Understanding soil inoculants
Biofertilizers contain live microorganisms that, when applied to the seed, plant or soil, inhabit the area around the roots (rhizosphere) or live in the roots. These microorganisms promote plant growth by increasing the supply or availability of nutrients, by stimulating root growth or by aiding other beneficial symbiotic relationships. Biofertilizers are also called plant growth promoters.
Legumes such as clover, peas and beans have root-colonizing rhizobacteria that can increase the availability of nitrogen to the plant by fixing nitrogen from the atmosphere. Each legume has a specific rhizobacteria that works best with that plant. Inoculating the legume seed with the correct bacteria ensures the legume will maximize nitrogen availability if nitrogen in the soil is low This is particularly important if you have not planted the legume species before, because the correct bacteria may not be present in the soil.
There are also free-living, nitrogen-fixing bacteria that can supply nitrogen to cereal plants such as wheat and corn. They live in the area right around the root (the rhizosphere). In general, nitrogen fixation with both the symbiotic and free-living nitrogen fixers is higher in nitrogen-poor soils.
In many soils, nutrients such as phosphorus, potassium and iron are present in large amounts but in forms that plants cannot use. Many bacteria and fungi are able to make these nutrients available to plants by secreting organic acids or other chemicals (siderophores) to dissolve the minerals. Mycorrhizal fungi that live in plant roots are well known for their ability to provide phosphorus to plants. Just like the situation with nitrogen fixers, mycorrhizal fungi are most effective when available phosphorus in the soil is low. When there are adequate nutrients available, plants do not seem to want to exchange their hard-earned products of photosynthesis for more nutrients.
Some bacteria and fungi produce plant growth hormones that can increase root growth specifically and plant growth in general. Increased root growth helps the plant utilize a larger volume of soil for nutrients and water and can help the plant to "outgrow" pathogen attacks. For example, fungi are known to produce gibberellins that are important for seed germination and cell growth, and some bacteria can reduce the amount of ethylene, which is a hormone that plants produce under stress.
There are many examples of soils that are naturally suppressive to plant pests. Suppressive soils are the result of interactions between certain microorganisms and pest organisms. Many of the most common soil inoculants are formulated with these suppressive microorganisms and are used as biopesticides or biocontrol products.