This article throws light upon the nine important groups of bio fertilizers. The groups are: 1. Azolla-Anabaena Symbiosis 2. Nitrogen-Fixing Bacteria Rhizobium 3. Diazotrophs 4. Frankia 5. Cyanobacteria 6. Phosphate Solubilizing Bacteria 7. Phosphate-Solubilizing Fungi 8. Plant Growth Promoting Rhizobacteria (PGPR) 9. Mycorrhiza for Sustainable Agriculture.
1. Azolla-Anabaena Symbiosis:
Azolla is the globally distributed small aquatic fern comprising of 7 living species. In their leaves a heterocystous di-nitrogen-fixing blue green alga Anabaena azollae is always present as symbiont. Azolla is an alternative N source.
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The agronomic potential of Azolla as bio fertilizer for rice has been recognized in many countries including India, Philippines, USA, Sri Lanka and Thailand. A trial under International Network on Soil Fertility and Fertilizer Evaluation for Rice (INSFER) conducted at 37 sites in 10 countries also confirmed the bio fertilizer potential of Azolla.
2. Nitrogen-Fixing Bacteria Rhizobium:
Symbiotic N2-fixation by Rhizobium with legumes contribute substantially to total biological nitrogen fixation (BNF). Rhizobium inoculation is a well- known agronomic practice to ensure an adequate nitrogen nutrition of legumes in place of fertilizer nitrogen.
The three rhizobial genera include Rhizobium, Brady rhizobium and Azorhizobium. Root infection by rhizobia is a multistep process initiated by pre-infection events in the rhizosphere.
Various events which lead to the formation of nodules include mutual recognition of host plant and Rhizobium species, rhizobial adherence to the root hairs, root hair curling, root hair infection, division of cortical cell to form root primordia and, finally, formation of nitrogen-fixing tissues in the nodules.
3. Diazotrophs:
Diazotrophs which comprise a large group of aerobic chemolithotrophs (Thiobacillus, Desulphovibrio), anerbic photoautotrophs (members of Rhodospirillaceae, Chromatiaceae and Chlorobiaceae), aerobic to microaerobic heterotrophs (members of Azotobacteraceae and Bacillaceae).
Corynebacteriaceae (Azotobacter) and Spirillaceae (Azospirillum) are the free-living as well as associative forms which fix nitrogen in the rhizosphere of a variety of crop species.
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Diazotrophs like Herbaspirillum sps. grow endophytically in the stems and leaves of sugar cane and rice. Azoarcus inhibits roots of Kallar grass (Leptochloafusca) and rice. Although many genera of bacteria are isolated from the rhizosphere of various cereals, many members of Azotobacter and Azospirillum genera have been widely tested to increase yields of cereals under field conditions.
4. Frankia:
Frankia is a member of Actinomycetes or ray fungi. It forms nodules with higher plants IikeAlnus and Elder. There is a need to determine the defence response of a crop to Frankia and technologies to engineer the plant to nodule with Frankia. It produces true mycelium and the spores are not produced.
5. Cyanobacteria:
Species of Anabaena, Nostoc, Scytonema, Calothrix, Aulosira, Nodularia, Lyngbya and Mastigocladus, etc., are able to fix nitrogen. Most of the crops which grow in plenty of water like rice, etc., have association of these blue green algae or cyanobacteria.
It is a common observation that if a soil is under rice cultivation there is native cyanobacterial flora and may not need fresh inoculation. If crops are changed—as is in most parts of the country—there is need to use fresh inoculum. Usually a mixture of these algae is used and not a monoculture. The cyanobacterial flakes are dried and mixed at the rate of 1015 kg ha–1 one week after rice is planted.
6. Phosphate Solubilizing Bacteria:
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A large number of heterotrophic and autotrophic soil microorganisms are now known to have the capacity to solubilize inorganic phosphates through their metabolic activities directly or indirectly. The solubilization of different types of insoluble phosphates varies with the type of microorganisms, the type of phosphates available and available carbon source.
Rhizosphere soil of wheat possesses a greater number of phosphate solubilizing bacteria than in the non-rhizosphere soil.
Bacillus sps., Pseudomonas sp., Brevibacterium sp., Acrobacteracrogenes, Serretia sps., Nitrobacter and Escherichia freundii are important phosphate solubilizing and phosphate mobilizing microbes. Phosphatic bio fertilizers were first prepared by (former) USSR scientists using Bacillus megaterium var. phosphaticum as phosphate solubilizing bacteria and the product was termed as ‘Phosphobacterin’.
7. Phosphate-Solubilizing Fungi:
Along with bacteria and actinomycetes certain fungi also solubilize phosphates. Aspergillus awamori has been found to be a better solubilizer than Bacillus polymyxa or Pseudomonas striata. There are several reports of found phosphate solubilizing bacteria {Pseudomonas) and fungi (Aspergillus) in the rhizosphere of coconut and cocoa.
8. Plant Growth Promoting Rhizobacteria (PGPR):
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The role of PGPR in plant growth and disease management has been studied in 1980s. Several strains in Bacillus have been patented for commercial production in developed countries. The plant growth promoters belonging to pseudomonads and bacilli play a major role in plant productivity.
These are usually non-colonizers. The spore-forming bacteria are promising organisms for microbial control. Pseudomonas fluoresces is registered as microbial pesticide against Pythium and Rhizoctonia.
9. Mycorrhiza for Sustainable Agriculture:
The root-fungus symbiosis is caused by mycorrhizae. The association of mycorrhiza is wide-spread in bryophytes, a large number of pteridophytes, most or all species of gymnosperms and some 90 per cent or more of angiosperms.
Frank was the first to describe association of fungus and roots of higher plants and coined the term ‘mycorrhiza’.
Rees reported the connection between the mantle of the mycorrhizas and the basidiomata or Elaphomyces granulatus. Forest crops may have ectomycorrhizae while endomycorrhizae are present in most agricultural crops. These mycorrhizae differ in their structure and in the systematic position of the fungi involved.
Trappe made a series of studies in Pseudotsuga menziesii (douglas fir) and Chilvers described 8 distinctive types of Eucalyptus ectomycorrhizas. Jayasinghe et al carried out observations on the mycorrhizas of Pinus patula in Sri Lanka and made an attempt to culture and identify the fungus. Typical ectomycorrhizas were synthesized on Pinus ponderosa by four fungi species viz.
Amanita muscaria, A. pantherina, Suillus granulatus and Lactarius deliciosus. Russula aeruginea is another fungus forming ectomycorrhiza in Picea. Ectomycorrhizal fungi grow best at pH 5-6. Excess of inorganic- fertilizers suppress ectomycorrhizal development.
It has been found that optimum development of ectomycorrhiza in chir pine nurseries occurs when nutrients are present in the soil at half the normal level. Shading suppresses ectomycorrhizal development.
The poor development of mycorrhizal root results in the stunted growth and chlorosis of leaves. The mycorrhizal plants are able to withstand better water stress condition that the non-mycorrhizal seedlings. The mycorrhizae are described as ‘Bio fertilizer’ and ‘Biocide’.