Biofertilizers are living microorganisms (blue-green algae, mycorrhizal fungi, and bacteria especially cyanobacteria) containing substances which when adding the nutrients through natural means of nitrogen fixation, synthesis of growth-promoting hormones, restores soil's organic matter, solubilization of micro and macro-nutrients.
Mechanism of Action in Biofertilizers:
Although it is still a mystery for scientists that how, but the mycorrhizal association that fungi develop with the roots of higher plants, stimulates the development of growth factors in the root cells.
Genome studies showed that mycorrhizae lack glucose production. It is dependent upon the plant for it and in response possesses the transporter genes that regulate the transportation of compounds like amino acids, polyamines, oligopeptides, etc.
Some of the major plant growth hormones like ethylene, gibberellins, and auxins are studied to be secreted by Azospirillum spp. A useful combination of various microbes could be made to regulate the synthesis of plant growth hormones because the excessive production of any one of the many suppresses plant growth. (Nosheen, Ajmal, & Song, 2021)
Types of Biofertilizers:
Free living-Nitrogen Fixing Bacteria:
These are the free-living saprophytic bacteria (that feed on the dead or decayed organic matter). They perform nitrogen fixation. Examples are Azotobacter, Clostridium beijerinckii, etc.
Symbiotic association-Nitrogen fixing Bacteria:
Symbiotic association means that the bacteria seek shelter from the plant, food from the organic matter and in return do nitrogen fixation for the plants. In atmospheric gases, the amount of N gas is 79% but plants are unable to fix or utilize this gas while Nitrogen Fixing Bacteria such as Rhizobium do this very efficiently for the plants. (Bhat, Ahmad, Ganai, & Khan, 2015)
Nitrogen Fixing Cyanobacteria:
Cyanobacteria or Blue-Green Algae also develop a symbiotic relationship with the plants they inhabit. Lichens, cycad roots, ferns, and liverworts are some of the very common nitrogen-fixing cyanobacteria. When they die, add organic matter to the soil i.e. their existence as well as death which is beneficial for the plants. (Xue & He, 2015)
A loose association formed by Nitrogen Fixing Bacteria:
Loose association means that the bacteria will remain around the roots of higher plants and fix nitrogen for them but do not develop any association. These are also called “Rhizosphere Bacteria” and they develop a mutual symbiotic association with the plants. (Yang, Kloepper, & Ryu, 2009)
Production/Manufacturing of Biofertilizers:
Different kinds of bacteria and fungi are being exploited in the production of Biofertilizers. Several Biofertilizer formulations are available in the market to directly facilitate the Farmers. The most important microorganisms are listed below:
Components of Biofertilizers:
Vermicompost is the end product of semi-decomposing activities by Earthworms. It is a Biofertilizer rich in micro and macro-nutrients, vitamins, antibiotics, organic carbon, hormones, minerals, in their simplest form/plant-available form. It is eco-friendly and a quick fix for restoring soil fertility.
Bio compost is a cheap, human-friendly decomposed product prepared by fungi, bacteria, and various plants by feeding upon the waste of sugarcane mills. It is a rich source of soil amending ingredients and pollution suppression.
A large collection/colony of gram-negative bacteria not only improves soil nutrient status but also helps in improving crop growth rate. Nature is always beneficiary in all its forms, so does the Azotobacter. One of its major and distinguishing roles is that it protects a plant's roots from the soil-borne pathogen.
A single Tricho-card contains more than 20,000 eggs (of a surrogate host overspread with Trichogramma larvae), these eggs release pests that act as antagonistic hyper parasitic. It is commonly used in preventing g 80-90 % loss (caused by shoot borers, leaf folder, paddy insects, and various other damage-causing pests) in sugarcane, maize, ornamental plants, vegetables, and pulses.
Phosphorus is immobile in the soil and one of the most important nutrients in every plant's growth as well as development. Biofertilizers contain a different kind of fungi and bacteria such as, Pseudomonas, Aspergillus, Bacillus, Penicillium, etc. that converts insoluble P into soluble form for uptake by roots.
Advantages of Biofertilizers:
Organic farming is a successful strategy towards sustainable agriculture and food production. It involves the utilization of all possible natural means including organic fertilizers, natural microflora of soil, cultural practices, and conventional methods of disease and pests control. (Panwar & Amit, 2016)
Biofertilizers are one of the cheapest yet efficient sources of keeping the soil fit for food production through its following functions:
· Nitrogen fixation
· Mineralization of phosphorus and potassium
· Release of PGRs (plant growth Rhizobacteria/regulators)
· AMF (arbuscular mycorrhizal fungi)
With the rapid increase in the human population, there is an urgent need for developing some useful strategies to overcome food shortages and make crop production sustainable within the available resources. The addition of naturally occurring microbes periodically improves the soil's physicochemical properties, soil's microbe population, suppresses soil erosion, loss of nutrients, soil drainage condition, and collectively lead to sustainable agricultural production. (Misra, Sachan, & Sachan, 2020)
Biological Control of Diseases:
With the increase in synthetic insecticides and pesticides, resistance in disease-causing pests and plants has been increased while the harm these are causing to the biodiversity is another issue. This shows that synthetic products may be cheaper and quick but these are not beneficial for the soil as well as the environment.
