Short Notes on Bio-Fertilizers

This article provides a short note on bio-fertilizers.

‘Bio-fertilizers’ or ‘Microbial inoculants’ are defined as preparations containing live or latent cells of efficient strains of nitrogen fixing, phosphate solubilizing or cellulolytic micro­organisms used for application to seed, soil or composting areas with the objectives of increasing the population of such beneficial micro-organisms and accelerate certain microbial processes to augment the extent of the availability of nutrients in a form which can be easily assimilated by plants.

Microbial inoculants or Bio-fertilizers are carrier-based preparations containing different micro-organisms which can fix nitrogen and solubilize phosphorus to increase soil fertility and help plant growth by increasing their numbers and biological activities.

In addition to these mycorrhizal plants (symbiotic association between plant roots and fungal mycelia) increase the surface area of the root system for better absorption of nutrients particularly of phosphorus.

Besides these, the different groups of micro-organisms like fungi, bacteria and actinomycetes which possess the ability to accelerate the process of composing as well as enriching the composts in order to improve the soil fertility and hence plant growth.

There are various types of bio-fertilizers like Rhizobium (for leguminous crops), A zotobacter and Azospirillum (non-leguminous crops), Blue-green bacteria (cyanobacteria), Azolla (water fern, forms a green mat over water which often becomes reddish due to the accumulation of anthocyanin pigments) which can fix nitrogen and acts as a bio-fertilizer augmenting nitrogen in the soil.

The fertilizer preparation with Rhizobium culture is known as “Nitrogen” and the bacterial fertilizer of Azotobacter Chroococcum is known as “Azotobacterin.”

In addition to nitrogen economy in soils, several soil bacteria like Pseudomonas and Bacillus and fungi like Penicillium and Aspergilluspossess the ability to bring insoluble phosphates in soil into soluble forms by secreting various organic acids namely formic, acetic, propionic, lactic etc. These acids lower the pH and bring about the dissolution of bound forms of phosphate.

Cultures of Bacillus megatherium var phosphaticum, termed as phosphobacterins are prepared commercially and distributed to the farmers as phosphatic bio-fertilizers. The use of phosphobacterins is undoubtedly of value in increasing crop yields where use of chemical fertilizers is still rather limited.

The most popular rhizobial cultures (R. Japonicum, R. mililotic etc.) should have the following specifications:

(i) The inoculant shall be carrier based.

(ii) The culture (free from contamination with other micro-organisms) should contain a minimum of 10⁸ viable cells of Rhizobium per gram of the carrier on dry-mass basis within 15 days of manufacture and 10⁷within 15 days before the expiry date (maximum 6 months).

(iii) The pH of the culture shall be between 6.0-7.5.

(iv) The carrier material shall be in the form of powder e.g. peat, lignite, peat soil, humus (neutralized with lime) etc.

The use of rhizobial cultures in the establishment of legumes has become popular and widely accepted. Cultures of both Azotobacter and Azospirillum are used for non-leguminous crops for the nutrition of nitrogen. Some important strains of Azospirillum like A. brasilense, Alipoferum etc. and Azotobacter like A. Chroococcum are used for the preparation of carrier-based bio-fertilizers.

Culture of blue-green bacteria (cyanobacteria) and Azolla are used for the low and rice cultivation as algal bio-fertilizers. Soil based mixed algal cultures containing strains of Tolypothrix, Nostoc, Anabaena, Aulosira, Cylindrospermum etc. are considered as dominant nitrogen fixers and hence can be used as algal bio-fertilizers for the growth of rice plants.

The application of soil based algal mixed culture at the rate of 10-15 kg/ha is generally recommended for rice.

Azolla can also be used as a bio-fertilizer and some important species are Azolla Carolinian, A. nilotica, A. fdiculoides, A. mexicana, A. microphylla and A. pinnata of which A. Pinnata is the most common in India. Azolla inoculum at 0.1 to 0.4 kg per sq m is most desirable for the rapid multiplication of the water fern in nursaries for production of 8-10 t/ha green matter in 20 days.

Generally two methods of Azolla application have been recommended in India:

(i) As a green manure applied in fields before transplanting of rice and

(ii) By dual cropping with rice when Azolla grows side-by-side with rice for some time.

The application of 10 t/ha fresh Azolla is as efficient as basal application of 25-30 kg N/ha.