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Biofertilizers, Types and Importances

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Biofertilizers are defined as preparations containing living cells or latent cells of efficient strains of microorganisms that help crop plants uptake of nutrients by their interactions in the rhizosphere when applied through seed or soil. They accelerate certain processes in the soil which augment the extent of availability of nutrients in a form easily assimilated by plants.Use of biofertilizers is one of the important components of integrated nutrient management, as they are cost effective and renewable source of plant nutrients to supplement the chemical fertilizers for sustainable agriculture. Several microorganisms and their association with crop plants are being exploited in the production of biofertilizers. They can be grouped in different ways based on their nature and function.

I. Nitrogen fixers
a. Free living : Aerobic – Azotobacter, Beijerinckia, Anabaena
Anaerobic – Clostridium
Faultative anaerobic – Klebsiella
b. Symbiotic : Rhizobium, Frankia, Anabaena azollae
c. Associative symbiotic : Azospirillum
d. Endophytic : Gluconacetobacter, Burkholdria
II. Phosphorus solubilizers
Bacteria : Bacillus megaterium var. phosphaticum, B. subtilis, B. circulans, Pseudomonas striata
Fungi : Penicillium sp., Aspergillus awamori
III. P mobilizers
a) AM fungi
b) Ectomycorrhizal fungi
c) Ericoid Mycorrhiza
d) Orchid mycorrhiza
IV. Silicate and Zinc solubilizers: Bacillus sp,
V. Plant growth promoting Rhizobacteria: Pseudomans spp.,and many more

Importance of Biofertilizers
Biofertilizers are known to make a number of positive contributions in agriculture.
• Supplement fertilizer supplies for meeting the nutrient needs of crops.
• Add 20 – 200 kg N/ha (by fixation) under optimum conditions and solubilise/mobilise 30-50 kg P2O5/ha.
• They liberate growth promoting substances and vitamins and help to maintain soil fertility.
• They suppress the incidence of pathogens and control diseases.
• Increase the crop yield by 10-50%. N2 fixers reduce depletion of soil nutrients and provide sustainability to the farming system.
• Cheaper, pollution free and based on renewable energy sources.
• They improve soil physical properties, tilth and soil health.

1. Rhizobium
Rhizobia are soil bacteria, live freely in soil and in the root region of both leguminous and non-leguminous plants. However they enter into symbiosis only with leguminous plants, by infesting their roots and forming nodules on them. Non legume nodulated by Rhizobia is Trema or Parasponia sp.
The nodulated legumes contribute a good deal to the amount of N2 fixed in the biosphere, (50-200 kg N/ha) varied with crops.
Clover - 130 kg N/ha
Cowpea - 62 – 128 kg N/ha
Beijerinck first isolated and cultivate a microorganism from the roots of legumes in 1888 and he named this as Bacillus radicola and latter modified as Rhizobium. Legume plants fix and utilise this N by working symbiotically with Rhizobium in nodules on their roots. The host plants provide a home for bacteria and energy to fix atmospheric N2 and in turn the plant receives fixed N2 (as protein).

Cross inoculation group (CGI)
It refers the group of leguminous plant that will develop nodules when inoculated with the rhizobia obtained from the nodules from any member of that legume group.
Genera/species - Principal and other reported hosts
RhizobiumR.etli - Phaseolus vulgaris, Mimosa affinis
R.galegae - Galega orientalis, G.officinalis
R.gallicum - Phaseolus vulgaris, Leucaena, Macroptilium, Onobrychis
R.giardini - Phaseolus vulgaris, Leucaena, Macroptilium
R.hainanense - Desmodium sinuatum, Stylosanthes, Vigna, Arachis, Centrosema
R.huautlense - Sesbania herbacea
R.indigoferae - Indigofera
R.leguminosarum
bv trifolii - 
Trifolium
bv viciae - 
Lathyrus, Lens, Pisum, and Vicia,
bv.phaseoli - 
Phaseolus vulgaris
R.mongolense - Medicago ruthenica, Phaseolus vulgaris
R.sullae  - Hedysarum coronarium
R.tropici - Phaseolus vulgaris, Dalea, Leucaena, Macroptilium, Onobrychis

Mesorhizobium
M.amorphae - Amorpha fruticosa
M.chacoense - Prosopis alba
M.ciceri - Cicer arietinum
M.huakuii - Astragalus sinicus, Acacia
M.loti - Lotus corniculatus
M. mediterraneum - Cicer arietinum
M.plurifarium - Acacia senegal, Prosopis juriflora, Leucaena
M.tianshanense - Glycyrrhiza pallidflora, Swansonia, Glycine, Caragana, Sophora
Sinorhizobium
S.abri - Abrus precatorius
S.americanus - Acacia spp.
S.arboris - Acacia senegal, Prosopis chilensis
S.fredi - Glycine max
S.indiaense - Sesbania rostrata
S.kostiense - Acacia senegal, Prosopis chilensis
S.kummerowiae - Kummerowia stipulacea
S.meliloti - Medicago, Melilotus, Trigonella
S.medicae - Medicago truncatula, M. polymorpha, M.orbicularis
S.morelense - Leucaena leucocephala
S.sahelense - Acacia, Sesbania
S.terangae - Acacia, Sesbania

Azorhizobium
A.caulinodans - Sesbania rostrata
AllorhizobiumA.undicola - Neptunia natans, Acacia, Faidherbia, Lotus

Bradyrhizobium
B.elkanii - Glycine max
B.japonicum - Glycine max
B.liaoningense - Glycine max
B.yuanmingense - Lespedeza, Medicago, Melilotus
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