Global food production has increased by many folds as compared to the past, but still, the pace of agricultural productivity is not adequate for a rapidly increasing population. Presently, chemical fertilizers used for crop yield improvements are not compatible because of their environmental hazards and cost. Indiscriminate use of synthetic fertilizers has led to pollution and contamination of soil and water basins. This has resulted in the soil being deprived of essential plant nutrients and organic matter. It has led to the depletion of beneficial microorganisms and insects, indirectly reducing soil fertility and making crops more prone to diseases. The increasing cost of fertilizers would be unaffordable to small and marginal farmers, thus intensifying the depleting levels of soil fertility due to the widening gap between nutrient removal and supplies. In the present scenario, there has been a real resurgence of environmental-friendly, sustainable agricultural products. Biofertilizers are formulations that contain living microorganisms that have the ability to colonize crop roots and promote growth by improving nutrient availability and acquisition.

Classification of Biofertilizers
Several microorganisms and their interactions with crop plants are being used to create biofertilizers. They can be grouped in different ways based on their nature and function.

Rhizobium: Rhizobium is a soil habitat bacterium, which colonizes legume roots and fixes atmospheric nitrogen symbiotically. Morphology and physiology differ from free-living conditions to nodule bacteroids. They are the most efficient biofertilizers as per the quantity of nitrogen fixed. They have seven genera and are highly specific for forming nodules in legumes, referred to as cross inoculation.

Azotobacter: Among various Azotobacter species, A. chroococcum happens to be the dominant inhabitant in arable soils capable of fixing N2 (2-15 mg N2 fixed /g of carbon source) in culture media. The bacterium produces abundant slime, which helps in soil aggregation. The numbers of A. chroococcum in Indian soils rarely exceeds 105/g soil due to a lack of organic matter and the presence of antagonistic microorganisms in soil.

Azospirillum: Azospirillum lipoferum and A. brasilense are primary inhabitants of the soil, the rhizosphere, and intercellular spaces of the root cortex of graminaceous plants. They develop associative symbiotic relationships with graminaceous plants. Apart from nitrogen fixation, growth promoting substance production (IAA), disease resistance, and drought tolerance are some of the additional benefits of inoculation with Azospirillum.

Cyanobacteria: Both free-living as well as symbiotic cyanobacteria (blue green algae) have been harnessed in rice cultivation.

Azolla is a free-floating water fern that floats in water and fixes atmospheric nitrogen in association with the nitrogen-fixing blue green alga Anabaena azollae. Azolla can be used either as an alternate nitrogen source or as a supplement to commercial nitrogen fertilizers. Azolla is used as a biofertilizer for wetland rice and it is known to contribute 40–60 kg N/ha per rice crop.

Phosphate solubilizing microorganisms (PSM): Several soil bacteria and fungi, notably species of Pseudomonas, Bacillus, Penicillium, Aspergillus, etc., secrete organic acids and lower the pH in their vicinity to bring about the dissolution of bound phosphates in soil. Increased yields of wheat and potatoes were demonstrated due to inoculation of peat-based cultures of Bacillus polymyxa and Pseudomonas striata.

AM fungi: The transfer of nutrients, mainly phosphorus, and also zinc and sulphur from the soil milleu to the cells of the root cortex is mediated by intracellular obligate fungal endosymbionts of the genera Glomus, Gigaspora, Acaulospora, Sclerocysts, and Endogone, which possess vesicles for storage of nutrients and arbuscules for funneling these nutrients into the root system. By far, the commonest genus appears to be Glomus, which has several species distributed throughout the soil.

Silicate solubilizing bacteria (SSB): Microorganisms are capable of degrading silicates and aluminum silicates. Several organic acids are produced during the metabolism of microbes, and these have a dual role in silicate weathering. They supply H+ ions to the medium and promote hydrolysis. Also, they promote the removal of and retention of organic acids like citric, oxalic acid, keto acids, and hydroxy carbolic acids, in the medium in a dissolved state.

