Technologies for production of microbial inoculants
Microbial inoculants are gaining popularity because of their multifunctional benefits to plants in sustainable agriculture. The living organisms (nitrogen fixers or phosphate solubilizers, or biocontrol agents) present in bio-fertilizers affect the plant growth and development directly or indirectly either by facilitating the nutrient uptake or by inducing increase in root surface or reducing the harmful effect of pathogens. They can partly replace or substitute the chemical fertilizers whose ever increasing cost is a major financial constraint for the marginal farmers. However, the success of microbial inoculants in the soil depends upon its formulation. Though, many products are commercially available but their poor quality results in loss of confidence among farmers for their use. Bio-fertilizers need to be produced, keeping in mind their acceptance by the farmers so as to harness the desired benefits for their crop.
Facts to be considered
1. The cost of bio fertilizer production should not exceed that of conventional fertilizer to assure the market sustainability.
2. Selection of effective strain is a prerequisite to get maximum functional benefit of inoculation.
3. The inoculant formulation should be able to protect the microbial strain from the harsh environmental conditions during storage and transportation.
4. Quality control of developed inoculants is a must and should meet the BIS standards.
5. It should ensure the prolonged survival and establishment of microbial strain after introduction in soil.
Inoculums production technologies
A. Carrier based inoculants
The carrier is usually an economical and easily available material that has good water holding capacity and is able to gradually release the viable cells in soil. Peat, charcoal -soil mixture, vermiculite, perlite, bentonite, compost, agro-industrial residues are some of the cost effective organic materials used for developing microbial inoculants. The selected carrier after sterilization is mixed with specific microorganisms (nitrogen fixer or phosphate solubilizer) under asceptic condition. The carrier based formulations ensure high cell density on storage only up to 3-4 months. Population decreases at a high rate if kept at room temperature.
B. Liquid formulations
Liquid formulations contain specific microorganisms in broth based or mineral based medium or organic oil based solution. They simplify the production and application for the farmers and may have some advantages over carrier based inoculants. As they are applied directly to the soil or seed, liquid inoculants allow direct contact of seed with the microorganisms and consequently increase the survival of microbial strain on plant roots. However, they fail to provide the microbial strain the protective environment. The possibility of contamination during storage and transport is also high that is not adequate for long term conservation. Moreover, the number of bacteria distributed on each seed is quite heterogeneous. Therefore use of liquid inoculants requires correct storage so that the test strain may not lose its efficiency and cell viability.
C. Use of Clays
Clays are widely used in agricultural formulations applied as granules, suspensions or powder. Clay due to its large surface area pore size distribution and total porosity can act as desiccant that can prolong the shelf life of microbial inoculants. Clay inoculants carrier provide protective microhabitat accessible to bacteria but not to predators.
Biofilms are formed as a result of microbial cell aggregate formation. There are four stages involved in its formation and these include (i) initial attachment (ii) irreversible attachment by the production of exo-polysaccharide (iii) early development and (iv) maturation. Beneficial biofilms developed by using bacterial and fungal strains have been found efficient. Biofilms allow the microorganisms to survive under stress conditions.
Bioencapsulation is based on the principle of providing protection to the microorganism from the biotic and a biotic stress of the environment and ensure their gradual and prolonged release. The additional advantages associated with bio-encapsulation of rhizo-bacteria include contamination reduction during storage and transport. The shelf life of dried capsule in some cases has been reported to be as long as 5 years. The degradation rate of encapsulation matrix depends upon the biological activity of the soil microorganisms.
Natural polymers including alginate, carrageenan, agar-agar and agarose and synthetic polymers including polyacrylamides, polystyrene and polyurethane have been widely used as matrix material. Starch, maltodextrins, corn syrup solid and acacia gum are used in spray- dried encapsulation. These materials due to their low viscosity and good solubility are preferred. However, alginate produced by brown algae Macrocystis pyrifera, Laminaria digitata, Laminaria hyperborean and Eklonia cava and bacteria such as Azotobacter vinelandii and Pseudomonas strains is one of the most commonly used polymer for bio-encapsulation of microorganisms.
Bioencapsulation of microbial inoculants is carried out in three steps
• Incorporation of active ingredient in to solid or liquid matrix
• Dispersion or spraying a solution on solid particles under mechanical stirring
• Stabilization by chemical polymerization
Increasing demand for economical and environment friendly agricultural practices promote the use of bio-fertilizers. As each method has its advantages and disadvantages but cost effectiveness, extended shelf life, high population count under field conditions, easy application and protection against soil environment are some of the issues that need special attention.
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