Biofertilizers act as a supplementary product for soil and agricultural production management through crop rotation, tillage maintenance, organic adjustment, soil fertility restoration, and conventional methods of insect pest control. All these operations would ultimately suppress plant diseases. (Itelima, Bang, Onyimba, Sila, & Egbere, 2018)
Overcoming Productivity Constraints:
Major constraints in crop production are the Abiotic and Biotic stresses. To overcome these stresses naturally, modern science has introduced many “Bio-Protectants”. PGPRs (plant growth-promoting Rhizobacteria) is one of them. They are the alternatives to the chemical treatments applied to avoid various stresses such as salt stress, heavy metal stress, pathogens attack, disease attack, drought stress, physiological, and germination restrictions.
Pseudomonas fluorescens MSP-393 has been found to induce osmolytes and proteins that overcome salt stress in cotton crops. In the same way, P. puttida RS-198, stimulates the synthesis and uptake of Mg2+, K+, and Ca2+, which suppress the absorption of Na+ in cotton plants grown in highly saline soils and also improves germination rate, plant height, fresh weight, plant dry weight, etc.
· Inoculation of Arbuscular mycorrhizae caused enhanced antioxidant response and photosynthetic activity in Rice plants grown in drought conditions.
· Heavy metal (cadmium, lead, mercury, selenium, iron, etc.) pollution is a major threat to future agriculture. Biofertilizers have the potential to overcome the stress that heavy metals induce in plants. For example, Glucanacetobacter sp., Azospirillum spp., and Phosphobacteria spp. isolated from rice fields were found to be more to heavy metals especially Iron. (Cajamarca et al., 2019)
Biofertilizers Control Soil Erosion:
Prevailing Soil Erosion is the most disused issue in agriculture. The only solution to control soil erosion is to shift towards the natural means of farming, use of organic products, and utilizing and promoting mini soil microbes. Biofertilizers are studied in increasing soil organic matter, conditioning the soil, and nutrient recycling. The nodules formation through symbiotic association and the biochemical activity of Biofertilizers binds soil particles together efficiently that eliminates the threat of soil erosion. (Riaz et al., 2020)
Sustainable agriculture is the need and desire of growers. Environmental stresses and increasing resistance to disease-causing agents are the major threats to sustainable production. Biofertilizers are the solution for the soil, plants, biodiversity, and environment. The need is to promote the production of Biofertilizers, enhance research on the hidden benefits of natural soil microbes, and encouraging the farmers, gardeners, landscapers to adopt organic products instead of life-threatening synthetic chemicals. Biofertilizers restores soil fertility, promote natural means of insect pests control, and cause no harm/pollution to the atmosphere.
Bhat, T. A., Ahmad, L., Ganai, M. A., & Khan, O. (2015). Nitrogen-fixing biofertilizers; mechanism and growth promotion: a review. J Pure Appl Microbiol, 9(2), 1675-1690.
Cajamarca, S. M. N., Martins, D., da Silva, J., Fontenelle, M. R., Guedes, Í. M. R., de Figueiredo, C. C., & Pacheco Lima, C. E. (2019). Heterogeneity in the chemical composition of biofertilizers, potential agronomic use, and heavy metal contents of different agro-industrial wastes. Sustainability, 11(7), 1995.
Itelima, J., Bang, W., Onyimba, I., Sila, M., & Egbere, O. (2018). Bio-fertilizers as key player in enhancing soil fertility and crop productivity: A review.
Misra, M., Sachan, A., & Sachan, S. G. (2020). Current aspects and applications of biofertilizers for sustainable agriculture. Plant Microbiomes for Sustainable Agriculture, 25, 445.
Nosheen, S., Ajmal, I., & Song, Y. (2021). Microbes as biofertilizers, a potential approach for sustainable crop production. Sustainability, 13(4), 1868.
Panwar, J., & Amit, J. (2016). Organic farming and biofertilizers: scope and uses of biofertilizers. Organic farming and biofertilizers: scope and uses of biofertilizers.
Riaz, U., Mehdi, S. M., Iqbal, S., Khalid, H. I., Qadir, A. A., Anum, W., . . . Murtaza, G. (2020). Bio-fertilizers: Eco-Friendly approach for plant and soil environment Bioremediation and Biotechnology (pp. 189-213): Springer.
Xue, Y., & He, Q. (2015). Cyanobacteria as cell factories to produce plant secondary metabolites. Frontiers in bioengineering and biotechnology, 3, 57.
Yang, J., Kloepper, J. W., & Ryu, C.-M. (2009). Rhizosphere bacteria help plants tolerate abiotic stress. Trends in plant science, 14(1), 1-4.