Plant growth promoting rhizobacteria (PGPR): The group of bacteria that colonize roots or rhizosphere soil and are beneficial to crops is referred to as plant growth promoting rhizobacteria (PGPR). The PGPR inoculants promote growth through suppression of plant disease (termed Bioprotectants), improved nutrient acquisition (termed Biofertilizers), or phytohormone production (termed Biostimulants). Species of Pseudomonas and Bacillus can produce as yet not well characterized phytohormones or growth regulators that cause crops to have greater numbers of fine roots, which have the effect of increasing the absorptive surface of plant roots for uptake of water and nutrients. These PGPR are referred to as Biostimulants, and the phytohormones they produce include indole-acetic acid, cytokinins, gibberellins, and inhibitors of ethylene production.

The formulation of biofertilizers is a crucial multistep process that includes the mixing of a suitable carrier with an inoculant, providing optimal conditions during storage, packaging, and dispatch, and ensuring survival and establishment after introduction into soil.

Methods of Application of Biofertilizers
Seed Treatment: 200 g of biofertilizer is suspended in 300-400 mL of water and mixed gently with 10 kg of seeds using an adhesive like gum acacia, jiggery solution, etc. The seeds are then spread on a clean sheet or cloth under shade to dry and can be used immediately for sowing.

Seedling Root Dip: This method is used for transplanted crops. For rice crops, a bed is made in the field and filled with water. Recommended fertilizers are mixed in this water and the roots of seedlings are dipped for 8–10 h and transplanted.

Soil Treatment: 4 kg each of the recommended biofertilizers is mixed in 200 kg of compost and kept overnight. This mixture is incorporated in the soil at the time of sowing or planting.

Advantages of Using Biofertilizers
Some of the advantages associated with biofertilizers include:

• They are eco-friendly as well as cost-effective.
• Their use leads to soil enrichment, and the quality of the soil improves with time.
• Though they do not show immediate results, the results shown over time are spectacular.
• These fertilizers harness atmospheric nitrogen and make it directly available to the plants.
• They increase the phosphorus content of the soil by solubilizing and releasing unavailable phosphorus.
• Biofertilizers improve root proliferation due to the release of growth-promoting hormones.
• Microorganisms convert complex nutrients into simple nutrients for the availability of the plants.
• Biofertilizer contains microorganisms which promote the adequate supply of nutrients to the host plants and ensure their proper development of growth and regulation in their physiology.
• They help in increasing the crop yield by 10–25%.
• Biofertilizers can also protect plants from soil-borne diseases to a certain degree.

Constraints in Biofertilizer Technology
Despite the fact that biofertilizer technology is a low-cost, environmentally friendly technology, several constraints limit its application or implementation. The constraints may be:

• Technological constraints like the unavailability of good quality carrier material and a lack of qualified technical personnel in production units.
• Infrastructural constraints like lack of essential equipment, power supply, etc.
• Financial constraints like non-availability of sufficient funds and problems in getting bank loans.
• Environmental constraints like seasonal demand for biofertilizers, simultaneous cropping operations, and a short span of sowing/planting in a particular locality, etc.
• Human resource and quality constraints, such as a lack of technically qualified personnel in production units and a lack of appropriate production technique training,
• Unawareness of the benefits of the technology due to problems in adoption of the technology by the farmers due to different methods of inoculation, no visual difference in the crop growth immediately as that of inorganic fertilizers.

Application of biological fertilizers is thought to be a key element in maintaining soil fertility and crop productivity at a sufficiently high level, which is indispensable to achieve the sustainability of farming. Biofertilizers may also help mitigate pitfalls arising from the growing demand of the global population for food and from the widespread chemicalization of agroecosystems. The changing approach to agricultural practices makes biofertilizers a vital part of modern-day crop production and emphasizes the significance of biological inoculants in forthcoming years.

References
DebmalyaDasguptaa,KulbhushanKumarbRashi,MiglanibRojita,MishracAmrita Kumari,PandadSatpal SinghBishtb.(2021). Microbial biofertilizers: Recent trends and future outlook:Toward the establishment of Microbial biofertilizers: Recent trends and future outlook.

S. Kannaiyan, Cyanobacterial biofertilizer technology for rice crops. In: Algal biotechnology (Trivedi, P.C. Ed.). Pointer publishers, Jaipur, India.2001. Rakesh Kumar, Narendra Kumawat,Yogesh Kumar Sahu.(2017). Role of Biofertilizers in Agriculture.

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Featured image – https://Anuvia aims to have its biofertilizer on 20m farm acres by 2025 (agfundernews.com)
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