What are biofertilizers? Explain them with suitable examples.

Hint: Fertility of soil is very important from viewpoint of agriculturist. The fertility of soil depends not only on its chemical composition but also on the quality and quantity of useful microbes in it. These materials are grouped under two broad categories –(i) manures and (ii) biofertilizers.

Step by step answer:Biofertilisers: these are the microorganisms which bring about soil nutrient enrichment, maximize the ecological benefits, and minimize the environmental hazard. For this purpose, the specific microorganisms are cultivated in scientific laboratories and then sup[plied to the farmers so that they can be utilized in the fields. Types of biofertilizers:
● Microphos biofertilizer: plants sometimes show deficiency of phosphorus even if they grow in phosphorus-rich soil. This is because most of the phosphates are present in the soil in the form of
immobile or bound forms. The plants absorb only available forms of phosphates. Microphos biofertilizer includes microorganisms which solubilise bound phosphate of rocks and soil to increase the availability of soluble phosphate to the plants. Eg Pseudomonas striata
● Rhizobial biofertilizers ( symbiotic nitrogen-fixing bacteria ): the bacteria Rhizobium multiplies as soon as it gets the rhizosphere of leguminous plants, enter into their roots and form specialised structures called root nodules. The bacteria fix atmospheric nitrogen only in the root nodules and when present in the bacterioid forms.
● A loose association of nitrogen-fixing bacteria: certain bacteria, living in close contact with roots of cereals and grasses, fix nitrogen. This association is a loose mutualism, called associative symbiosis. Some of the fixed nitrogen is absorbed by the roots and sometimes enter the roots.
● Cyanobacterial biofertilizers: these organisms grow well in symbiotic association with other plants or as free-living individuals on the surface of moist soil or under logged condition. Eg. Anabena, nostoc, etc

Note: Azolla anabena symbiosis, mycorrhiza are some of the other biofertilizers. Some of the biofertilizers enhance crop yield even under ill irrigated conditions where chemical fertilizers are of not much advantage. They do not cause atmospheric pollution.