The main source of biofertilizers 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.