Received: October 27, 2018; Published: October 30, 2018
*Corresponding author: Nida Tabassum Khan, Department of Biotechnology, Faculty of Life Sciences and Informatics, Pakistan
Biofertilizers are constituents of microbial cells that colonizes rhizosphere of the plants to improve nutrients uptake by increasing the rate of mineralization of insoluble nutrients in the soil through numerous natural process such as nitrogen fixation, phosphorous fixation etc.
Keywords: Arbuscular Mycorrhiza; Bacillus subtilis; Bacillus circulans; Azolla; Azos Azospirillum lipoferum
Biofertilizer is a biotic constituent of specific microbial cells which when applied, stimulates plant growth by accelerating the rate of nutrient release through nitrogen fixation, phosphorus cycle etc . Invitro cultures of specific microbes are selected for the preparation of biofertilizers to fulfill the specific nutrient requirements of different plants . Biofertilizer is categorized in to different given below
A. Nitrogen fixing
i. Free-living: Clostridium, Azotobacter, Nostoc, Anabaena etc 
ii. Symbiotic: Rhizobium, Azollae, Anabaena etc 
iii. Associative Symbiotic: Azos Azospirillum lipoferum 
B. Phosphorous solubilizing
i. Bacteria: Bacillus subtilis, Pseudomonas striata, Bacillus circulans etc 
ii. Fungi: Aspergillus awamori, Penicillium sp etc 
C. Phosphorous mobilizing
i. Arbuscular mycorrhiza: Glomus sp., Acaulospora sp, Gigaspora sp & Sclerocystis sp .
ii. Ectomycorrhiza: Laccaria sp., Boletus sp., Amanita sp .
iii. Ericoid mycorrhizae: Pezizella ericae 
iv. Orchid mycorrhiza: Rhizoctonia solani 
D. Biofertilizers for Micronutrients
i. Silicate and Zinc solubilizers: Bacillus sp 
E. Plant Growth Promoting Rhizobacteria
i. Pseudomonas: Pseudomonas fluorescens 
a. Rhizobium: Rhizobium is a nitrogen fixing bacteria that colonizes the root nodules of legumous plants and is an effective biofertilizers . They are referred as cross inoculation group for being specific to form root nodules in legumous plants and has seven genera [15, 16]. For all legumous plants it is applied as seed inoculant .
b. Azotobacter: Another nitrogen fixing bacteria inoculant that produces ample slime which aids in soil accretion e.g. A. chroococcum .
c. Azospirillum: Nitrogen fixing bacteria that colonizes the non-leguminous graminaceous plants rhizosphere and intercellular spaces of root cortex e.g. Azospirillum lipoferum, A. brasilense, A. amazonense . Besides these have the ability to reduce nitrate, denitrify etc . Azospirillum is inoculated through seed, seedling root dip and soil application methods 
d. Cyanobacteria: Free-living /symbiotic cyanobacteria is used as a biofertilizers for rice .
e. Azolla: Azolla is a free-floating water fern that fixes atmospheric nitrogen in association with cyanobacteria . It is used as a biofertilizer for wetland rice .
f. Phosphate solubilizing microorganism: Microorganisms such as Pseudomonas striata., Bacillus polymyxa, Penicillium, Aspergillus etc secretes organic acids that causes dissolution of bound phosphates in soil .
g. Arbuscular Mycorrhiza: Intracellular obligate fungal endosymbionts that possess vesicles for storage of nutrients and arbuscles for directing phosphorus, zinc and sulphur into the root system .
h. Silicate solubilizing bacteria: Some microbes are capable of dissolution silicates by secretion of organic acids like citric, oxalic acid etc . e.g. Bacillus sp .
i. Plant Growth Promoting Rhizobacteria: Such inoculants are either bioprotectants (suppresses plant disease), biofertilizers (improves nutrient acquisition) or biostimulants (phytohormone production) . e.g Pseudomonas and Bacillus species .
a. Long shelf life
b. No contamination
c. No loss of activity
d. Easy documentation
e. Cost effective
f. Quality control protocols are easy and quick
g. Better performance
h. Easy to use
i. Less dosage required
j. High marketable profit
k. High distribution potential
l. High enzymatic activity [31-36]
a. Fixing nutrient availability in soil
b. Improves soil fertility
c. Readily converts complex organic compounds into simple soluble forms
d. Accelerates mineral uptake by plants
e. Increases crop yield
f. Stimulates plant growth
g. Provide resistance against drought and soil-borne diseases
h. Cost-effective [37-40]
i. Treatment of seed
ii. Root dipping of seedling
iii. Main field use 
Though this technology is cost effective, environmental friendly but still there are numerous limitations that restricts its implementation 
a. Use of less effective strains
b. Absence of competent technical staff
c. Synthesis of poor-quality inoculants
d. Short shelf life of inoculants [43-45]
i. Deficiency of appropriate production facilities
ii. Absence of crucial production equipments
iii. Availability of space for production or storage, etc.
iv. Lack of cold storage facility for inoculants [46-48]
a. Insufficient funds
b. Low profit generation in smaller scale units 
i. Cyclical demands for biofertilizers
ii. Instantaneous harvesting
iii. short duration of sowing/planting
iv. Soil characteristics [50-54]
a. Lack of technical expertise
b. Lack of appropriate training
c. Manufacturer ignorance regarding product quality
d. Unapproachability of quality management and control methods
e. Lack of strict regulations [55-59]
i. Lack of knowledge about the technology assistances
ii. Reluctance to practice
iii. No apparent change in the growth of crop
iv. Lack of knowledge on the environmental impacts of inorganic fertilizer [60-62]
a. Unavailability of right inoculant
b. Lack of preserve channels or markets for the producers [63,64]
Thus, biofertilizers proves to be an effective way for supplementing the plants with soluble essential nutrients for the sake of good growth and commercial benefit.
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