Microbial inoculants: An approach to sustainable agriculture
By: Sunita Gaind

The green revolution though made India self sufficient in food production, but at the cost of soil health. Persistent use of chemical fertilizers and low input of organic material in soil reduced its organic matter content, resulting in stagnation of food grain production by 1.5 %. To restore the productivity of soil, efforts need to be focused on use of natural resources that can be an alternate to costly chemical fertilizers and restrict soil impoverishment. Current developments in sustainability involve the rationale exploitation of soil microbial activities and use of less available sources of plant nutrients. Nitrogen and phosphorus are the macronutrients that limit the plant growth. To meet the crop need, these are generally supplemented through chemical fertilizers. Soil inhabits microorganisms that possess the particular trait for nitrogen and phosphorus transformation. Their application to soil under crop cultivation can improve the nutrient availability; reduce the input of chemical fertilizer and a way to sustainable agriculture.
What are microbial inoculants?

Microbial inoculants are the formulations of beneficial living microorganisms that when added to soil, directly or indirectly, improve the nutrient availability to the host plant and promote plant growth. Microbial inoculants for biological nitrogen fixation are both strain and crop specific. However, phosphorus solubilization and mineralization can be mediated by potential isolates of bacteria and fungi. The latter being the key components of soil plant system can be developed as phosphate solubilizing microbial inoculants.

Fungi vs Bacteria as Phosphate solubilizeres
 Fungi maintain their P dissolving efficiency even on repeated sub culturing
 The extracellular production of phosphatase and organic acid is higher with fungi compared to bacteria. Therefore, fungi are more effective phosphate mineralizer/ solubilizers compared to bacteria.
 Their hyphae can travel long distance in soil more easily than bacteria and can prove more beneficial for solubilization of phosphorus in soil.
 They can tolerate low moisture, high temperature, heavy metals and agrochemicals.
 Their spore forming nature is an additional advantage for their survival under environmental stress.

How do microbes improve the availability of nutrient?
Soil microorganisms are involved in large number of processes that affect the P transformation and influence its availability to plant roots.
 By the excretion of hydrogen ions.
 By release of organic acids.
 By production of phosphatase enzymes that can mineralize soil organic P.
 Chelating metal ions that may be associated with complexed forms of P or may facilitate the release of adsorbed P through ligand exchange reactions.
 By displacement of sorption equilibria that results in increased net transfer of phosphate ions into soil solution or an increase in the mobility of organic forms of phosphorus.
 Growth stimulation through production of phytohormones.
 By production of siderophores.
 Phosphate dissolving fungi may also provide micronutrients for formation of polyphenol and other aspects of phenolic metabolism.
 Phosphate dissolving Trichoderma harzianum has shown the ability to accelerate the oxidative dissolution of metallic Zn.
 They also provide disease resistance to plants due to production of antibiotics and protection against soil borne pathogens.

Phosphate dissolving microorganisms used as microbial inoculants
Fungi: Aspergillus awamori, Aspergillus niger, Penicillium digitatum, Pencillium radicum, Penicillium bilaiae, Trichoderma koningii

Bacteria: Pseudomonas striata, Bacillus polymyxa, Bacillus megaterium, B.subtilis, B. circulans

Carriers: Charcoal- soil mixture, vermiculite, press mud, peat, cow dung cake powder, farm yard manure, wheat bran etc. Amendment of charcoal soil - mixture carrier with calcium alginate resulted in better retention of moisture.

Pre-requisites for developing Microbial formulations
 Selection of efficient strain through rigorous screening.
 Selection of easily available carrier material that may aid in the stabilization and protection of microbial cells during storage and transformation.
 Carrier should have good water holding capacity.
 Analysis of carrier for pH, pore space and nutrient content is a must as it has to maintain the population load of inoculated culture.
 Developed inoculant should be safe, easy to handle and apply. It should be stable during storage and transformation.
 It should be cost effective.
 It should have a good population load of x 10 7 cfu/ g carrier and shelf life of more than six months.
 It should be free from contaminants and meet the specific standards of BIS.
 It should be able to perform well under different climatic zone.

Once the microbial inoculant is developed, it is packed in sterile polythelene bags labeled with culture code, instruction for its use and manufacturing and expiry date. It should be stored at low temperature to maintain the cell viability.

Why do inoculants fail to perform in field?
A number of factors are responsible for failure of microbial inoculants in the soil atmosphere. These include
 Failure of the added microorganisms to compete with native flora of soil.
 Physico-chemical properties of soil including pH and moisture content.
 Composition of root exudates.
 Presence of environmetal pollutants as pesticides.
 Lack of association between inoculated culture and host plant.
 Difference in type and level of organic acid produced by the inoculant culture.
 Power of organic acid to complex with cation and liberate inorganic P varies with citric and oxalic acid. Gluconic acid has a limited ability to chelate and release P complexed with calcium.

In order to draw the full benefit of microbial inoculants, the initial laboratory screening of the strain should be rigorous.

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