What are biofertilisers? Write the main sources of biofertilisers and explain their functions.

Explanation

Biofertilizers are the organisms that enrich the nutrient quality of the soil. The main source of biofertilizers are bacteria, fungi and cyanobacteria. Bacteria and cyanobacteria have the property of nitrogen fixation, while mycorrhizal fungi help in the mineral uptake by the plant.
Rhizobium forms symbiotic association with the roots of leguminous plants. They develop the ability to fix nitrogen when they are present inside that root nodules.
Note : Some non-legume plants also show symbiotic association with some other nitrogen fixing bacteria.
Example : Frankia is associated symbiotipally with the root nodules of several non-legume plants like Casuarina, Alnus.
Symbiotic nitrogen fixing cyanobacteria : A number of cyanQbacteria (blue green algae) form symbiotic association with several plants, e.g., Lichens, hornworts, Azolla (fern), Cycas roots. Azolla is a small fast growing fern that occurs floating on water. Anabaena azollae, (acyanobacterium) lives in the cavities of Azolla leaves. Azolla - Anabaena symbiotic association is of great importance to agriculture. The fern can grow in rice fields, because it does not interfere with the growth of crop plants.
Mycorrhiza : Mycorrhiza (pl. mycorrhizae ) is a symbiotic association of certain fungi with the roots of certain seed bearing plants. Many members of the genus Glomus form mycorrhiza. The fungal symbiont in these associations absorbs phosphorus from soil and passes it to the plant. Plants having such associations show other benefits also, like (a) resistance to root borne pathogens, (b) tolerance to salinity and drought, and (c) an overall increase in plant growth and development.
Mycorrhizae can be broadly classified into two types: ectomycorrhizae and endomycorrhizae.
(i) Ectomycorrhiza: The mycelium of the fungus forms a mantle on the surface of the root. Ectomycorhizae commonly occur on the roots of trees such as pine, oak, peach and eucalyptus.
(ii) Endomycorrhiza : The fungus lives between and within the cells of the cortex of the roots.