In a previous post, we talked about biofertilisers: Bio” means “life”. Therefore, by definition, biofertilisers are living organisms that enrich the quality of soil nutrients. It refers to the use of microbes instead of chemicals to improve soil nutrition. As a result, it is also less harmful and does not cause pollution.
We will now look at the main classification. The term biofertilisers includes bacteria, fungi and cyanobacteria.
Rhizobia are bacteria that contribute to nitrogen fixation, thus helping to replenish soil nutrients and act as biofertilisers.
You may have already studied nitrogen-fixing nodules on the roots of legumes. This is a great example of a biofertiliser. The nodules are formed by the association of ‘Rhizobium’ bacteria with the roots of these plants. This association is beneficial and is therefore called ‘symbiotic’.
The nodules help fix atmospheric nitrogen into organic forms that can be used as food by the plants. The addition of Rhizobium cultures to fields has become a common practice to ensure an adequate amount of nitrogen in the soil.
Other examples of bacteria that act as biofertilisers are Azospirillum and Azotobacter. These bacteria live freely in the soil. Azotobacter is often used with crops such as cotton, wheat, mustard, maize, etc.
There are also symbiotic associations between plants and fungi. These associations are called “mycorrhizae”. The fungus in this association absorbs phosphorus from the soil and provides it to the plant. Plants grown with these associations also show other advantageous characteristics such as:
– Tolerance to drought and salinity conditions.
– Resistance to root-borne pathogens.
– An overall increase in plant growth and development.
– Increased efficiency in absorbing other nutrients in addition to Phosphorus.
These are blue-green bacteria found in water and soil. They also help fix atmospheric nitrogen. Examples are Oscillatoria, Nostoc, Anabaena, etc. The symbiotic association between the aquatic fern Azolla and Anabaena is very important for rice fields. In this association, Anabaena receives carbon and nitrogen from the plant in exchange for fixed nitrogen. This adds organic matter to the soil and improves the fertility of the paddy fields.
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