Microbial Inoculants: an Approach to Sustainable Agriculture

originally an article published here by Sunita Gaind, i thought i'd repost it, given there's a wealth of good information on the subject, which gives a broad overview:

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 nutrients?
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.