Microbial Bio-Fertilisers are biologically active (living or temporarily inert) inputs and contain one or more types of beneficial microorganisms such as bacteria, algae or fungi. Every microorganism – and hence each type of biofertiliser – has a specific capability and function. It would be relevant to mention that vermicompost is not a biofertiliser as is propagated by some, but merely an improved form of compost.
There are broadly seven types of biofertilisers :
Rhizobia is a group of bacteria that fixes nitrogen in association with the roots of leguminous crops. Rhizobia can fix 40-120 kgs. of nitrogen per acre annually depending upon the crop, rhizobium species and environmental conditions. They help improve soil fertility, plant nutrition and plant growth and have no negative effect on soil or the environment. Every leguminous crop requires a specific rhizobium species.
Azotobacter is also a group of nitrogen-fixing bacteria but unlike rhizobia, they do not form root nodules or associate with leguminous crops. They are free-living nitrogen fixers and can be used for all types of upland crops but cannot survive in wetland conditions. In soils of poor fertility and organic matter, azotobacter need to be regularly applied. In addition to nitrogen-fixation, they also produce beneficial growth substances and beneficial antibiotics that help control root diseases.
Like azotobacter, azospirillum species also do not form root nodules or associate with leguminous crops. They are however not free-living and live inside plant roots where they fix nitrogen, and can be used in wetland conditions. This group of microorganisms also produce beneficial substances for plant growth, besides fixing atmospheric nitrogen. Azospirillum does well in soils with organic matter and moisture content, and requires a pH level of above 6.0.
4) Blue-Green Algae
Blue-green algae or cyanobacteria are free-living nitrogen-fixing photosynthetic algae that are found in wet and marshy conditions. Blue-green algae are so named for their colour but they may also be purple, brown or red. They are easily prepared on the farm but can be used only for rice cultivation when the field is flooded and do not survive in acidic soils.
Azolla is a free-floating water fern that fixes nitrogen in association with a specific species of cyanobacteria. Azolla is a renewable biofertiliser and can be mass-produced on the farm like blue-green algae. It is a good source of nitrogen and on decomposition, a source of various micronutrients as well. Its ability to multiply fast means it can stifle and control weeds in (flooded) rice fields. Azolla is also used as a green manure and a high-quality feed for cattle and poultry.
6) Phosphate-Solubilising Microorganisms
These are a group of bacteria and fungi capable of breaking down insoluble phosphates to make them available to crops. Their importance lies in the fact that barely a third of phosphorous in the soil is actually available to the crop as the rest is insoluble. They require sufficient organic matter in the soil to be of any great benefit.
Mycorrhiza is a sweeping term for a number of species of fungi which form a symbiotic association with the plant root system. Of these, the most important in agriculture is vesicular-arbuscular mycorrhiza or VAM. Plants with VAM colonies are capable of higher uptakes of soil and nutrients and water. VAM strands acts as root extensions and bring up water and nutrients from lateral and vertical distances where the plant root system does not reach.
A point to consider before using biofertilisers produced by commercial units is the issue of using microorganisms native to another area or region. It is possible to isolate the required species of microorganisms from a farm’s soil and mass produce them. Besides allowing a better chance of survival of the organism(s) used, this ensures that local species of microorganisms alone are used. There are a few crude (but fairly effective) as well as standard laboratory procedures for isolation and mass production of biofertilisers.