Microbes as a biofertilizers

MICROBES AS BIOFERTILISERS

Organic farming is the raising of unpolluted crops through the use of biofertilisers that provide optimum nutrients to crop plants.

Organisms which can be used to improve the nutrient quality of soil through biological activity are known as biofertilisers.

The main sources are bacteria, cyanobacteria and fungi. In paddy fields, cyanobacteria serve as an important biofertiliser.

(a) Symbiotic N₂ fixing bacteria like Rhizobium leguminosarum fixes atmospheric N₂ in root nodules of legumes.

(b) Frankia (Actinomycetes) in root nodules of non-legume plants (e.g., Casuarina and Alnus).

(c) Symbiotic cyanobacteria (blue green algae) like Anabaena azollae fixes atmospheric N₂ in leaves of Azolla (water fern). Azolla pinnata (a pteridophyte) is used as an excellent fertilizer in rice field.

(d) Anabaena cycadae lives in coralloid root of Cycas (a gymnosperm).

(e) Aulosira is most active, non symbiotic nitrogen fixer in rice field in India.

(f) Free living nitrogen fixer like Azospirillum and Azotobacter enrich the nitrogen content in soil.

(g) Mycorrhiza : It is symbiotic association between the fungus and roots of higher plants (seed plants). Many members of the genus Glomus form mycorrhiza. The fungal partner absorbs phosphorus from soil and passes it to the plant. Plants having mycorrhizal associations show resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in growth and development. It is of two types :

(i) Ectomycorrhizae (Ectotrophic or Ectophytic) : Hyphae of fungus only form mantle on the outer surface of the root, increasing absorption of water and minerals e.g., Pinus, oak etc. Mycorrhiza absorb and store nitrogen, phosphorus, potassium and calcium.

(ii) Endomycorrhizae (Endotrophic or Endophytic) : Fungal hyphae penetrate into cortex and cells of root e.g., orchids, coffee and woody plants. These are also called as vesicular arbuscular mycorrhizae or VAM, because cortical cells swell and form vesicles or arbuscles. It has significant role in phosphorus nutrition in plants.